M20 cutting out, possible ECU problem?

[Diablo]

I have some strange symptoms with my 87 325i cabbie. A local shop traced it back to the ECU so i purchased one with a warrenty from a wrecking yard but the car still has the identical problems.

The car will randomly cut out when going down the road, when it cuts out the check engine light comes on and the tach drops a large amount, down to zero if it cuts out for long enough (1 sec or so). The hesitation is more noticable when one is light on the throttle, though if one is hard on the throttle the check engine light wil come on and the tach will blip but its not physically noticable.

The second symptom is at about 5500rpm with i am full on the throttle. If the throttle is all the way down it will feel like it is hitting the rev limiter, but if i back off the throttle and ease it through you can continue to redline as long as the you dont put the throttle all the way to the floor. When this happens the check engine light doesnt come on and the tach only drops about 100-200rpm.

When looking at the engine compartment yesterday i noticed that the wire that leads to the crank position sensor (i think) looks slightly damaged. (the insulation is pulled up a bit to expose a bit of the wire).

I have a 90 day warrenty from AAA so think i will try a different ECU, but i find it hard to believe that a new ECU would give exactly the same symptoms as the old one. So given these symptoms do any of you have ideas of what else to look at? Another note this car has the "525" ecu not the "153" or "173" would a "173" be useable in this car?


Thanks

[nando]

test your throttle position switch, and check the injector harness under the throttlebody for corrosion (commonly caused by a leaky throttlebody coolant gasket).

other things to test are your crank position sensor, coolant temperature switch, and AFM.

when this was happening to me it was caused by coolant in the injection harness. it caused the motor to randomly cut out momentarily (check engine light flashes, RPMs drop); really annoying especially if you are cruising at 75mph on the freeway.

[James Crivellone]

Check the speed reference sensor on the front of the engine

As your tachometer is dropping to Zero, as you are moving with the engine in gear, then its def an electrical issue with that sensor (or ECU/wiring failure, but you replaced that).

The ECU passes the tach feed from that sensor to the gauge, if the sensor fails, the ECU sees Zero RPM and freaks out, putting the ECU in an idle mode which will usually only allow the engine to idle.

Check that

[Diablo]

thanks for the ideas guys.. ill let ya know what i figure out


D

[Diablo]

I havnt done much engine work on these cars and my bently is buried since my recent move... is the speed reference sensor on the left hand side of the engine, near the front, if one is facing the front of the car?

Thanks

[nando]

I havnt done much engine work on these cars and my bently is buried since my recent move... is the speed reference sensor on the left hand side of the engine, near the front, if one is facing the front of the car?

Thanks

yes, it's down by the crank pulley on the left.

[mikeedler]

I would say replace the AFM(air flow meter). I had a similar problem years ago, and that fixed it.

[Diablo]

Well just on a hunch i ran the tank down to empty and filled it back up (supreme of course) and the random hesitation has disappeared.

However when driving at WOT it is impossible to rev the engine past 5500, but if i back off the throttle just a tad it pulls strong all the way to redline. Also when starting the car after it has sat over night it idles rough and somtimes dies in the first 30 seconds or so. After that it runs fine. For those that replied above, does your advice still hold? or was it in reference to the random hesitation?

Damian

[BMW2ner]

I had similar issue with running to much fuel