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Thread: 3.2 V6 Petrol GSi Misfire Under Load

  1. #1
    Regular Member marcdale's Avatar
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    Default 3.2 V6 Petrol GSi Misfire Under Load

    Guys, any help appreciated on this as it's starting to cost me big time!

    Situation is this - Car drives fine under 3k RPM after that I get a really bad stutter or misfire. I'm getting various error codes regarding 'misfire in cylinder 1' 'short term fuel trim malfuntion' 'generic misfire on bank 1' 'low fuel level on bank 1' etc. None of them happen at the same time if i'm gentle with the throttle i'll get 1 type of error and if I do it under load i'll get another and so on. If I drive it so that I don't get the misfire I eventually get warnings about fuel trim levels.

    I did notice when I cleared the error codes the last few times, when I watch a live graph of short term fuel trim(STFT) on banks 1 and 2, when I press the accelerator bank 2 STFT fluctuates slightly but generally stays balanced as you would expect with a closed loop system. However bank 1 STFT goes through the roof, straight up to +25 as soon as I press the pedal. and when I lift off it returns to around 0% as expected.

    So, in short I have a problem which is causing the fuel system to demand a massive increase on bank 1 as soon as it's under load.

    I don't know if there is a vaccum problem on bank 1, or some sort of cam sensor, knock sensor, TDC sendor or what ever.

    I can exclude the injectors because I have an after market gas system fitted and the problem occurs on gas or petrol and they use seperate injector banks. So the problem is not blocked or dirty jets. It's slightly worse on gas, but that again is to be expected because gas always shows fuel/ignition problems up more than petrol because it doesn't explode quite as well as petrol.

    I was using an induction kit, but have since put the VX airbox back on. I think it made it slightly worse with the standard air box, but there's not much in it. I did think MAF sensor, but that wouldn't just effect one bank, would it?

    Help! Throw anything you like at me so I can soak it all up and and decide what to do next. I'm really stuck guys, I need help!

    Thanks for your help!

  2. #2
    Full Member Big Sig's Avatar
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    Christ! thats all gone over my head. I don't know if its possible but can you swap the coil packs from bank one to two to see if the problem changes?
    Now I need a turbo to keep up!

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    Regular Member marcdale's Avatar
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    Quote Originally Posted by Big Sig View Post
    Christ! thats all gone over my head. I don't know if its possible but can you swap the coil packs from bank one to two to see if the problem changes?
    I've replaced the coil packs anyway. But to answer your question - no you can't. I really wanted to try that first, but when I took them out I saw that the legs down to the spark plugs are angled and the wiring block connection doesn't have any slack so you can turn em round to angle the legs the other way.

    Hence replacing them...

    Thanks anyway

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    Regular Member marcdale's Avatar
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    Quote Originally Posted by marcdale View Post
    the wiring block connection doesn't have any slack so you can turn em round to angle the legs the other way.
    I meant to say...


    "You CANNOT turn them round"

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    How's the compression?

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    Regular Member marcdale's Avatar
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    Quote Originally Posted by shunter View Post
    How's the compression?
    Not sure, but it doesn't use oil or water so i'm assuming there is no issue with the rings or pistons. it's as smooth as you like below 3k RPM

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    Still worth checking the compression to compare the compressions between cylinders. Could also show up a valve problem.

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    Regular Member marcdale's Avatar
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    Ok guys, I want to thank you for your help on this one, but I think it's sorted!

    I took the plenum chamber off the car again cause it's been off a few times recently for plug changes, coil pack changes, LPG installation etc. (For the benefit of others, the plenum chamber is the big air induction system on top of the engine which needs to be removed to change the plugs).

    I could see that despite all of the times the chamber had been removed the original instant gasket was in place around the 6 inlets which are laid flat between the two banks on the V6.

    So, I put some new instant gasket around them and installed the chamber again and it's running perfect again. In fact I would say it's running better than before I noticed there was a misfire. I'm thinking maybe there was a small leak there from a previous service which has got worse every time the chamber has been lifted off.

    Had a little misfire as soon as I put it back together, but remembered that I hadn't cleared the error codes from last night. As soon as I did this and the car started using the O2 sensors instead of a default fuel map it started running like a dream!

    Please be aware, certain error codes like misfires will flash your engine warning light, but it will not stay switched on. But the codes are still stored and the engine will continue running on a default fuel map with out showing the engine light. Were as other error codes like an O2 sensor malfunction which will switch the engine warning light on and leave it on. So if you ever get just a flashing engine warning light, YOU STILL NEED TO GET THE ERROR CODES CLEARED to allow the engine to start running in closed loop (using the O2 sensors to control fuelling and emissions = economy)
    Last edited by marcdale; 18th March 2008 at 16:57.

  9. #9
    Full Member Big Sig's Avatar
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    As you say, probably sucking in or leaking air through the gasket. Nice find mate!
    Now I need a turbo to keep up!

  10. #10
    Regular Member marcdale's Avatar
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    This was a problem with an intake manifold leak on my 3.2 GSI petrol.

    http://www.vectra-c.com/forum/showthread.php?t=40210

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