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
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.
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
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.
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.
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!
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?
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.
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.
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
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 email@example.com London SW
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.
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.
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
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?
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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