525 Check Engine Light On

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Trying to find possible causes for why my Check Engine Light goes on and off. Car is 1991 525 with 149,000 miles.
Non mechanical BMW lover, who is hoping for some friendly advice in layman
terms.
Car alternates from running rough when Check Engine light is on to running smoothly when not on. At traffic light, have to put car in neutral to keep it from wanting to surge forward. When green light, car dies or stalls and then suddenly surges forward.
Help, please.
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Turn the Ignition ON, not Start. Fully press the gas pedal five (5) times. The Check Light will flash a trouble code.
The flashing will be a couple of long flashes to indicate that is is ready to flash the code, then it will flash a series of short flashes, a space, and some more short flashes. The pattern will be something like _ _ . - . . - . . - . - - . _ _ . - .. - .. - . _ _ <repeat> to indicate 1221. There are other codes as well, but yo uwill easily determine the pattern, and all codes start with 1, so observe the long flashes, then jot down the short flashes.
If you are not able to do the repairs, then simply go to an independent BMW mechanic; I suggest one that is in the phone book.
My guess is you have an O2 Sensor that is going bad, and the car is runing rich, and poorly, when the O2 Sensor decides to go offline. Having said that, you can also be having trouble with an ignition coil.

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Thanks for the quick response. I will try the recommendations you suggest.
Do you know approximately I am looking at for repair costs if is one of the problems you suggested?
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I didn't actually suggest anything, I only described with ASCII code what the flashing light might look like. A short flash followed by a pause, then two short flashes and another pause, and two more short flashes another pause and one final short flash would represent 1221. 1221 may or may not mean anything, but we need to know what your codes are to give an idea of what needs to be done.

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An O2 sensor is about $90-$125, installed another $50-$100 depending upon who you go to.
Floyd
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go to http://www.bimmer.info/forum/forumdisplay.php?f=4 they are the best forum for your model... but Jeff is likely right about the o2 sensor, the codes are a bit confing at first... it gets easier!
wrote

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Hi all, thanks for your quick replies. Repair shop says it is my ecu -electronic control unit that is my problem. Costs over $600 for a used one and over $1000.00 for a new one, plus new one has to be coded for my car - another $250.
What do you all think?
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take it for a second opinion, could be a scam find good indy mechanic and let him hook it up again.

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I think that, if the ECU goes out the car usually won't run at all. Find someone that really knows BMWs (or Mercedes/Porsche/etc.) and pay them the hour labor rate to check - $100 is way cheaper than $1000.
Floyd
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justalaguy wrote:

I think this guy is blowing smoke up your skirt. It is almost *never* the ECU that causes the CEL. Get the codes and figure out what it is first then take it to a knowledgable BMW specialists shop.
--
-Fred W
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I think you should expect a C-note and a dozen roses after that because somebody is bending you over and coming in from the back.
If you drove the car to the "mechanic", then it doesn't need an ECU. Well, I suppose it _could_ need an ECU, but since ECU means Engine Control Unit, and if it was failed then there would be no means of engine control, then it stands to reason that the engine wouldn't run, and therefore you won't be driving the car to the mechanic for him to tell you that the engine controller is bad. If the engine controller was bad, then your car would be taking trips around town on the back of a flat bed truck.
The ECU is not a known trouble spot, this means it generally is pretty robust and can take a bit of abuse before it releases the Magic Smoke that makes it work. I don't know what, exactly, it does, but I know that if the magic smoke leaks out, it stops doing it. If it is doing stuff, then the magic smoke is still there, and it shouldn't need any particular attention.
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It's possible for an input buffer etc to the microprocessor(s) in the ECU to fail - so it gives the same symptoms as a faulty sensor. And for dry joints etc on the PCB to give intermittent problems as it heats up.

In general, I'd agree. There's a UK firm which does reconditioned exchange ones, and a large proportion of those they get back after exchange have no faults.
I have had a faulty ECU - although not on a BMW. The symptoms were exactly that of a worn track on a TPS - wrong fuelling at just above idle where the pot will spend much of its life. Idle was fine, as was were wider throttle openings. Replacing the ECU cured it. Putting back the old one restored the problem - so it wasn't just the act of replacement cleaning the connector. On opening up the ECU, it had many through PCB connections to transfer things from the tracks on either side, and these are notorious for giving problems. Resoldered the lot and it was fine again. However, this was an early EFI ECU. Repairing modern surface mount stuff isn't as easy.
--
*Happiness is seeing your mother-in-law on a milk carton

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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All of you who have so far replied, you all are the best. Thanks for taking the time to help this non-mechanic understand why his 525 is sick, haha.
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As Dave points out, you could need the ECM and still be able to drive the car. But of all of the ways for the ECM to fail, not many of them include allowing the engine to run, and fewer still will let the engine run well one day, and crappy the next. If you drove the car to the mechanic, I'd put money on the ECM being a remote point of failure. My money is on one of the components the ECM is looking to for information, and the O2 Sensor is high on my list. The O2 sensor is replaced in much the same way as a spark plug, except it lives in the exhaust manifold -- there might be a second one that lives further down the system just behind the CAT, but it is replaced with the same tools as the other one. There is always an O2 sensor before the CAT, and sometimes there is one after the CAT.
Generally, the O2 sensors do not cause the motor to run poorly -- that is, not so poorly that you would notice anything more than a higher rate of fuel consumption. If youare feeling the motor running poorly, then the odds favor something in addition to the CAT, or other than the CAT.
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Jeff Strickland wrote:

The one after the cat is only present in OBD2 cars and only to inform the ECU if you are fouling the atmosphere. It has nothing to do with engine management, only a means to determine if the cat is doing its job.

Well, sort of. I've had some O2 sensors give errors tha tthe mixture was "out of range" (never any clue which way) and I've had really bad sensors throw the engine into open loop mode, which means it will run like crapola. So, yeah, sometimes you can't feel it, but sometimes you can.
Either way, assuming I had not had the O2 sensor replaced recently, if you are getting a code that points to a mixture out of range - go for the sensor. It's cheap and probably needs to be replaced anyway.
--
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Hi All, Tried finding my codes today by pressing 5 times in 5 seconds on the gas pedal. Key was in the on position, without engine running. Must have been doing something wrong because check engine light never "flashed", thus no codes were ever revealed.
I did add some Valoline fuel treatment yesterday before I filled up. I did notice today when I turned the engine on, and rev'ed up the engine to the red line, the car seemed to run better than before, but only for a few seconds.
I read where it may be tricky at first to get the codes to read, so any thoughts on what I may have done wrong to get the codes?
Thanks,
Larry
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justalaguy wrote:

Refer to this: http://www.bmwe34.net/e34main/trouble/engine_codes.htm
You need to do the pedal stomp immediately upon moving the ignition switch from position 1 to 2.
--
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That's "fully depress the gas pedal five (5) times within five (5) seconds."

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Jeff Strickland wrote:

And to be even more precise:
"fully depress and fully release the gas pedal 5 times in 5 seconds."
It has to sense both the full throttle and closed throttle position switches. The 5 times in 5 seconds thing is a bit tricky too. You don't want to do it in 3 seconds, nor 7 seconds, but try to get a rhythm where you hit just about 5 seconds.
--
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Thanks very much for this info. Now how do we find the list of codes??

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