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Thread: Newbie here with a 2+ year problem

  1. #1
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    Newbie here with a 2+ year problem

    Greetings,

    Your mission, should you choose to accept it, but I warn you, it will be tiring.

    Hoping to find someone who can read GM TBI logs and know what they mean. Long story VERY short: '93 Suburban, new engine. On a cold start, regardless of ambient temperature it will run for 20 seconds just fine, then it goes way rich for about 45 seconds then back to normal and we're good to go with no further issues. I have replaced everything I can think of...twice. I'm at my wits end....anyone?

    I have a thread on a GM forum if anyone really wants more detail. I've worn out the GM people, they are stumped.


    https://drive.google.com/open?id=175...VtAvN5pIBK6U4H

    Thanks!

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    Hello,

    Sounds like a tough problem to fix. No promises from me but if you help by providing some basic information you might get some suggestions. Year/make/model of vehicle along with powertrain specs, ecm #, and broadcast code are some of the basics. If you could explain what the logs are showing (I looked and only one clearly indicates 20 seconds of run time) and post a link to the other thread that might help further.

  3. #3
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    Quote Originally Posted by 1project2many View Post
    Hello,

    Sounds like a tough problem to fix. No promises from me but if you help by providing some basic information you might get some suggestions. Year/make/model of vehicle along with powertrain specs, ecm #, and broadcast code are some of the basics. If you could explain what the logs are showing (I looked and only one clearly indicates 20 seconds of run time) and post a link to the other thread that might help further.
    Sure thing: 93 Suburban, 5.7, 4L60-E, 4WD, 3:73's I dont have ecm or broadcast code. Its still a daily driver at this point in time. Yes, thats how long it took for the thing to start running correctly, just that log.

    This is the newest thread: https://www.chevytalk.org/fusionbb/s...d/353434/tp/1/

    This is an older one, but still the same: https://www.chevytalk.org/fusionbb/s.../post/2701171/

    Here's and older video, again the same thing: https://youtu.be/L5FVl_sLXVY

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    Whew. There's a lot of reading to go over. And I have a feeling that you're not going to like the "by the book" suggestions you're likely to continue to receive. But the people who do a good job diagnosing EFI system problems also understand that the systems do not like seat of the pants changes and good enough for me settings.

    Diagnosing troubles like the Suburban's takes accuracy and observation. For example, you said in your first post here that the problem occurs 20 seconds after startup. I looked through the log you posted for signs of a problem 20 seconds after startup. In a post in one of your links you say 15 seconds, and then in a third you have a different time again. For people who work with computers, if you give a specific time or value we often rely on that number to be accurate. And when we respond with specific numbers we mean exactly those numbers. It's not because we're that way. It's because computers are that way. This way of looking at problems can be tough for non-computer folks to work with but it can make all the difference between finding and missing an issue. Now, on to the help.

    There were many good suggestions in posts that your links lead to but there weren't enough questions imo. Let's try to make sure there's a comprehensive picture of what's going on.

    Why was the original engine replaced? Did this outfit run fine prior to the swap? Were you the owner before the engine replacement? Was the engine replaced at a shop?

    Is the picture of the invoice from Sunwest Engines for your engine? Did you choose this company to supply the engine? Why? Did you select this part number for the engine or was it recommended for you? Did you get the base cam or the optional RV cam?

    Is your distributor now set at the value indicated on the sticker under the hood? I believe it should be 0 degrees? Or did you leave it at 6 degrees?

    Have you ever confirmed that the timing mark is correct for the timing tab? Not that the part number is correct, but that the piston is at TDC when the mark indicates TDC.

    Have you ever measured the fuel pressure? You said the injectors were rebuilt. Have you ever confirmed you weren't sent the wrong injectors?

    What are you using to scan and save the computer data? Have you ever recorded computer data while driving? A recording of computer from a spell on the highway when the truck is running correctly could be helpful.

    You say the problem is rich running. Is this because you can smell fuel fuel from the exhaust? Anything else? Do you have a cat installed? Does the problem clear up completely when the engine runs better?

