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Thread: crazy fuel trim problem

  1. #1
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    crazy fuel trim problem

    any input from anybody would be greatly appreciated. thanks in advance. the car is a 95 f body with a lt1 and a 0411 pcm and a 24x conversion head work, cam, long tubes, injectors and so on. one day bank 2 runs crazy rich and I checked a few things. assumed it was the injectors. swapped the out with some spares and same problem. So I swapped o2s and widebands side to side and same problem . test wiring. all seems well so I run a scan and find when the car warms up I lose o2 signal on bank 2 and that's when it goes wacky. trims spike and wideband maxes out. test, swap and retest everything again. same thing every time, car warms up a bit, meaning I drive it a mile or so and then it loses o2 signal on bank 2 and trims spike. swapped o2 wires at pcm pins to see if it is a pcm problem and if it changes to the other bank, during that I sat in the car with the scanner on it and looking at the data, I pushed the throttle and when the tps changed the trims on bank 2 spiked when the car wasn't running. since trims are calculated from various other inputs I assumed I had found the problem and we have a short circuit between the tps and something. started unplugging sensors 1 by 1 and no change . down to everything unplugged but the tps and it still does it. So I checked pcm powers and grounds, voltage drop tests, the works, no issues, opened up the harness and inspected it all, no problems, double checked all of my harness all of my battery, body and engine grounds, nothing, went home slept on it and in the am it wouldn't even start!!! wtf injector pluse width was over 26 trying to start??? found a blown injector fuse and it runs. So i ordered a pcm and installed it, programmed it, and BAM, same thing
    I'm sure im forgetting details but that should be enough to start with
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    Last edited by jason k; 01-11-2015 at 09:18 PM.

  2. #2
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    Could you please use proper writing.

    Sorry, but at least for me - a non-english person - your post is quite difficult to read. Or understand.

  3. #3
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    Ok. Cleaned it up. try that

  4. #4
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    Quote Originally Posted by jason k View Post
    ...I drive it a mile or so and then it loses o2 signal on bank 2 and trims spike...
    Did you find the problem with bank 2 narrowband? And why did you blow the fuse?

    I would start with the basics, i.e. fix the physical problems first.

  5. #5
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    As I was looking into the narrow band issue I found the fuel trims on bank 2 move when I stepped on the accelerator pedal and I assumed they were related and moved my focus to that search.
    As for the blown fuse I did not see a reason for that either and ASSUME that I must have blown it during all the testing. But I don't know how I would have its not like I was applying voltage to the pcm pins.


    Did you look at the log? I have never seen anything like that ever

  6. #6
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    I did swap narrow bands from side to side. I also ohm checked the wires and tightened all of the terminals
    it seems strange that i loose the voltage after it warms up a bit so i thought maybe high resistance in a power or ground circuit. So I hooked up a headlight to the circuit and powered that to put a high load on the wires and the light shines bright
    I am going to run new wires from the pcm to that o2 with tonight and hopefully I can road test it this weekend

  7. #7
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    I am also running the 24x conversion on a 1996 LT1 and have issues with fuel trims bank to bank acting up. Bank 1 runs -3.9% and then Bank 2 runs 15.5%. I have also checked wiring and swapped o2 sensors, no misfires, no exhaust leaks before o2 sensor, fuel injectors are brand new flow matched, new plugs, wires, good fuel. Do you have any update with your problems yet, that might could be related with my issue.

  8. #8
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    Quote Originally Posted by Mark@TuckerPerformance View Post
    I am also running the 24x conversion on a 1996 LT1 and have issues with fuel trims bank to bank acting up. Bank 1 runs -3.9% and then Bank 2 runs 15.5%. I have also checked wiring and swapped o2 sensors, no misfires, no exhaust leaks before o2 sensor, fuel injectors are brand new flow matched, new plugs, wires, good fuel. Do you have any update with your problems yet, that might could be related with my issue.
    no your issue is completely different from mine you could have a vacuum leak on bank 2 causing your problem or it could be in your tune my trims max out out 50 % on bank 2 with tps input totally different

  9. #9
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    sounds like the same problem that I had. the BBC is a 1-8-4-3-6-5-7-2 firing order which is different to LS1 1-8-7-2-6-5-4-3, so you need to rewire 4 injectors.

    1-8-7-2-6-5-4-3
    1-8-4-3-6-5-7-2

    having them wrong makes the fuel trims go spaz. mine richened out so much that the cats started glowing red hot and almost set fire to the floor.

    there is an injector assignment table under the engine/fuel tab, but it doesn't change the firing order, rather it just seems to report to the pcm which injector is on which bank so the pcm can adjust accordingly
    1966 impala. L29 454. 0411 pcm. 2.5" twin system. comp cams XR282HR. ported heads. 1.75 rollers. 60lb deka injectors. bosch 044 pump. standard intake (cos' looks cool in a 66')

  10. #10
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    ari666, I did rewire the 4 injectors that needed to be switched for the correct. So you say that I need to also change the injector bank select table under Engine/Fuel/General/InjectorControlGeneral? And then how about changing Firing Order table under EngineDiagnostics/General? Should that be messed with?
    Thank you

  11. #11
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    You should match the injector bank select table to a 2002Chevy Express Van tune (it's in the repository, runs a 0411 PCM, but is an L31 classic style small block with the same firing order as the Gen 2 LT1/LT4/L99).

    When you wire the injectors as noted above, you also need to wire the ignition coils in the same manner, or they're firing at the wrong time.

    Gen 2 engines were always notorious for fuel trim splits. Check for vacuum leaks, but 99% of the time, it was due to the IAC. The IAC on an LT1 dumps into passages below the plenum, not the plenum itself like the newer engines. Those passages loved to clog or not breath properly...the problem would always get worse with larger camshafts...search older tuning websites for "split BLM's" and you'll find a wealth of info.

    As for the trims changing when you touch the TPS...shut the engine off but turn the ignition on, connect VCM Scanner and reset the fuel trims...the TPS changes which fuel trim cell the PCM looks at...it's possible it's just learned horrendous data in one cell but not in another, and moving the TPS is showing you that.
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