Intermittent rough engine w/O2 Sensor Issue

Spaz325ic · 09-29-2011, 01:33 PM

New to the board and own an '88 BMW e30 M20 325ic (over 300K miles) with an intermittent rough running issue associated with a OBD O2 sensor "check engine" light. This is second time I've had this issue (last time 4 months ago; about 3K miles). I replaced O2 sensor then and now problem is back. Details:

- Problem normally presents after driving for a few minutes (before reaching operating temperatures) with the check engine light on. Car seems to run fine, with maybe a slight hesitation at idle

- After vehicle is shut off, next time I start it, it will run rough (poor idle; seem like a misfire; rich fuel mixture [based on exhaust smell])

-- If driven, any RPM below 2000-2500 rpm exhibits the rought running, but if I keep the RPMs over ~2500, the engine seems normal and smooth
-- However, it just drinks the fuel...1/8th of a tank in just 10 miles or so

- Often then the next time I start the vehicle, the "check engine" light resets and car runs smooth as silk, then it becomes a matter of waiting until the next time the fault shows it's ugly head.

I've got the Bentley manual and my initial thought is to start with vacuum lines, spark plugs, wires, distributor, etc., but as it's so intermittent I just don't think it's the standard "tune up" issues (tune up done professionally about 7 months ago to include valve adjustment).

I'm fairly handy mechanic (done timing belts; brakes; wheel bearings; distributor replacements; etc) so not worried about attacking the issue, but looking for a good starting point.

Any suggestions are greatly appreciated.

Cheers,

Spaz

1988 325ic

Spaz325ic · 09-30-2011, 04:08 AM

O.K., been doing a lot of scouring the interent on this one and here's my plan of attack:

- Check for vacuum leaks (repair as necessary). Don't expect to find any, as virtually all the hoses look in excellent condition

- Pull the intake and clean both the MAF and Idle control valve

- Check the Throttle Position Switch and the Coolant Temp Switch

Of course, I'll be checking the plugs / wires to ensure they haven't been fouled by the rich fuel mixture.

Of course, as the problem is intermittent, makes it hard to find the "smoking gun" to determine if my actions fix utnil it doesn't show up again.

If anyone has other suggestions, let me know.

Thx,

Shawn

Jared at Pelican Parts · 09-30-2011, 09:08 AM

I would suggest removing the glovebox to gain access to the ECU and start testing all of the inputs at the ECU connector. The Bentley has a diagram which will show how to do this.

My immediate thought would be that you have something going on with the MAF. However this does sound a bit strange. Do the obvious stuff first, check vacuum lines, clean ground points, etc..

Jared at Pelican Parts · 09-30-2011, 09:09 AM

One other thing, when you check the plugs, make sure they are not platinums. Using platinum plugs causes all sorts of weirdness with the M20.

Spaz325ic · 09-30-2011, 09:28 AM

Jared, thx for the input. I'm got my list and Bosch Super (non-platniums) or NGKs are on it. Figured I'd just replace them.

I'll do the obvious as you suggest (vacuum, clean MAF, ICV, TSP, etc.), but your though on ECU seems valid, especially as it is intermittent.

I'll post results as I get to it.

Spaz325ic · 10-03-2011, 01:25 PM

Did the starter fluid trick to find vacuum leaks...nothing found. Pulled the ICV and cleaned it (appeared to be operating normally). After that the ECU threw a MAF "check engine" light which then subsequently reset. Not sure whats going on still. I'm going to get a new multimeter (current one died) so I can start checking all the e-part resistances to determine if they are within specs.

More to follow.

Jared at Pelican Parts · 10-03-2011, 01:27 PM

Sounds like the MAF is going out. Whereabouts are you in CA? Looks like the high desert of SoCal.

Spaz325ic · 10-03-2011, 02:25 PM

Yeah, leaning towards MAF as well. Gonna check all the key component resistances, just to be sure nothing else is an issue, then order a MAF (found remanufactured one for $130 incl $40 core).

The photo is actual Colorado Springs (I'm military). Car was purchased Feb 2008 in CA, moved to CO, now in Alabama.

Spaz325ic · 10-04-2011, 01:43 PM

Cleaned MAF (including the bypass port and temp probe on intake side), verified ICV resistance (within spec) and the thermal control sensor (within specs). Cleaned all electrical connections with electrical parts cleaner and put it all back together.

Car started up fine (with minor bobble) then purred and ran great on test drive. However, when I started it up, it threw a TCS "check engine" light (3 flashes). Rechecked the TCS resistance and appears to be at specs (at cold and full operating temp).

Car runs like a champ now, no hesitation, no poor idle, no running rich smell, etc., so I don't want to replace anything until I get the poor performance again.

Is there a way to reset the ECU error to see if it is a transient TCS error?

Frajo · 10-04-2011, 09:07 PM

Where did you get those wheels?

Manolito · 10-04-2011, 09:30 PM

Intermittent problems are tough. I suspect the heater circuit on the oxygen sensor. The problem could be a poor connection anywhere between the fuse box and the sensor itself.

Those are nice wheels. Actually, that is a REALLY nice car.

Spaz325ic · 10-04-2011, 09:49 PM

Frajo, wheels are made by a company called Beyern. They made wheels specifically for BMWs. I got mine thru a local Discount Tire (special order). Very reasonable (whole package was under $1000, tire/wheels. If you go to the Discount Tire webpage, select Beyern on the wheels page and I think you can order and have them shipped to you in HI.

Manolito, thanks for the input. Yeah, gotta love the intermittent part. I'll just wait until it remanifests...will focus on the some soft-top repairs for the next few days or so. The cars been a labor of love/hate these last 3.5 years. Bought it on the cheap, but it hasn't been cheap. Keeps me occupied and mostly out of trouble with my better half. I've done quite a bit (some planned, others not). Car was lowered when I got it, but here's my list:

- New steering rack; heater core; A/C everything accept the compressor (coverted to R-134)
- New front/back seats
- New wheels/tires
- replaced the head bolts to the torx-type
- Rebuilt the VDO dash (including Speedo gears)...tach/mpg gauge is intermittent (suspect the SI batteries, but haven't gotten to it)...bigger fish to fry
- New timing belt/H20 pump/cam & crank seals
- Coverted to HELLA Euro-spec "Smiley" headlights
- Lots of clean-up in trunk from water leaks
- New paint (went cheap on this--National chain)--but I did a lot of prep/stripped the car. It's not flawless, but looks 1000 times better than the peeled paint it replaced
-...and of course a better stereo (nothing fancy but play MP3s now)

Still have more to do, but don't plan on getting rid of it anytime soon.

pokface · 10-06-2011, 10:53 AM

the last 7 of the vin would be helpful. i believe yours is late enough to have a 55 pin ecu, 1 crank sensor on the timing cover, 1 sensor on #1 wire and a troublesome connector under the intake manifold. the heater hoses that go to the throttle body or the heater itself leak and the coolant gets into the round connector. it' a bear to get to, much less take apart to clean and/or repair. pull the boot of the bottom and make sure it's dry. it'll collect coolant and the wires wick it back to the ecu and up into the connector, after you've cleaned it.

if you have any early car, with a 35 pin ecu and 2 sensors on the bellhousing the above does not apply. i believe you don't as they didn't have a check engine light iirc, been a while. good luck, chuck.