    BTW, I made a living working on these trucks in a GM dealership in eastern MT when they brand new. I also worked nights and weekends repairing equipment on the ranches in the surrounding county. Unlike folks that live near town, I know what it's like when a trip to the store for a tool or a part is an all day affair.

  5. #5
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    Quote Originally Posted by 1project2many View Post
    Whew. There's a lot of reading to go over. And I have a feeling that you're not going to like the "by the book" suggestions you're likely to continue to receive. But the people who do a good job diagnosing EFI system problems also understand that the systems do not like seat of the pants changes and good enough for me settings.

    Diagnosing troubles like the Suburban's takes accuracy and observation. For example, you said in your first post here that the problem occurs 20 seconds after startup. I looked through the log you posted for signs of a problem 20 seconds after startup. In a post in one of your links you say 15 seconds, and then in a third you have a different time again. For people who work with computers, if you give a specific time or value we often rely on that number to be accurate. And when we respond with specific numbers we mean exactly those numbers. It's not because we're that way. It's because computers are that way. This way of looking at problems can be tough for non-computer folks to work with but it can make all the difference between finding and missing an issue. Now, on to the help.
    I should have said "when you see the numbers change, thats when it happens". :) Actually, it varies on ambient temperature but you are right, I was giving you a rough number.

    There were many good suggestions in posts that your links lead to but there weren't enough questions imo. Let's try to make sure there's a comprehensive picture of what's going on.

    Why was the original engine replaced? Did this outfit run fine prior to the swap? Were you the owner before the engine replacement? Was the engine replaced at a shop?
    Developed a knock.
    Ran as fine as a TBI can run.
    I was the owner
    I replaced the engine.

    Is the picture of the invoice from Sunwest Engines for your engine? Did you choose this company to supply the engine? Why? Did you select this part number for the engine or was it recommended for you? Did you get the base cam or the optional RV cam?

    My engine invoice
    I chose them on reviews
    Parts were what they gave me
    Base cam

    Is your distributor now set at the value indicated on the sticker under the hood? I believe it should be 0 degrees? Or did you leave it at 6 degrees?
    0 wont work, its a dog. I am at 6 now, have been at 4 and 8 as well. 6 seems to be nice. I have done the 0 thing though and its the same.

    Have you ever confirmed that the timing mark is correct for the timing tab? Not that the part number is correct, but that the piston is at TDC when the mark indicates TDC.
    I asked the engine people about it, they said it was correct. The cam/crank gear were dot to dot when I put the cover on. I think we pulled a plug and did the TDC thing too.

    Have you ever measured the fuel pressure? You said the injectors were rebuilt. Have you ever confirmed you weren't sent the wrong injectors?
    Fuel pressure is to spec, injectors were the ones I sent to be cleaned and tended to. I do believe the numbers are correct. Its been a while and I am 57 so.....

    What are you using to scan and save the computer data? Have you ever recorded computer data while driving? A recording of computer from a spell on the highway when the truck is running correctly could be helpful.
    I am using this: http://www.1320electronics.com/12pin_ALDL_BT_MK2.html and his ALDL app. I have all sorts of logs, long drives, hot weather, etc: [brace yourself] https://drive.google.com/open?id=0B5...ndnMmlqcDJEVDA

    You say the problem is rich running. Is this because you can smell fuel fuel from the exhaust? Anything else? Do you have a cat installed? Does the problem clear up completely when the engine runs better?
    Its running rich because it smokes, you can smell fuel, and my experience tells me. No cat, new o2 sensor. Completely, that is when I stopped logging, when it cleared up to normal.

    BTW, I made a living working on these trucks in a GM dealership in eastern MT when they brand new. I also worked nights and weekends repairing equipment on the ranches in the surrounding county. Unlike folks that live near town, I know what it's like when a trip to the store for a tool or a part is an all day affair.
    I'm a ranch guy, the big city is 120 miles. We both know what thats like. I'm a pretty good wrench, until you through electronics in the mix. Hope I wasnt short with you but I thought I would be brief and to the point. I appreciate your input.

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    Ok...

    No problem with your answers. You've been fighting this long enough.

