Unstable Idle, Only When Warm

[jbruch694]

Hi,

I have a 1987 325is, and have just brought it out from the East Coast to Colorado where I go to school. Car made the drive without a hiccup, and had been running well. Right before I came out I did a valve adjustment. In the last two weeks I have noticed that at warm idle (fully warmed up) the idle hunts between about 550-850 rpm, back and forth. It doesn't seem to be misfiring or anything, as the engine itself runs smoothly when it does this, other than the constant rpm changes. The engine runs great anywhere above idle and has no issues. The idle is perfectly stable at cold and warming-up temperatures.

What I have done:
-Vacuum leak test with starting fluid - no discernible leaks found.
-Stomp code test - car threw code 1211 (faulty DME) and nothing else.
-I replaced TPS ~2 years ago (Bosch) for other reasons.
-I replaced coolant temp sensor ~2 years ago (Bosch) for other reasons.
-Last time I tested the AFM, about a year ago, all the values were within spec.
-Checked spark plugs, they look as they should, no black / sludgy residue, just light brown color like they should be, no evidence of running lean or rich.

Other things to note:
-Cat is gutted (not my doing, will be fixing as soon as the dough happens).
-I run 87 octane gas.
-Car does not ever die or stall out when doing its funky idle thing, but the RPM does drop low enough for the lights to dim a bit.

Main question was, is there some other circumstance that would cause the DME to throw a 1211 code, other than it failing? I have always been taught that the computer should be the last thing you look for as a problem source, and was wondering if anyone knew of something else that could cause it to throw such a code. What else should I check on this thing?

One more thing: When looking at the DME, I noticed it does not have the typical "153" or "173" number on it, and this unit is stamped as made in Spain, not Germany. Since it stores codes, I am pretty sure it is a 173, but has anyone seen a DME with labels like this before? Possibly an old reman?

Thanks in advance!

http://i.imgur.com/Boy2NKG.jpg

[Kershaw]

It really sounds like you have a vacuum leak. Using starter fluid isnt the best method. Where in Maryland are you? I built a home made smoke tester for vacuum leaks and I want to see if it works.

[jbruch694]

I'm actually at school in Boulder CO right now, unfortunately, so my tools are limited. I am from Bethesda. I actually was going to use a fog machine to test for leaks like you have a photo of, but both the ones I had were broken! I might see if I can get a shop to test for leaks, but I was still curious about the DME fault code. I reset the codes, and that one still showed up.

[5Toes]

test the TPS for continuity for idle and WOT cold AND HOT. Ive seen cars before that the TPS would test fine cold, but warmed up it would be skewed and cause a headache similar to this.

have you cleaned the ICV?

[dnguyen1963]

Code 1211 can show up when a new chip is installed and the ECU fails its check sum. Disconnect the negative battery cable for about 10 - 20 min and see if the code comes back.

[jbruch694]

Code 1211 can show up when a new chip is installed and the ECU fails its check sum. Disconnect the negative battery cable for about 10 - 20 min and see if the code comes back.

I did do this already, and the code came back immediately. I am gonna try the other suggestions probably tomorrow night when I have time, and report back.

Thanks!

[jbruch694]

Update:

-Car now misfires under load at low rpms, like when taking off at a stoplight. No power during this time, I stalled it when taking off once. Was pulling onto a mountain road from a turnout expecting the rears to spin on the gravel but nothing. Occasionally misfires at idle. Runs great above ~1800 rpm.

-Did smoke test, no vacuum leaks found.
-Checked resistance on AFM for the door and the temp sensor. Smooth change in resistance when moving door, no dead spots. Brought airbox inside to my ~68 degrees apartment and the resistance spec at 68 degrees is exactly where it should be.
-ICV rattles when you shake it, spins freely, and resistance is perfectly within spec.
-<5 ohms resistance across O2 sensor heater circuit.
-Resistance on each of the 6 injectors was about 16 ohms, where it should be, though this doesn't mean I don't have a leaking injector.
-TPS continuity was correct at closed and WOT.
-Temp sender resistance values were within spec.
-No resistance in plug wires.

One thing to note:
-Coil resistance was about .75 ohms between + and -, and about 6500 ohms between + and center. However, I just changed this out with a new Bosch unit 2 or 3 years ago. I may clean the contacts and take another reading, but is this of concern?

-Did not get a chance to check voltage on O2 while running, will try tomorrow.
-Did not think to check evap purge valve, will try tomorrow.

