Tuning help needed - '91 TBI 383 w/ Vortec heads

Hi all, could use some tuning advice.

I've built and installed a vortec-head 383 in my '91 squarebody suburban. 3/4 ton, 4wd, 4L80e tranny. 16147060 PCM, using $85 mask. Here's my intro thread for some more info:

http://www.gearhead-efi.com/Fuel-Inj...ortec-head-383

Considering my complete lack of experience with tuning, it's running decent. I know it's running rich and I'm continuing to play with the BLM table using my log files and an adjustment spreadsheet that @Dave W provided me. But other than that, I'm unsure of where to go next with tuning, e.g. spark, IAC, choke, etc. My two biggest concerns (other than still a bit rich) are,

1. Significant surging during compression braking. I can hear it out the tailpipe, see it in the tach (300-400 rpm surge range, cycle time is about 1 second) and feel it in the seat of the pants. It might be doing the same thing, but very slightly, during steady, low speed cruise, too. But going down hills with foot off throttle and brakes, it's pretty bad.

2. High idle at cold startup. 1200-1300 rpm, doesn't match target idle in .BIN. Takes several minutes before it will even come down to below 1000.

Do these issues sound like they're related? Vacuum leak? I've attached all the files I'm using in TunerPro, including my two latest log files and my latest modified .bin file - just in case anyone has the desire to take a look.

Oh, and EGR has been deleted via RPM limits (Dave W helped with that on initial .BIN creation) and no EGR valve physically installed. EGR is complicated on Vortecs - need to route exhaust gas from DS exhaust manifold to intake manifold.

Thanks much!

It looks like three weeks and no one responded. Do you have a wide band o2 hooked up? If so what's it reading during your decel problem? You could have a vacuum leak, the only way to tell is to check. A smoke machine is what I prefer. There are instructions online on how to make one for not much money. There are other ways, but sometimes they miss leaks.

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Originally Posted by donf

It looks like three weeks and no one responded. Do you have a wide band o2 hooked up? If so what's it reading during your decel problem? You could have a vacuum leak, the only way to tell is to check. A smoke machine is what I prefer. There are instructions online on how to make one for not much money. There are other ways, but sometimes they miss leaks.

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Thanks for the response. I haven't been able to do anything for a couple weeks due to a severely leaking tranny, but that is resolved now so back to the tuning. I'm leaning away from vacuum leak since it's now running pretty good with the only exception being surging at deceleration. Decelerates smoothly down to about 1500 rpm and then between there and about 1100 rpm there's a crazy oscillation between the two, back and forth on the tach and audible. Then once it's down below 1100 and all the way to 650 idle - smooth as butter. I found another thread on here that may hold some insights. Going to start working through some of those issues. I'm still running pretty rich, too. I also recently did the adjustable FPR conversion (using the stock regulator and making the spring cup adjustable) -- I only had about 8 psi before that, now I'm back up to 12-13.

No WB O2 sensor. I may go that way, but would prefer to stay stock if I can get it to work. I did get a new Y-pipe made recently and I had him put in additional bungs but not sure they'll work for wide band. Stock O2 is just after the two legs of the Y come together, but he added bungs upstream of that, one in each leg of the Y. Would that work?

Great progress! The other thread you found has some great info. The reason the factory had the unheated o2 in the exhaust manifold was heat, when they relocated it to the y pipe in later years they installed a heated three wire o2 from the factory. The wide band is also heated and can be mounted pretty much anywhere in the exhaust. I wouldn't waste my time without a wideband. It can be done but really the tune should be more accurate with the wideband and dataloging. I see it all the time, people spend thousands upgrading then compromise the results with a shifty tune. A wide band is cheap and can be sold or rented to friends once your done with it.

Thanks, I'm definitely considering the wideband. Going to start though with just converting existing O2 to three wire. I agree with the reasons you state about why they changed to 3-wire, but I don't think they had made that change yet in '91. As best I can tell, it came stock in the current location (after the Y pipe junction, before the cat) and was never heated. But it seems like a good start to just swap out that one wire unit for a 3 wire. Then, if I want to add WB, I can add it into one of the banks, before the y-junction, since I have a bung in each bank now.