    I don't see anything in the data that suggests a specific part to change or system to test. Like others, I see large RPM fluctuations in some of the logs when the engine is running poorly. But there are a couple of ways the scantool could be calculating the RPM so it's possible it's just how the scantool shows it and not what the PCM is seeing. You've changed computers and distributors a few times which makes it difficult to blame either of those parts. It's possible that you've gotten more than one part with the same problem but I think the odds are against it. It seems to take longer than usual for the coolant temp to heat up but that shouldn't make it run badly.

    One question that I didn't think to ask last time is did you change the transmission or torque converter at any point? A converter with a different stall speed will change everything about how the truck feels. I'm sure you know this, and would mention it if you had, but I'm just covering all the possibilities.

    I've spent time researching the engine parts. I have to assume the heads are stock TBI heads as you haven't mentioned needing a different intake manifold. The pistons are generic rebuilder pistons which are .020" shorter than stock but they shouldn't make the truck feel like a dog. There could be a problem with piston quench but I didn't notice substantial knocking in the logs. Did I miss anything there? The cam appears to be sourced from Delta, a cam grinder in Tacoma. There's no specs for a DEL274 cam on their web page so I used specs for a CS274 Sealed Power cam. This cam is a copy of the GM cam used in most of their 350s for 15-20 years. The Sealed Power cam is close to the TBI truck cam and should be a decent match in terms of power although it might benefit from a custom chip. But is this the same as the DEL274 cam in your engine?

    Knowing the BCC of the chip might help some. For '93 and '94 trucks GM decided that people were complaining too much about piston noise caused by the short skirt pistons they were using. They also decided they didn't want to keep blowing the rod bearings out of engines with less than 100k miles. So they made a few replacement calibrations. These chips pulled timing among other things. Customers complained they lost power and lost mileage. But the engines lasted and they didn't knock so GM was happy. If your scantool shows the PromID code we might be able to look up the BCC.

    There is one other potential cause that I've seen a few times. Sometimes the computer goes into closed loop too early due to O2 sensor activity. The sensor is cold so the numbers the ecm uses for O2 value are not accurate. I didn't see closed loop as one of the data values in the logs you posted but I might have missed it. It's easy to test whether or not it's the O2 by disconnecting the sensor before starting the engine. The engine should never go into closed loop with the sensor unplugged. If the truck idles and runs poorly with the sensor unhooked then it's not a problem caused by O2 sensor activity.

    With the research and history of this rig, if it came to me to fix, I'd probably start by confirming the bad idle and watching my own scantool to look for problems. If nothing looked unusual I'd do a visual check in the engine compartment and underneath looking for problems. I'd make sure all the ground straps are connected and tight. I'm make sure the wiring harnesses are in all the factory clips and brackets and are routed in the original locations. I'd look at the four wire connector at the distributor and make sure the terminals all look ok. If no problems were found I'd reset the timing to stock and take the truck for a drive. If it was a dog compared to other TBI engines I'd use my piston stop to confirm the balancer is marked correctly. I'd take cranking compression readings at multiple cylinders and since I had the equipment connected I might do a leakdown test. If everything looked normal I'd take the driver's side valve cover off and set a dial indicator up to measure intake and exhaust valve opening and closing events and the cam lift. I'd probably check more than one cylinder just so I had the data. Then I'd compare the numbers to a stock cam. Of course this whole set of steps assumes that nothing jumps out as a problem along the way.

    I don't believe the problem is unfixable. And I don't believe everyone on the other forum is stumped. But I believe that the people who diagnose problems every day rely on their senses and have learned not to trust anyone else's. Which makes diagnosis over the internet really tough. My hunch is that the engine build is at the root of the problem. An old cam design with large overlap will slow the fuel combustion and cause unburned fuel to exit the engine. The engine isn't running rich, but the exhaust indicates a problem. GM used to use AIR injection in the manifold combined with a catalytic converter to clean this up. Eventually they figured out how to make better cams.

    If your block is set up for a factory roller cam you could find plenty of good replacements for not a lot of $$. The factory roller lifters used in cars and later truck engines are good for hundreds of thousands of miles and can be re-used with different cams. Ebay isn't as good as it used to be for used parts but there are still some deals to be found. And there are still good flat tappet hydraulics available inexpensively that are a good match for your outfit.

