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S4 losing boost

Discussion in 'A4/S4 forum(B5 Chassis)' started by fatarnie, Jul 27, 2003.

  1. fatarnie
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    fatarnie New Member

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    [Jul 27, 2003]
    I have a late year 2000 S4 Avant.

    Shortly after buying it I took it to Kim Collins for a 300bhp chip and a Miltek cat back exhaust.

    The upgrade made an immediate improvement, and the car went fairly well.

    Recently it has started to be very sluggish with minimal boost in the gears. I took it back to Kim who changed the airflow meter - no effect, then changed the lambda probes - also no effect.

    He agreed performance was flat and the VAG box he uses to plug into the car showed (i think only one) cyl bank running weak.

    Changing the lamda probes mde a short term difference, but today I have discovered that disonnecting and reconnecting the battery restores full performance for about 40 miles or so before it starts to drop off again.

    So I guess the fault is somthing to do with the learning ability of the closed loop ECU, but I know little about the ignition and control managment on an S4 or any other modern turbo car.

    My first question is: HAs anyone else experienced this and what could be wrong...????

    My second question is: Does anyone know of a way to reset the ECU to the default map? (I read on a US site that if you leave the ign on for 3 minutes it resets the map to default - is this true and should it be doen hot or cold?

    Thx in advance - help greatly appreciated.

    AW
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  3. Gambba
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    Gambba Active Member

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    [Jul 28, 2003]
    I can't pretend to know anything about your fault from experience, but regarding your last question, yes the reset/learn procedure does work, and the engine does have to be warm IIRC. I also recall there are functions within VAG-COM that allow you to re-teach the ECU, amongst other things....so maybe this is another option you could look at to try and identify your fault.

    Sorry I can't be much more help...I'm an A3T owner though.
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  4. Dippy
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    Dippy New Member

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    [Jul 28, 2003]
    I am surprised that Kim can't fix this, but be warned - it is important. Running lean (which is what I assume you mean) causes overheating which can have a number of results including turbo death.

    What you describe is 'limp' mode where the ECU limits boost to 0.4 bar above atmospheric. It does this to protect the engine. When you disconnect the battery, you erase the DTCs (diagnostic trouble codes) and so indeed the ECU can go back to full boost. If it happens again and again then all you are doing is bypassing the protection and damaging the engine.

    From what I know (based on the experience of other S4 owners), MAF and lambda sensors are the typical culprits. If Kim has replaced these then your problem may be deeper. If Kim can't solve it then go to someone who can.
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  5. Patrick
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    Patrick Member

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    [Jul 31, 2003]
    Dippy is correct. I have similar issues with my 97 S4 with an AMD remap. Although I get a DTC in the engine module of VAG COM stating 'Intermittent boost exceeded' However, the frequency of the problem. Is about 6 times now over 10 months.

    I have my suspicions that the Hyperboost Diverter valves (DV) may be contributing to this as there is distinct turbo lag on changing gear. To this end, I am going to reinstall my Bosche units this weekend to see what, if any response I get with both the lag during shifting gears and the intermittent problem.

    With respect to resetting the ECU I've found that on different occasions I've been able to restore full boost by either blipping the thottle, (this worked twice) where this hasn't worked I simply switched off the ignition for about 2 seconds and switched the engine back on. The fault was still recorded but the the boost was restored.

    Now of course my problem could be entirely different from yours.

    One other experience of low boost / limp mode I had with my car was due to faulty exhaust temp sensors (NOT the lambda probes.) There are two of these. They both resulted in different faults from the car.

    The first one activated the EPC light. However, it didn't result in an error code suggesting the exhaust probe was faulty every time. But on two other occasions when the dealer tech checked the error about the exhaust temp probe was noted. (I'm not sure if a specific code came up) This probe was replaced and no more EPC light problems. Then some months later. The car was running flat again. But no EPC light. The dealer maintained the car was running fine. I was convinced otherwise a trip to AMD revieled a burst throttle body boot and the second exhaust temp probe was faulty but no error code was noted. It was the strange output readings that gave the game away that it was malfunctioning.

    The only advice I can give is if Kim can't sort it. Take it to AMD those lads are as good as it gets in my opinion.

    Good luck and let us know how you get on.
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  6. fatarnie
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    fatarnie New Member

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    [Aug 3, 2003]
    Another visit to Kim and the airflow meter has been changed again in conjuction with a reset of the ECU and all now seems well again.

    The fact that the car actually has a low boost limp mode (as opposed to a default sensor value) is a good indictaion of what has happened, although I was not sure in speaking to QS if the limp mode activation was logged.

    Why can manufacturers do not ditch airflow metering is beyond me. 3D mapped throttle position sensors combined with a barometric sensor are a much better way to achive the same critical data.
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