I think I finally got the surge at deceleration issue resolved today. Minimum idle air was misadjusted. Fortunately my re-man and bored throttle body already had the cap over the throttle stop screw removed, so I didn't have to worry about that. It made following the procedure here really easy. After getting the base idle adjusted I checked the TPS voltage at idle and had to adjust it slightly - ended up right at 0.51 volts which is pretty much optimal. (Used ALDL cable and TunerPro R/T to look at TPS voltage) Took it for a short drive and surge at decel seems to be gone. Fingers crossed.

If Decel surge continues, try reducing spark advance by 2-3 degrees in the cells affected by decel.

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Originally Posted by 1project2many

If Decel surge continues, try reducing spark advance by 2-3 degrees in the cells affected by decel.

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Thanks for this. I guess I was wrong - the decel surge is still there although it seems to be a little softer. Now I'm wondering if there is an IAC problem that could be related. Most of the time it will idle at or near target idle speed (650 when warm), but it will occasionally idle higher. I drove it 45 miles at highway speed a few days ago, at the end it was idling over 1000. Shut it off and restarted a minute or two later and was back to normal. IAC sticking? I have a spare IAC that I'm going to swap in to see if anything changes.

If IAC does not solve problem, I will try the timing reduction in the affected cells - thanks!

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Originally Posted by Daveo91

Thanks for this. I guess I was wrong - the decel surge is still there although it seems to be a little softer. Now I'm wondering if there is an IAC problem that could be related. Most of the time it will idle at or near target idle speed (650 when warm), but it will occasionally idle higher. I drove it 45 miles at highway speed a few days ago, at the end it was idling over 1000. Shut it off and restarted a minute or two later and was back to normal. IAC sticking? I have a spare IAC that I'm going to swap in to see if anything changes.

If IAC does not solve problem, I will try the timing reduction in the affected cells - thanks!

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I am not sure if it helps but there is idle overspeed spark retard and idle underspeed spark advance tables at least with the 95 tbi computer. My cam is very mild but even with it, the timing was bouncing all over the place like a yo yo with the factory settings and the idle had a little lope/instability. I neutered the tables for advance or retard up until a 50 rpm difference then after that decreased the adjustment range slightly. The idle even with the almost stock ramjet cam smoothed out and the timing is very stable at idle now. Even the definitions list the problem " Idle overspeed/ underspeed corrections .... This is why timing is constantly moving around at idle." Not sure if it will help your problem or not but since your IAC adjustment seem to help. I figured I would mention it. I did not remove all the correction, just toned it down a bit.

I've been wondering for ages what was causing my spark timing to bounce around at idle so much!
It would be much appreciated if you could give me an example of how your tables vary from my stock ones (shown in the attachment).
I'm keen to try this out :-)

Idle Overspeed Spark Retard vs. RPM Error 0.0 0.0 0.0 0.0 1.1 2.1 3.2
Idle Underspeed Spark Advance vs. RPM Error 0.0 0.0 0.0 0.0 2.1 2.1 3.2
It's not my tip, someone that is very familiar with the later LS computers told me to check that when I was wondering why the timing was so unstable at idle. My $0D computer has it, not sure about the earlier ones.

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Originally Posted by donf

I am not sure if it helps but there is idle overspeed spark retard and idle underspeed spark advance tables at least with the 95 tbi computer. My cam is very mild but even with it, the timing was bouncing all over the place like a yo yo with the factory settings and the idle had a little lope/instability. I neutered the tables for advance or retard up until a 50 rpm difference then after that decreased the adjustment range slightly. The idle even with the almost stock ramjet cam smoothed out and the timing is very stable at idle now. Even the definitions list the problem " Idle overspeed/ underspeed corrections .... This is why timing is constantly moving around at idle." Not sure if it will help your problem or not but since your IAC adjustment seem to help. I figured I would mention it. I did not remove all the correction, just toned it down a bit.

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Thanks for the tip, but, yeah, I don't think my older ECM has those tables. Searched for overspeed and underspeed with the parameter finder and nothing. Searching for "decel" gives me two: Decel Enlean Multiplier Map Diff (Scalar) and Decel Enleanment Coolant Factor vs Coolant temp. I've been looking at what DFCO is set at - looks like it's a pretty narrow RPM range that is a little outside where I'm seeing the surge, so not sure that would be the problem?