    Feel free to check or go over anything else you'd like and post results/questions here for further discussion. This has been a long fight. Hope you find a solution.

  7. #7
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    Quote Originally Posted by 1project2many View Post
    Ok...

    One question that I didn't think to ask last time is did you change the transmission or torque converter at any point? A converter with a different stall speed will change everything about how the truck feels. I'm sure you know this, and would mention it if you had, but I'm just covering all the possibilities.
    Negative

    I've spent time researching the engine parts. I have to assume the heads are stock TBI heads as you haven't mentioned needing a different intake manifold. The pistons are generic rebuilder pistons which are .020" shorter than stock but they shouldn't make the truck feel like a dog. There could be a problem with piston quench but I didn't notice substantial knocking in the logs. Did I miss anything there? The cam appears to be sourced from Delta, a cam grinder in Tacoma. There's no specs for a DEL274 cam on their web page so I used specs for a CS274 Sealed Power cam. This cam is a copy of the GM cam used in most of their 350s for 15-20 years. The Sealed Power cam is close to the TBI truck cam and should be a decent match in terms of power although it might benefit from a custom chip. But is this the same as the DEL274 cam in your engine?
    I talked to the builder at length, it is a copy of the GM cam. He said there was nothing changed or special.

    If your scantool shows the PromID code we might be able to look up the BCC.
    It doesnt, but it is the OEM one.

    There is one other potential cause that I've seen a few times. Sometimes the computer goes into closed loop too early due to O2 sensor activity. The sensor is cold so the numbers the ecm uses for O2 value are not accurate. I didn't see closed loop as one of the data values in the logs you posted but I might have missed it. It's easy to test whether or not it's the O2 by disconnecting the sensor before starting the engine. The engine should never go into closed loop with the sensor unplugged. If the truck idles and runs poorly with the sensor unhooked then it's not a problem caused by O2 sensor activity.
    I have done multiple o2 sensors. Negative on roller cam. When it does this, it is in fact very rich. The loop in the log I believe is in the MALF sheet, column DM

    Mind you, I am fussing over this for a mere 20 seconds worth of running poorly. If it were mechanical, it would be consistent. If you listen to the video, you can here it begin and end, just as if someone threw a switch. I've checked grounds. Then again, you know what kind of a ground it takes to turn a starter so I would think I'd know about it in other areas. Personally, I dont think it has anything to do with the engine, it is surely electronic. Like I said, after this 20 seconds or so, the thing runs great.

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    I talked to the builder at length, it is a copy of the GM cam.
    The engine is sold for a very wide range of years. Is it possible the cam is a copy of an old GM cam design? Is there a chance the cam was ground with the wrong lobe centers (most rebuilders never check this)? You could contact the cam grinder and ask them directly. http://deltacam.com/

    If your scantool shows the PromID code we might be able to look up the BCC.
    It doesnt, but it is the OEM one.
    Hmm... There's more than one OEM one. Some caused complaints about low power.

    Like I said, after this 20 seconds or so, the thing runs great.
    Well you've set the timing up by six degrees so I'd argue it seems like the thing doesn't run great. They don't usually need that much extra timing to run well. Did you have to set your old engine at six degrees for it to run well?

    Personally, I dont think it has anything to do with the engine, it is surely electronic.
    Well, ok. What do you recommend checking next? What part can you change or check that you haven't already? The scantool readings appear to match the operating conditions except for the strange RPM swings. The computer doesn't see anything that it would flag as a problem. So what would do you think should be tested?

  9. #9
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    If it was the cam, why would it only last for 20-30 seconds? If I could somehow edit the first minute of running time, I'd have it licked. Maybe I should create a vac leak to test.

    I never touched the old engine timing, never had a need to. There is surely a difference between 0 and 6. At 6 it isnt holding back like it is at 0. From what I have read online, most people dont like 0 and are happier at 8. Thats excessive to me, I am not expecting too much out of this sled.

    I am trying to find someone with a spare MEMCAL that I can try.

