E30 M20 Engine Hesitation during acceleration - no one can find the fault!

Hello,
I have a problem with my e30 325ic. Under hard acceleration at almost exactly 4500RPM (although sometimes it does it at 5000) the engine sort
of "stalls" for a second, and then picks up again and pulls all the way up to 6000 Rpm. It is exactly as though I just took my foot off the pedal for a second (but I don't, lol), and happens consistently every time. Other than that the car runs perfectly.
I have tried changing the plugs, but no change. I have checked for vacuum leaks, but can find none. I took it to a BMW "specialist", and he had it for 3 days, and has no idea what the cause is. He tried changing the index ring for the crankshaft position sensor, but no change. The only thing he could tell me was that it keeps registering "a lot of different fault codes", but nothing specific.
Anyone else got any ideas? Any help much appreciated.
Thanks Rob
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I'm not sure how sophisticated the M20 engine is but in my experience, acceleration related problems point to something along the lines of a worn distributor cap, althoguh thte fact that it only does it a certain RPM makes me think it might be something else!
I had something similar on another car where it would cut out completely at 4000 or thereabouts and it was the MAF but like I say I dunno how the M20 engine works.
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JoshIII responds:
Change out your Engine Control Unit (ECU), which controls shift points, ingnition/timing, and electronic fuel injection system.
Modern cars are like desktop computers. Its less expensive to replace components than to analyze and repair faulty components.
If that doesn't resolve the problem, ship the new ECU back for a refund.
Good luck!
JoshIII josh3i at hotmail . com
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He'll need it if he tries to return an ECU for a refund...
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win.ntli.net:

Coil? Its possible that the coil fails at a certain firing frequency. Also once on an m10 I had a bad engine ground that would only fail when the engine was at a certain RPM (the shakey shakey m10 would pull the wire just enough so it would cut out just at 3700 rpm .. but if you quickly went past 3700 or stayed below it was fine. Boy that was an annoying one).
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Robert wrote:

Thanks for the advice. I will try the coil this weekend, as it will take me a while to get another ecu.
I will let you know how it goes.
Robert
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Assuming you are a typical E30 owner, you no doubt have met a few others and stay in contact. One of them might pull thier ECU and test it out with you. We've done a lot of this kind of troubleshooting around here. Isolated a few problems that would have otherwise been expensive to fix..

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I had a similar problem several years ago on a volvo V40 T4 (200hp turbo). Very similar symptoms. Volvo chief mechanic found it on the third visit by watching the engine at high revs, in a dark room.
He found an ignition component shorting to the engine. I can't remember which component, but his method may help you physically locate a short.
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What is the ECU model number? I have 2 ECU's from an '88 325is (one Programa and another stock). What state are you in? (No, confusion doesn't count) Bill in Omaha
wrote:

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Bill wrote:

Thanks for the offer, but I am in the UK, probably a bit far.
I tried changing the coil this weekend, but no change. One thing I did notice though, is that after changing the coil (even after going back to the old coil) the engine ran rough for a while (hunting and un-responsive), but then after a short run, it was ok again, which is strange, considering the only thing I touched was the coil.
Still no closer
Thanks for the advice so far.
Robert
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I was wondering Rob if you had solved the engine hesitiation problem. I have a 325ix 88' 175k that has a similar problem at 5krpm. It started right after I cleaned the airfilter for the first time. But I dont' know if it is coincidence or not. Would a Mass air flow sensor be the coulperate? Hope its not an ecu. Also, any help appriceiated :)
Thanks, Darren
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Just a guess, but could it be bouncing off the rev limiter?

B.S. flag on the play! Index ring? That version of the m20 has a front crank position sensor that reads from a pin in/on the harmonic balancer. See where this is going?
but no change. The

10 yard penalty! That car can post maybe, what, 10 codes? I believe they are all read from blinks on an instrument cluster light, check engine, perhaps? Your BMW specialist is an idiot and playing you for a fool, straight up. Ask him what the OBD Code Reader says. If he says anything except "You can't use an OBD Code Reader," he's lying. Fault codes on the e30s were read from the check engine light flashes.

Somebody correct me, please. My caffeine is wearing off. Then again, the AFM is a good culprit, as well as the fuel pump or fuel pressure regulator. But if the pressure is there, most likely AFM is binding or out of "adjustment" (carbon track worn) in that rpm range.
Bill in Omaha '86 535i
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REV limiter much higher than this

There is still an index ring (actually it's an entire plate) but there is NO way it could cause this...

There are actually a few more codes than that depending on which Motronic version the car has and the codes are very specific. Either way, they can be read from either the check engine light flashes or a code reader. You are correct in the statement that it's not an OBD reader but that point would be missed by this quack mechanic. Replace the fuel filter (always a good idea) and check the air filter. Neither of these are the full cause but they could be contributing to it. Is the check engine light operational and is it on or off? If it flickers under acceleration, the O2 sensor is going and this WILL cause these symptoms. This is not a part you want to change for the hell of it, it's a bit pricey. You provide details and we'll get this sucker figured out...
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Rob, I had a very similar problem with my 318i. Turned out it was a bunch of grime in the intake snorkel from the airbox back. Made no sense to me but a thorough cleaning savrue but e me buying a new ecu and various other parts. Sounds too simple to be true but it worked for me.Don.
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