I just found two more exhaust leaks upstream of the O2 sensor tonight. I had a custom y-pipe made but have found three pinhole leaks in two different welds so far. A little bummed. Going to get those fixed before I do anything more with tuning or chasing out the surge. If surge still there after exhaust tightened up, I'll swap IACs and see if that changes anything. Then I'll dial back timing just a little in cells affected by decel. Thanks for the help!

I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

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I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

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Great read!

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Originally Posted by PlayingWithTBI

Great read!

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It's one of the better sources of info I have found. Its missing the last part, the tuned examples according to the index. Even so it's worth saving a copy.

I tried out zeroing out the overspark and underspark vs RPM error as you said and it did hold my idle spark at a steady 27 degrees @725 RPM in Park, before that it would bounce around a couple of degrees.
It didn't seem to work so well idling in gear at 575 RPM, the spark still moved around a bit (21-24) and where previously the RPM would stay pretty constant between 600-575 it was now occasionally dropping to 550 RPM. I guess that's part of the job of overspark/underspark vs RPM to try and help hold the RPM near it's commanded value, I'm also guessing there's more than just overspark/underspark trying to do that job as well.
Great tip but once again when it looks like I might have finally solved something the final answer only ends up getting more complicated.

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Originally Posted by Kitch

I tried out zeroing out the overspark and underspark vs RPM error as you said and it did hold my idle spark at a steady 27 degrees @725 RPM in Park, before that it would bounce around a couple of degrees.
It didn't seem to work so well idling in gear at 575 RPM, the spark still moved around a bit (21-24) and where previously the RPM would stay pretty constant between 600-575 it was now occasionally dropping to 550 RPM. I guess that's part of the job of overspark/underspark vs RPM to try and help hold the RPM near it's commanded value, I'm also guessing there's more than just overspark/underspark trying to do that job as well.
Great tip but once again when it looks like I might have finally solved something the final answer only ends up getting more complicated.

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There is an idle speed table for both in gear and out. Even with the small cam in my van, I have the hot idle speed at 600 and it stays there without wavering at all.

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Originally Posted by donf

I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

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Thanks for the link, ton of valuable info in there. Saved it to my iPad and am part way through reading it. It doesn't seem to cover my specific ECM (PCM actually since I have 4L80e) but still pretty applicable on the engine side.

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Originally Posted by donf

I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

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THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.

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Originally Posted by Redtires019

THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.

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Glad to share, even with the 95 computer it's helpful. I actually tried to track the author down. I wanted to see if I could get all the pages. From the index its missing the tuned examples, I think that would be educational. The website mentioned in the pamphlet is still registered to someone since early 2000's but it's got the info blocked for privacy.
Edit: I did some searching and actually found the complete pamphlet here zipped in docs, under CustomEfis Setup. The tuning examples were never in the info, probably why the others cut it off. It's just a link to a long gone website. It is a cleaner copy though.

So I've made some more progress on my tune. I'm down to two problems, one has been persistent since the beginning.

1. A very quick bog or stumble right off idle, light accel only. It's worse when it's cold, when warm it is still there, but very slight. I've hunted around in my log files looking for the problem, but have not had much luck. Definitely spending more time on #2.....

2. Annoying, cyclic surge at deceleration. Most persistent at 1600-1400 rpm, but will happen higher or lower than that as well. very consistent - virtually always occurs when there's a negative load (overrun?) on the engine. RPM will drop 200-400 and then immediately come back up. Entire cycle takes about a second or even less. It will just keep repeating that until it gets to an idle state or close or the negative load is gone. Idles fine and steady. I've always thought it was fuel or spark, but I've started wondering if this could be a 4L80e problem? Overrun clutch? I live on a hill and have put it in 1st and accelerate up to 2500 or 3000 and then let it decel on it's own. Very pronounced when I do this - feels like something is letting go for split second and then reengaging. It does happen whether I select a lower range manually, in D, and in OD so that kind of leads me away from it being a tranny problem. Any advice appreciated!

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Originally Posted by Daveo91

So I've made some more progress on my tune. I'm down to two problems, one has been persistent since the beginning.