    I think you should tell me you have a dual plane intake, 4 bbl carb and a HEI distributor sitting in the corner. :)

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    f it was the cam, why would it only last for 20-30 seconds?
    The cam determines the airflow through the engine. It also determines the amount of air that stays in the cylinder while the piston comes up on compression (dynamic compression ratio) and how much chance there is for exhaust to dilute the intake charge. Hard to tell everything that cam needs but it certainly seems to need more timing everywhere.

    So it's possible for 20 - 30 seconds the spark timing isn't right for the engine, and maybe the fuel isn't right as well. Too little timing makes the exhaust seem rich. And we can see the computer pulling fuel once it goes into closed loop. If you disconnect the O2 and force the engine to stay in open loop you might see it run rough for longer. And that would help confirm the cam is different enough from stock to be an issue. And if you can get it to run rough longer with the O2 disconnected then you can see if it runs better at idle with a little more or a little less timing.

    There is surely a difference between 0 and 6. At 6 it isnt holding back like it is at 0. From what I have read online, most people dont like 0 and are happier at 8. Thats excessive to me, I am not expecting too much out of this sled.
    I would sometimes set timing at 2 - 4 advance for guys that wanted a change. You could do it if the truck was running 87 octane fuel instead of the 85.5 cheap stuff. I never had to go as high as six.

    I am trying to find someone with a spare MEMCAL that I can try.
    I might have one here. I would want to make sure it's the right one for your truck which means, matching the code to the one you have installed. But I'm thinking this engine really wants a custom chip.

    I think you should tell me you have a dual plane intake, 4 bbl carb and a HEI distributor sitting in the corner. :)
    I hear ya. I've been down that road before. At least you'd be able to make changes yourself. You'll need to separate the dash wiring from the computer wiring. And you'd need a fuel pressure regulator to keep the pressure around 7 psi. Use an intake from an '86-'87 Monte Carlo, a carburetor from an '84 truck with 350, and an aftermarket distributor for a strong combination. Or change to a nice, cheap roller cam. ;)

    And just to swap a story, around '94 I put EFI on top of a 302 chevy I built in a '57 pickup. I had a mix of parts that wasn't too bad but they were all old designs. Heads from an '83 305, block from a '63 327, crank from a '67 283, and cam from a 66 327. The stock computer barely ran. I paid a guy to make a chip (there weren't really any tools to do it yourself back then) and what he made worked a lot like what you've got. But the rough running lasted about 45-50 seconds. I drove that outfit all over the place like that because the guy that made the chip said my cam wouldn't work with EFI and I needed a different one. I know the cam works because I learned to make my own chips. But the final result was quite a bit different than the 350 chip I started with.

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    On older things I would set timing by ear mainly, or by vacuum. I know, you cant do that on this but its tempting. I'll disconnect the o2 and see what I get. Should I disconnect the timing wire too and mess with the timing by ear, if in fact it stays running? Get it to where it runs smoothly or how should i go about it?

    A custom chip would be nice but at this point, I have thrown all the money I possibly can at it. Its become a sink hole with this issue. I was under the impression that with the exact vehicle specs, you had a couple different chips that would work.

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    You can still set it by ear. Race guys do that at the track and it can be helpful when making your own custom chips. But the computer covers such a wide range compared to an old distributor that it's easy to get it right in one place and wrong in a bunch of others.

    I'd leave the timing wire connected. The idea is to see if a little more or a little less timing at idle helps.

    Most of the chips are made for a stock engine. I might be able to find an older one with more spark advance but if your truck still has an old one there's not much to be gained in the swap.

    Keep us posted.

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    I think my new workout is going to be climbing up on the thing and stretching for the distributor!

    I tried, all it did was make things worse, either way. Where its at is actually the sweet spot when you are doing it by ear.

    I'd say try different injectors but I had them gone through. It had the issue before that though.

    Could I unplug an injector when it starts acting up, to see if it clears out?

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    You can unplug an injector. I would expect it to clear up pretty quickly then go the other way.

    I think my new workout is going to be climbing up on the thing and stretching for the distributor!
    That's funny. Make sure you give each arm an equal workout.

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    The more I think on this, the more I am thinking prom. If I can find one to try, anyone that will run a 350. I dont even care about drivability. Just something to try.....

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