These options, and a messed up DME are my only options. Has anyone ever had a dying DME give symptoms like this? I'm about to cave and take it to a shop for the first time!

Thanks!

[dnguyen1963]

Do you have another DME to test? One more place to check is the aluminium elbow going into the throttle body. It can get loose when the engine is warm. Wiggle it to see how loose it is.

[jbruch694]

Do you have another DME to test? One more place to check is the aluminium elbow going into the throttle body. It can get loose when the engine is warm. Wiggle it to see how loose it is.

I don't have another DME unfortunately. I will try to source one. The aluminum elbow is solid (I think there are 2, actually) as I JB welded them in. They used to be loose, but they are good now.

Thanks!

[5Toes]

did you check the plug wire resistance engine hot or cold? this really seems ignition related, so perhaps the coil lead wire is getting too much resistance when hot

or the fuel injectors are tired and gross and in dire need of a rebuild, if it is getting worse over time then that could be the problem. granted, MEPEH on the forum here can rebuild injectors for about $40 all said and done and has done great work for me over 3 times now.

the AFM may need some work too. If you are able to find one to swap out Id try that as well, however the misfire while idling should not be caused by a bad afm...

wish I was more help :(

[jbruch694]

I checked it cold. Will check again hot.

Injectors are rebuilt units I put in about 3.5 years ago. I won't rule that out, but they aren't original to the car or anything. Fuel filter is a year old. The AFM tested as it should as far as ohms go.

I found a DME (type 380 I think, from a 1990 325is, but Motronic 1.3 and functionally the same as 173) from some dude on CL here, swapped it in (with the engine warm), and initially it idled fine when warm. But as I drove it and I guess it "learned" it, the idle got worse. Not as bad as it was, but not perfect. Also, the car now throws Check Engine lights when it senses poor idle, throwing code 1222. So I think something was wrong with the old DME seeing as it didn't pick up on the O2 sensor rich / lean code or throw a CEL like the new one does.

Will check again for vac leaks. Gonna buy some black n milds and blow into a vac line. lol

[jbruch694]

Update - did smoke test again, with black n milds blown into the FPR hose (wine flavor lol) with several people watching with bright flashlights. Found a leak at the intake boot where the hose clamp wasn't tight enough. Tightened it and no other leaks found.

Tested resistance of plug wires when hot. Coil to dist was about 2k ohms, and each of the plug wires was about 6k ohms. The #3 plug wire had some evidence of compromised insulation around the plug boot, presumably because this plug is right above the gap in the manifold heat shields. Didn't get a pic, but it was like the rubber outer layer of the boot had shrunk and broken in one spot. We wrapped that in gorilla tape for shits and gigs to make sure the spark wasnt jumping out of the boot to the head or anything.

Tested resistance on heater circuit of O2 sensor, and got 5 ohms. Tested for current for the heater circuit and got battery voltage. With the car running and at operating temp, I tested voltage of the O2 output. At idle, the numbers were all over the place, from .1 to .8, and occasionally evened out and then got crazy again. I held it at about 2k rpm, and the numbers were bouncing all over too. Would this say the O2 is getting bad readings, if it isn't reading a constant voltage when running at a constant RPM?

After this, I reset the ECU, and went for a drive. After resetting, the car runs perfectly as it should, like it was new. Then, as you drive, the CEL comes on, and it gradually runs worse. After shutting it iff and letting it cool, it runs great when cold, and then very suddenly gets bad when it hits operating temp. The misfire has not come back yet after taping the questionable plug wire, but I haven't driven that far either.

To me, this points to a bad O2 sensor, because the computer "learns" from the bad inputs, and because it doesn't use O2 inputs when cold, which would be why it runs fine. I don;t want to throw parts at the car, but does this sound reasonable?

Thanks!

Edit: to clarify, the car runs gradually worse at idle after resetting the ECU, but anything above 1500 or so seems fine. No loss of power on hard acceleration, and th eCEL goes away during hard accel.

[Kershaw]

Can you try running it with the o2 sensor unplugged after you reset the ecu?

[jbruch694]

Can you try running it with the o2 sensor unplugged after you reset the ecu?

I will try that today and see what happens. Thanks!

[jbruch694]

Update: ran car without O2 sensor. Car idles perfectly, but (predictably) runs quite rich. This should point it to being the O2 sensor if I'm not mistaken? To save a few bucks, I picked up a sensor from a late 80s Econoline van, and am going to splice this to the old connector. Saved about $80.