1. A very quick bog or stumble right off idle, light accel only. It's worse when it's cold, when warm it is still there, but very slight. I've hunted around in my log files looking for the problem, but have not had much luck. Definitely spending more time on #2.....

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I am not sure how similar the early computers are to the 94-95 but it sounds like an Acceleration Enrichment problem since its worse when cold. Do you have a wideband o2 logging data as you drive? If the wideband data looks OK, then try a slight timing increase in the same cells you are having the problem in.

Thanks, donf. No wideband yet. In the master plan, but I'm trying to get all the other issues ironed out first. For a while I had both exhaust leaks and an intake leak that were causing all sorts of problems - those are all fixed now so on to the other stuff.

My #2 issue has me concerned. Any thoughts from anyone whether the cyclic surge on decel could be caused with fuel/timing parameters on the engine side, or is consensus that this is a tranny issue?

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Originally Posted by Daveo91

Thanks, donf. No wideband yet. In the master plan, but I'm trying to get all the other issues ironed out first. For a while I had both exhaust leaks and an intake leak that were causing all sorts of problems - those are all fixed now so on to the other stuff.

My #2 issue has me concerned. Any thoughts from anyone whether the cyclic surge on decel could be caused with fuel/timing parameters on the engine side, or is consensus that this is a tranny issue?

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The wideband info can help on both. We are on this side of the computer screen, so its very hard to diagnose a problem. You have to start with what you know and your flying blind on a lot of this without logging wideband readings. If it starts acting weird you can log that and know for sure its not or it is fuel related. With a narrow band you may know its rich or lean, but they are basically Stoic sensors and nothing more accurate than that. If you are looking at the block learn averages to change to spot something like this, it may if it happens for long enough, but its not the best way. If you positively cant afford a wide band I would look at the logged data in spreadsheet form anyway without it. See what it changing frame by frame over that particular problem. Whether its IAC counts, or timing, or the very limited fuel mixture info you already have access to. Post the info in spreadsheet form for the few seconds when the problem is happening and maybe others can take a look too.

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Originally Posted by donf

The wideband info can help on both. We are on this side of the computer screen, so its very hard to diagnose a problem. You have to start with what you know and your flying blind on a lot of this without logging wideband readings. If it starts acting weird you can log that and know for sure its not or it is fuel related. With a narrow band you may know its rich or lean, but they are basically Stoic sensors and nothing more accurate than that. If you are looking at the block learn averages to change to spot something like this, it may if it happens for long enough, but its not the best way. If you positively cant afford a wide band I would look at the logged data in spreadsheet form anyway without it. See what it changing frame by frame over that particular problem. Whether its IAC counts, or timing, or the very limited fuel mixture info you already have access to. Post the info in spreadsheet form for the few seconds when the problem is happening and maybe others can take a look too.

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thanks again Don, I’ve started shopping for WB. I’m convinced. In the meantime I’ll see if I can do what you suggest with the log files frame by frame in excel. I haven’t done that yet but I think is should be able to figure it out.

If you have an android phone and the blue tooth ALDL, Johns free app "ALDL Scan Tool" works well for this too. The wide band requires a totally different input setup, but for ease it saves the log file as a csv and seems to be just as accurate as all the others. Sometimes I just hit the record button and look at the data when I get home.

You could always do what I did before cheap, good commercial widebands were available. Disablw the BLM and tune off the integrator alone. Integrator reponds much more quickly.

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Originally Posted by donf

If you have an android phone and the blue tooth ALDL, Johns free app "ALDL Scan Tool" works well for this too. The wide band requires a totally different input setup, but for ease it saves the log file as a csv and seems to be just as accurate as all the others. Sometimes I just hit the record button and look at the data when I get home.

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No android phone, I use an iphone. I'll just stick with windows laptop/TunerPro/ALDL cable. But thanks for the tip.

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Originally Posted by Fast355

You could always do what I did before cheap, good commercial widebands were available. Disablw the BLM and tune off the integrator alone. Integrator reponds much more quickly.

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Hmm, thanks for the tip. Might try that.

Guys, I've thought about disabling closed loop just for diagnostic purposes. I'm curious to know if these decel surge issues are still present in open loop - seems like if they're not that would help me zero in on the problem at least a little more. Can I just unplug the O2 sensor or should I do the disabling in the BIN? If BIN do I just increase the timer a big amount?

