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

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  1. #1
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    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!
    Attached Files Attached Files

  2. #2
    Fuel Injected! donf's Avatar
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    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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    Quote Originally Posted by donf View Post
    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.
    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?

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    Fuel Injected! donf's Avatar
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    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.

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    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.

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    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.

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    Fuel Injected! donf's Avatar
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    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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    Quote Originally Posted by donf View Post
    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!
    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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    Fuel Injected! donf's Avatar
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    Quote Originally Posted by Daveo91 View Post
    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.
    Thats too bad. It has to be disappointing.

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

  11. #11
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    Quote Originally Posted by PlayingWithTBI View Post
    Bummer - hope it's something simple like the rings' gaps lined up in a row vertically (I've seen it before).
    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.
    7C1153E7-6C07-446A-8F23-5B702E61D86C.jpg17A07925-4A18-4A12-B9BE-B7D27B20DE57.jpg

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