How many O2 sensors on '95 525i ??? M50

[volvodefector79]

Hello! It's been about two years since I've been on UnitedBimmer, as we parted with our '96 328i a while back ('twas a sad day). But now I'm back with an OLDER Bavarian auto - a stock '95 525i (e34 M50 motor).

I've only had the car for about a week, and I've been busy with summer classes, my baby, and acquiring new Bilstein struts for the car (the front right is actually BROKEN). So, yeah - I've been prioritizing. And though the CEL is on, the car has run well enough to get around. I'm now just getting around to addressing the CEL.

Using the "tube" (Peake code reader), I originally pulled two fault codes: OXYGEN SENSOR and LAMBDA CONTROL. I'm assuming the LAMBDA CONTROL was tagged because of the O2 sensor (the CEL has been on for a while, according to the previous owner).

This may also be relevant, as I know the O2 sensor (multiple???) was replaced one within the last year... The previous owner made MANY short trips of 2 minutes or less, as she lived down the street from the school where she worked. Condensation build-up around O2 sensor & premature aging? They replaced the O2 sensor and a few months later CEL was back on, indicating a fault of the very same sensor(s) that had just been replaced...

ANYHOW, back to the present tense. I pulled the two aforementioned fault codes, then reset the CEL and waited for the light to reappear.

Sure enough, after about 4 minutes of smooth driving today, the CEL is back on. I popped the "tube" on again after running a few errands (about 15 minutes of city driving) and only the OXYGEN SENSOR code was present. I should mention that in park my idle hunts/bounces slightly back and forth between 600-700 RPMs.

Anyhoo, does any of this ring a bell to current/previous e34 M50 drivers? Is the hunting idle typical of O2 sensor failure? AND how many oxygen sensors are in this vehicle (just one, or one upstream and downstream)??? I've ordered the Bentley repair manual, but have yet to receive it, and haven't put the thing up in the air yet.

IN ADDITION to the oxygen sensor fault code, the thermostat is stuck open (during hot summer days in Illinois it takes a while to reach normal operating temperature, and on the highway at night it hovers just above the blue/cold side). Is it possible THAT could throw off the ECU and cause the mixture to be too rich, triggering the O2 code? Probably not... either way, that issue will be fixed by this Wednesday.

ONE other thought - could a faulty MAF be causing the slightly jumpy idle/slight acceleration hesitation, and somehow show up as "oxygen sensor," since that, too, affects the correct air/fuel mixture? (based on anybody's experience, is that typical? Or does O2 sensor ALWAYS mean O2?)

I'm trying to get her back to optimal running condition. We paid $1600, and considering she's pretty clean with 115,000 miles, I think we made out pretty well.

THANK YOU to anybody who reads through all this and has any input. I REALLY appreciate it. I'll be looking forward to your knowledge/advice... Happy driving, and Auf Wiedersehen!

[Big Evil]

You are right to ask the question. An O2 sensor code does not always mean a bad O2 sensor. Most of the time actually it isn't even the O2 sensor that has the problem. You say the one that's in there is a year old. Even with nothing but short hop driving it SHOULDN'T be bad. What an O2 code means is that the sensor is giving the ecu a read that is out of range. This can be cause by ANYTHING that the ecu can't compensate for to maintain air fuel mix. Here's a list starting with something else you mentioned, stuck open thermostat. The thermostat is designed to get the vehicle up to operating temp quickly (within a predetermined time frame). If the car doesn't heat up quickly enough the O2 sensor doesn't read accurately within the specified time frame....bam, you get a code. Too lean a mixture, caused by weak pump, bad regulator or clogged/restricted filter, bam, O2 sensor code. Vac leak or air leak btw maf and throttle plate, causes lean condition and crappy idle quality also.............you guessed it O2 sensor code. Is your MAF bad? it's possible but you're getting a minor rolling idle, chances are the O2 isn't giving the ecu an accurate read and the computer is just trying to compensate. I'd look to the thermostat first and try it again, by the way do a full coolant system flush and use the factory recommended nuclear green stuff, same with oil plugs etc. Always use oem replacement parts, all the new synthetic stuff and crazy rare metal spark plugs didn't exist when these cars were designed and it DOESN'T help them be any better, in fact most of the time it'll just kill them slowly. Good luck

[volvodefector79]

Big Evil ~

Thanks for the advice! I, too, was thinking that I should wait until the thermostat is replaced before venturing into troubleshooting other potential electronic gremlins that may be causing the O2 fault codes. Always start with the simplest problems, and you end up with simple solutions. ;-) Let's hope for now the thermostat is the culprit!

I'll post the results later this week. Here's hoping this thread will end itself there... Take care, and thanks again!

[volvodefector79]

Big Evil ~

Hey! Did coolant flush, replaced thermostat, t-stat housing (used NEW OEM plastic housing, as crack independent mechanic said there are many cheap aluminum cast housings, and that they are actually less reliable), etc. And drove it a few cycles. CEL still on...

So I used Peake's "TUBE" and reset the CEL. Low and behold, it showed up again after 5 minutes of driving. Well, no big deal, as it needed a thermostat, anyway. But as opposed to throwing parts ($$$) at the car, I took it to the aforementioned shop (Euroquipe in St. Charles, IL - excellent). They read DME, and were able to test the O2 sensor independently (maybe easy, but beyond my knowledge) and they said there was absolutely no response at all - it was DEAD.

So, the sensor I could definitely do myself. I replaced that yesterday. I should mention that the "old" O2 sensor was indeed quite new, but it had broken pieces of ceramic-looking substance inside the sensor tip (viewable through the narrow slots). Evidence, perhaps, of why it was dead. ANYHOW, after replacing I reset the CEL, and viola! No more check engine light. And it starts with hardly any cranking, idles smoothly, and has better acceleration. The MPGs are already improved as well. Awesome!

Thanks again for your advice. I guess that after the most simple solution, it was the next (in this case). I'm sure I'll be on this site again, looking for advice, but hopefully not for a while!!!

Take care, and happy driving. Auf wiedersehen!

[Big Evil]

Nice work man, sounds like you did VERY well. Congrats on the fix,

K