And one other thing I'm trying: have a new MAP sensor on order. I'm not big on throwing parts at problems, but I found a NOS GM on ebay for $15. MAP is one of the few things I reused after my rebuild and I just want to eliminate that being the problem.

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Originally Posted by Daveo91

No android phone, I use an iphone. I'll just stick with windows laptop/TunerPro/ALDL cable. But thanks for the tip.



Hmm, thanks for the tip. Might try that.

Guys, I've thought about disabling closed loop just for diagnostic purposes. I'm curious to know if these decel surge issues are still present in open loop - seems like if they're not that would help me zero in on the problem at least a little more. Can I just unplug the O2 sensor or should I do the disabling in the BIN? If BIN do I just increase the timer a big amount?

And one other thing I'm trying: have a new MAP sensor on order. I'm not big on throwing parts at problems, but I found a NOS GM on ebay for $15. MAP is one of the few things I reused after my rebuild and I just want to eliminate that being the problem.

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There are different ways to end up with different results. Look at the voltage shape of the typical narrow band o2 sensor. You may be able to get you block learn dialed in and its fine for that, but what about your WOT mixture that's not suppose to be Stoic?? After all you have put into you project do you really want to skimp with $160 and a few wires to log the data? If money is that tight, sell it on craiglist when you are done. No one will ever say a narrow band is superior for tuning, you may be able to "get by" but really for the money involved, why? I did not want to bother with mounting a gauge or cutting holes so the new AEM line has one that just data logs to a computer. The accuracy is very close to the big $$ Haltech setup on my chassis dyno.

Attachment 14167

Have you blended your main and idle spark tables in the areas where they crossover (I'm assuming you have tracing turned on)?

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Originally Posted by donf

There are different ways to end up with different results. Look at the voltage shape of the typical narrow band o2 sensor. You may be able to get you block learn dialed in and its fine for that, but what about your WOT mixture that's not suppose to be Stoic?? After all you have put into you project do you really want to skimp with $160 and a few wires to log the data? If money is that tight, sell it on craiglist when you are done. No one will ever say a narrow band is superior for tuning, you may be able to "get by" but really for the money involved, why? I did not want to bother with mounting a gauge or cutting holes so the new AEM line has one that just data logs to a computer. The accuracy is very close to the big $$ Haltech setup on my chassis dyno.

Attachment 14167

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I get it that WB is superior for tuning outside of closed loop. Not arguing and, no, money is not that tight. I just haven't gotten to it yet. I have other projects and I stagger the time and $$ I put into all of them. Suburban is drivable as is so it's not number one priority. Am looking around online for a WB kit, but in the meantime I don't see the harm in spending $15 on a new MAP to see if I can rule that out, same with forcing it into open loop and taking a short test drive. I'm still concerned I have some unresolved mechanical issues, too.

So some beginner questions on WB.... You mention you're not using a gauge and just a single wire into a computer? How does that work, is it a USB? Does it sync with the log file from the ALDL/ECM output? If not how would you reference it back to time-based events like a throttle stab?

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Originally Posted by Kitch

Have you blended your main and idle spark tables in the areas where they crossover (I'm assuming you have tracing turned on)?

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Thanks, no, I have not been using tracing. But I just read the help section in TP on that and I'm definitely going to start using that feature, assuming I can get it to work. Sounds like the ADX and XDF have to have values linked between them - not sure if the ones I've been using have that.

I have my sensor wired into the ecm. The instructions are the same for logging even though there is no gauge. Its important that you download the zip and follow ALL the instructions. I have had several people here contact me privately saying their wide band is not working. I ask them to follow the instructions listed in the zip file and never hear from them again. Ha ha. I think people just think its hooking up a wire and your done. I also had to redo some of the adx file. The A217 OD file has some quirks. Some stuff led me to change things and now I like it better. One peeve is they have extra data points that look cool for the fuel table but have no value as the bin does not have those same points. I kept getting data from the useless columns/cells mixed up with the ones I needed. Now my ADX does not have those!

