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  1. #1
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    Loss of acceleration when fully warm

    As a relative í88 M5 newbie, Iíd appreciate some diagnostic help from the greybeards here (thoí mine is now pretty grey!). My car, with 165k miles and a Conforti chip, has plenty of power but loses its zippy acceleration when it gets fully warm. With the temp gauge about a quarter of the way up, the car pulls very nicely, but as it gets fully warmed up, the acceleration grows more leaden. Itís most noticeable ca. 5k RPM. Itís not terrible, mind you, but it increases speed like an old six-banger, not an M car. Iíve had Bavarian Professionals in Berkeley (Calif.) check it over, but theyíve found nothing to explain this, and theyíre skeptical thereís any problem at all. They did suggest an explanation for the relative lack of pulling power high in the range might be a bad fuel tank pump, which it was and which they replaced, but it hasnít eliminated the problem.

    So what do you think? Iím wondering if one of the cold start system switches might be bad, since a slightly over-rich mixture might yield these symptoms. But I really donít know the car well enough.

    Iím also wondering if anyone knows of a truly great diagnostician for these cars in either the East Bay (I live in Berkeley) or the San Mateo area (where I work during much of the week). Back in my Tii days, we had some awesome mechanics in Berkeley, but times have changed!<g>

    Thanks!

    Alan Kay
    í88 M5


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    Re: Loss of acceleration when fully warm

    replace the in tank pump (easy) . Then check you cat. I had the same problem and it turned out to be my cat. Eventually it got clogged up broke turned sideways in side the pipe and I could only rev it to 5k. the result Now I am only runnning on 5 cyclinders.

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    ilovecow
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    Re: Loss of acceleration when fully warm

    I can't offer much about the alleged power loss, but I have to say if you don't think Bavpros are great, you're not going to find anyone else in your area, or anywhere.
    Bavpros knows their M5s, especially the E28 and E34. Everytime I go there for service, they always have five or six E28 M5s waiting to be serviced. There aren't that many E28 M5s to begin with, you think a whole bunch just shows up at Bavpros by accident? Bavpros have been featured numerous times in "Roundel" and "Bimmer" and other publications, I'm sure they have some clue of what they're talking about.
    This is not to say they can fix 100% of everything, but perhaps you should consider other things. Have you tested the car with a stock chip? What kind of gas have you been putting in? The car has 165k, when's the last time you had a compression/leak down test?
    There are plenty of possibilities, but my first choice would not be saying Bavpros are not great.

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    Re: Loss of acceleration when fully warm

    hot day or cool day ? the air temp sensor in the AFM could also be a suspect and not readily checked

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    Re: Loss of acceleration when fully warm

    Hi Alan..., ever get the rear calipers installed ?

    Regarding your power issue...., I've noticed the following, after startup the ECU goes through an enrichment phase where if you try to drive the car it will stuble and choke. But if you let the car warm up a bit (just out of the blue in the temp) the car seems to be super zippy. Power off the start is awesome and it seem to be a completely different engine..., but then as the car warms a more sedate engine feel and power starts to creap in.

    What's happening is as the engine warms the ECU reads the 02 sensor which forces (as much as possible) the engine to run at 14.7 AFR. During the warmup cycle the 02 is ignored and the car runs at around 13.2 - 13.8.... so the richer AFR is giving you that M feel up until the engine is warm. I wondered about this exact problem myself until I went to an Ind. BMW shop in the SF Valley that is well respected..., I saw a 14k mile 88M6 there that was a customer's car.., brand new baby.., it was beatiful. Anayway, the owner said "nice M5.., but they run lean." So I wondered... Fahey has always commented the same.

    So I wanted to test this and one day a muffler shop install a wideband 02 as a monitoring AFR system just forward of the cat. Yup.., in the warmup phase the AFR starts at 12.0 AFR..., too rich to drive.., then as the engine warms she leans out a bit... and that magic period of 13.0-13.7 AFR is where the engine pulls really good in the lower RPMs.

    I believe.., not 100% sure, that the reason the car runs a bit lean is due to the technology of the time.., non progressive TPS that car enrich things at part throttle.., AFM monitoring, etc. The E34 ECU and monitoring systems solved this problem to a degree.., and the cars of today just seem to run perfect no matter what the engine or ambient temp are.. they just run. Processor speed is much faster then 1985 when the Motronic 1.1 came out. My wife jumps in her Ford mini SUV and just goes... engine cold.. who cares she says. She always asks .. "why do you have to wait for that old bimmer to warm up.." I say.., "all race cars have to do that.... ;) "

    Your chip should have solved some of the problem so if you notice a big dip in power there might be something else at play. Try this..., un plug the 02 and go for a drive. You should have that power back. You can also fit a fuel pressure gauge to your fuel line via a T fitting and long tube that weaves under the hood and then held down by one of the wind sheild wipers so you can see the gauge through the wind shield.., then go for a drive and see if the fuel pressure acts up while driving. I saw a different Ind. BMW shop do this..., hooked it up to a customer's E28 535is (he had stumble issues) and said "go drive for an hour and see what happens.

    Anyway.., I think you just need to live with everything as-is unless you want to swap out the ECU with a programmable unit. I'm in the process of doing this (have been for a year now) and it's a bunch of work. New engine harness, new sensors, adapt the air inlet and remove the AFM..., MAP sensor.... a bunch of work.

    My best time of day to drive my car is early morning when the air is dense and the car isn't fully warm..., it's a whole different experience.

    Philo.
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    Re: Loss of acceleration when fully warm

    Thanks, Eric. I'll check the state of the exhaust system.

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    Re: Loss of acceleration when fully warm

    Whoops, that wound up under the wrong comment. Sorry.

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    Re: Loss of acceleration when fully warm

    Boy, I ask for help and I get attitude. Oh, well. Compression has been slipping a bit of late, but it's good across. Gas is Chevron. No point in swapping the chip; the car ran well with this chip installed.

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    Re: Loss of acceleration when fully warm

    Can't say as I noticed a difference related to temperature, but I'll try to watch for that.

  10. #10
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    Re: Loss of acceleration when fully warm

    Hey, Philo! Brakes are installed, and are great; with the 850 fronts, they definitely do the job. Thanks!

    You've accurately described the warmup pattern. My issue is that layered on top of that is an intermittant problem of loss of zip that's got me stymied. Maybe I'll try that fuel pressure monitoring option. And yes, it would be nice to be able to program the ECU to enrich the mixture just a bit. But it shounds like too big a job to undertake.

    If you have any other thoughts, do let me know.

    Alan

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