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Thread: Stock engine idle. This should be easy for everyone but me.

  1. #1
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    Stock engine idle. This should be easy for everyone but me.

    Problem: Poor bouncy idle

    Engine Application: 1951 Chevy Suburban with 2005 GMC Envoy Denali, 5.3L V8, LH6, 4L60e, completely stock engine except for an AEM dry filter. Has DBW throttle and DOD.

    History: In 2005 I purchased the engine/trans from a GM Special Event Vehicle sent to the wrecker with 10 miles. I installed the engine with a new harness and computers from Speartech. Everything worked great. Smooth idle, excellent throttle response. Then I started restoring the Suburban and time/life flew by. Fast forward to 2020 and I'm finally starting the engine again. I think I did everything possible to bring it out of hibernation safely.

    Lightly oiled cylinders and turned over by hand.
    Changed oil and filter.
    Prepressurized the system with an external pump.
    Cleaned the fuel tank.
    Replaced the in-tank pump and filter.
    Cleaned the fuel rails and installed brand new stock injectors.
    Replaced a missing vacuum cap on the manifold.
    Reversed the coil harnesses that I had on backwards.
    Cleaned all spark plugs.
    Checked continuity of the O2 sensors.

    Current status: After 3-4 starts that sputter and die after about 3-10 seconds the engine will finally sputter, speed up and then settle into an idle that is rough and jumpy. If it catches, sometimes it will run for a couple minutes. I haven't run it longer because I'm waiting on a drain plug for the radiator so no cooling right now. I'm hoping the computer just needs to run longer for the computer to learn the parameters again?? When it does catch and run I've tried tapping the throttle and it'll rev and sputter and then idle down and die.

    Might also need help learning to attach my log file.

    11-11-2020.1.hpl
    Last edited by LH6-DOD-54-GMC; 11-30-2020 at 09:50 AM.

  2. #2
    Very lean. Or has a massive exhaust leak before the o2's

  3. #3
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    Proxses, thanks for the reply. I'll check the fuel system again from back to front.

  4. #4
    Tuning Addict 5FDP's Avatar
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    Make sure the exhaust is complete if you are running o2 sensors, they don't like open headers. If the exhaust is just open unplug the sensors to force open loop.

    And have 58psi of fuel pressure.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  5. #5
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    Full exhaust with mufflers. I can hear a slight exhaust leak downstream about two feet after the stock manifolds. I have a connection there where the exhaust goes behind the running boards. I'll have to figure out how to tap in for a fuel pressure gauge. I did another run after verifying that the coil harnesses were not damaged or reversed. I also replaced the fuel filter/regulator and the accessory drive belt for a tighter one. And I verified there were no rodents in the intake.

    I did notice the throttle position is staying in the 20% range rather 1-2% for an idle. I verified nothing was hanging up on the throttle pedal. On this run I got codes for being lean on both banks. It did stay running on the first turn of the key though. Didn't think it would with the initial stumbling. At the end of the run I cracked the throttle a pinch and there was a dead spot, then she stumbled up rpms. I let off and she died.
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    Last edited by LH6-DOD-54-GMC; 11-13-2020 at 09:50 AM.

  6. #6
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    I also checked for any other exhaust leaks or vacuum leaks and found none. Only leak I did find was the damn upper radiator outlet from the water pump. The stupid pressed in metal pipe. Who thought that one up!?

  7. #7
    Tuning Addict 5FDP's Avatar
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    Drive by wire will not idle at 0-1% because the blade must be open to run the engine. Seeing like numbers in the teens is pretty normal for a DBW vehicle.

    Figure out the fuel pressure and we can go from there.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  8. #8
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    THANK YOU EVERYONE! Wouldn't have known where to start without the guidance here. It looks like I got a low pressure pump! No bother. Too much trouble to argue with them. I only care that I can get it running so I'll order a new pump and hopefully hit 58 psi.

  9. #9
    Tuning Addict blindsquirrel's Avatar
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    I'm surprised more people don't use carb/brake cleaner as a fuel source when diagnosing issues like this. An engine with no fuel supply will run surprisingly well off a can of brake cleaner. Good enough that I used to drive cars and trucks into the shop off the lot that had dead fuel pumps using nothing but brake cleaner and a long piece of vacuum hose snaked up front and aimed at the TB inlet. (In a flat-rate shop it's sometimes hard to collect 3 or 4 people all at the same time willing to help you push one inside - if nothing else, it's a good motivator for finding shortcuts.)

    It's an excellent way to answer the question, "Would it run better if it got more fuel?" If the answer is yes, you can then move on with figuring out why it's not getting enough.

  10. #10
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    Confucious say, Squirel not so blind. I requested help from numerous sources across the spectrum from hot rod builders, tuners, motorcycle mechanics and even my daughter. Everyone pointed in the direction of fuel. Even my daughter said "Since it ran well before, you have to look at what has changed, to make it run poorly now." Boom!

  11. #11
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    Dang, still not working!

    Well, the problem hasn't gone away. I installed the new fuel pump and I'm maintaining 58psi. I still have the ignition timing bouncing a bit but my throttle angle has dropped from 24% down to about 15% at idle. The throttle body is clean since it only has 10 miles on it. I tried the ignition off/on for 3 minutes TB relearn. Didn't change a bit. I also still have codes for running lean in both banks. Could it be a bad gasket or vacuum leak?

    P0171 System Too Lean Bank 1
    P0174 System Too Lean Bank 2
    P0650 Malfunction Indicator Light Control Circuit
    Last edited by LH6-DOD-54-GMC; 11-30-2020 at 09:06 AM.

  12. #12
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    Here are the .hpl and .csv logs.
    Attached Files Attached Files

  13. #13
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    .hpl
    Attached Files Attached Files

  14. #14
    Tuner in Training LH6-DOD-54-GMC's Avatar
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    I tried carb cleaner around the engine and gasket areas and it doesn't seem to have any vacuum leaks. I can't road test the car because it's on stands in the barn. Could it be a drive cycle issue?
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  15. #15
    Tuning Addict 5FDP's Avatar
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    I prefer to smoke test the intake to find leaks instead of brake/carb cleaner, it usually works better. I'd double check all your injector o-rings too.

    Was the part number on the injectors you bought the same ones that came with the engine?
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  16. #16
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    I agree with 5FDP. Smoke test the intake. Make one real cheap with a bicycle pump. Injector orings or wrong injectors is what I'm thinking too. If it ran when it parked and not now it has to be something you recently replaced.

  17. #17
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    Injectors were replaced with same part number new injectors. I'll work on the smoke testing. Thanks for the tips!!!

  18. #18
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    UPDATE:
    I fabricated the smoke tester and found that the vacuum fitting for the power brake booster was loose. Fixed that. Extended smoke testing fouled the plugs so I cleaned them and scanned it again. Then the scan showed a bad 02 sensor so I just replaced them both. Now it idles well around 600-675 rpm range. The only problem it still has is the throttle position still shows 15-17%.

    Could this just be a drive cycle issue since it's been a long time since it last ran? It's currently up on stands until I plumb and bleed the brakes. Is there any way to simulate a drive cycle?

  19. #19
    Tuning Addict blindsquirrel's Avatar
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    If the spark timing is -5* then the throttle will have to be open a bunch to reach the target idle speed. And the other way around, if the throttle is open too far the idle timing will get pulled down way low to reach target idle speed.

    A log of it running in its current condition will help with figuring out what it is doing now after all the recent changes. Logs of how it was 6 months ago are probably no longer relevant.

  20. #20
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    I'll try to run a log tomorrow.