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Originally Posted by donf

I have my sensor wired into the ecm. The instructions are the same for logging even though there is no gauge. Its important that you download the zip and follow ALL the instructions. I have had several people here contact me privately saying their wide band is not working. I ask them to follow the instructions listed in the zip file and never hear from them again. Ha ha. I think people just think its hooking up a wire and your done. I also had to redo some of the adx file. The A217 OD file has some quirks. Some stuff led me to change things and now I like it better. One peeve is they have extra data points that look cool for the fuel table but have no value as the bin does not have those same points. I kept getting data from the useless columns/cells mixed up with the ones I needed. Now my ADX does not have those!

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Thanks for the great info. Unfortunately, my project has hit a major mechanical stumbling block - my new 383 seems to be using significant oil. And it's not going on the ground. The only explanation I have is that I have the wrong oil rings or they didn't seat correctly. I haven't talked to the machinist yet that did the work (I assembled) but I'm guessing I'm going to have to pull the motor and tear it down. Obviously tuning will go on hold if all this is accurate.

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Originally Posted by Daveo91

Thanks for the great info. Unfortunately, my project has hit a major mechanical stumbling block - my new 383 seems to be using significant oil. And it's not going on the ground. The only explanation I have is that I have the wrong oil rings or they didn't seat correctly. I haven't talked to the machinist yet that did the work (I assembled) but I'm guessing I'm going to have to pull the motor and tear it down. Obviously tuning will go on hold if all this is accurate.

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Thats too bad. It has to be disappointing.

Bummer - hope it's something simple like the rings' gaps lined up in a row vertically (I've seen it before).

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Originally Posted by PlayingWithTBI

Bummer - hope it's something simple like the rings' gaps lined up in a row vertically (I've seen it before).

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Thanks to both of you. Yeah, I’ve been hoping for something simple for a while now, have actually been in denial. But I can’t deny 1/3 to 1/2 quart in 300 miles any longer. Funny thing is plugs do not look oil fouled to me at all, and the (hot) compression is good and consistent in all cylinders (approx 180 at three hits and 200-210 at five hits). And there are not clouds of smoke going down the road, even when I get on it with high load. It will puff smoke if I gun it at idle, though.

As for ring install, I followed the instructions that came with the Hastings rings to a T. There’s a backstory tho: I had to switch machinists mid-build. The first one was hitting the sauce pretty hard and he had about five missed deadlines. He also bought pistons that would have given me 10:1 CR with vortec heads. So I found a new machinist and bought some KB pistons that lower my CR half a point. Second guy, who has a good rep building high end motors, said the rings would be fine with the new pistons but now I have some doubts.
Attachment 14191Attachment 14192

Some rings take a different grit hone. Its possible if between the two machinists, the wrong grit was used the rings may take longer to seat or may not seat at all. The plugs look really good, so not a lot of oil is getting in there. I made the mistake of ordering Jegs crappy vortec heads, the guide seals leak even with under a 1000 miles on them. i will have to tear them down this winter.

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Originally Posted by donf

Some rings take a different grit hone. Its possible if between the two machinists, the wrong grit was used the rings may take longer to seat or may not seat at all. The plugs look really good, so not a lot of oil is getting in there. I made the mistake of ordering Jegs crappy vortec heads, the guide seals leak even with under a 1000 miles on them. i will have to tear them down this winter.

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Thanks for the info. I know there was something with the hone. The first guy did his own, but I know the final machinist did a cross-hatch hone too. He didn’t want to rely on the other guy’s work. I’m going to change the oil one more time and see if that helps. Initial start up and first few hundred miles were on conventional oil (Chevron Delo, I think) but then I went to Mobil 1 synthetic. Sounds like there’s at least a possibility that’s making things worse. I’m going back to probably Shell Rotella, put a couple hundred miles on it and see if there’s improvement. Also ordered a new AC Delco PCV valve tonight - just in case the parts store one in there now is adding to the problem.

how are you breaking in the engine? with a flat or hydraulic tappet cam, after initial break in i will dump the oil, but why are you changing oil after a few hundred miles? i normally goto a "normal" oil change interval and just add oil to make sure it stays at full until next oil change. i would do at least 2 normal OCIs befote switching to synthetic, but that's me. im also a fan of "beating on it" vs the old school way of driving like grandma.