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Old 01-04-2017, 06:35 PM   #1
Andy.B
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Surging idle: bad IAC or DME?

The car is an 87 325i with a m30b34. Engine management is motronic 1.3 from a m30b35. Running a 178 DME through a modified m20 25 harness. I replaced all intake gaskets, boots, and hoses when I did the swap. It's been a daily driver, with about 200 miles a week the past 18 months. It's run trouble free this whole time. Suddenly, halfway through my morning commute, while driving a steady 55 mph, the throttle became very unpredictable, and it became impossible to modulate a smooth speed. When I came to a stop light in the next town, the rpm was rapidly surging from 1500 to 2000 rpm. I liked it home and started diagnosing the issue.



Stomp test only revealed 1 code: 1262 Idle Speed Actuator:
1262 - Possible causes for Idle Speed Control fault are:
•damaged wiring to the idle speed actuator
•wire from DME is defective
•output stage of DME is damaged
Also if engine stalls while it is above 600 RPM, code 1262 will be set

I've tested the AFM, CPS, TPS, and temp sensor. I tried a known good IAC valve from friends car. I took off all boots and hoses and inspected for leaks or cracks. I pulled the valve cover to check for leaks/damage. I reassembled everything and tightened down all hose clamps. I checked the torque on the intake manifold studs. I sprayed every connection and flange with brake clean with it running to try and find a leak. I would suspect a pretty significant leak wood be present to cause such a sudden, unmanageable drivability issue.

I wish I had found something, but short of a smoke test everything seems leak free. The only thing I can think of at this point is a faulty DME. (Which is a potential cause for the only code the DME throws).

Idle is perfectly smooth and the car is perfectly drivable with the IAC disconnected (if the DME is left disconnected at the right position). You can see the immediate change when I disconnect it in the video. It seems like the IAC is rapidly switching between full open/closed. If I disconnect it with it fully closed, it stalls. If I disconnect it fully open, the car revs towards redline. If I get it in the middle, it holds that rpm.

Is there anything I need to check besides trying a replacement DME? Something I'm missing? Something else that can cause this behavior?

Last edited by Andy.B; 01-04-2017 at 06:47 PM.
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Old 01-04-2017, 07:44 PM   #2
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Are you using the C101 connection still? Check those connections for damage/wear/weathering. How did you modify the harness? Have you run continuity tests between the IAC and the ECU? (Pin 4 white/yellow cable, and pin 22 white/green cable. unless your wiring changed something). I really don't think it's the ECU in this situation
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Old 01-05-2017, 03:27 AM   #3
Andy.B
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The only modifications I made was to cut the sheathing off, route the wires where they needed to be, and re wrapped everything. I believe the only wire I cut was one of the AFM wires that was too tangled to reach where I needed it.

I am still using the c101. I will open it up and check for signs of damage.

I will also check continuity between the DME and IAC. The fact that it is able to actuate it (buzzes key on engine off, clicks open/closed when running) but not modulate it properly made me assume the wiring was OK, but I guess I shouldn't rule anything out.

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Old 01-05-2017, 07:11 AM   #4
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The situation your described leads me to believe there may be a loose/bad wire. If it's not getting consistent signal or power or ground, that may cause the wild idle swings, since it seems you've been pretty thorough about it already
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Old 01-05-2017, 08:17 AM   #5
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When the fuses on my E28 528e (which uses an older inferior fuse box design) were making poor contact one of the symptoms was a badly surging idle. If you haven't pulled all of your fuses to check and clean them, you should do so before doing anything else.
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Old 01-05-2017, 05:41 PM   #6
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I spent as much time as I could before I got too cold out in the garage. A little electric space heater is no match for the lack of insulation, and the 12 degree cold outside.

I pulled all the fuses and relays, checking for corrosion or loose/damaged connectors. Everything looked good, so I opened up the fuse box to check for internal corrosion or damage. Everything looked good, so I put it back together.

I also disconnected the halves of the C101 and looked for any obvious damage. Here again, everything looked good.

I worked through all of the pinout tests from the DME connector listed in the Bentley manual. I got proper power/ground/resistance everywhere, except for the CPS. Acceptable results are 540 +/- 54 ohms. I measured 470 ohms at the DME connector, and the CPS connector. I'm surprised with a failing CPS it would run at all, but I suppose it makes sense that it could cause the idle issues. My understanding is that the only input for the idle control logic is the engine speed, and if it isn't seeing an accurate engine speed from the CPS, it can't modulate properly.

I will try and borrow a CPS from a friend to see if that solves the issue.

With everything testing out properly in the harness and all the sensors (except the CPS), all I can think of at this point are a bad CPS, a vacuum leak I can't find, or a bad DME.

Does it sound like I've covered all my bases? Am I overlooking something?
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Old 01-05-2017, 06:04 PM   #7
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Have you checked for cross talk on the IAC wires? Disconnect the ECU/IAC and test continuity between the wires, they should all be infinite open.

The surging idle seems to be a vacuum leak, but they usually don't idle quite that high when searching for idle.

Also I have had CPS's work fine all the way down to 420ohm, and on up over 600 - even though they are out "out of tolerance". Not going to rule it out, but the stock ECU is pretty forgiving on resistance and air gap.
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Old 01-05-2017, 09:36 PM   #8
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Quote:
Originally Posted by Andy.B View Post
With everything testing out properly in the harness and all the sensors (except the CPS), all I can think of at this point are a bad CPS, a vacuum leak I can't find, or a bad DME.

Does it sound like I've covered all my bases? Am I overlooking something?
I think you've got it nearly figured out at this point; these electrical issues are incredibly frustrating to troubleshoot, especially in poor working conditions.

I'd be surprised if the problem with your car is a vacuum leak given how it's idling, as it sounds like an electrical fault. If you take apart your D.M.E. and carefully look it over I'll bet you'll spot failed soldering or other problems with it, and that would be consistent with a sudden change in temperature combined with high humidity.
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Old 01-07-2017, 08:08 AM   #9
Andy.B
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After ruling damn near everything else out, I got a known good DME from a local community member. My car runs and idles silky smooth again.

Thanks everyone for your input and advice. I never advocate shotgunning parts at a problem, but sometimes, your hunch is correct.

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Old 01-07-2017, 08:12 AM   #10
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Glad you got it fixed! Did your ever take apart your ECU to find out which part failed?
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Old 01-07-2017, 07:29 PM   #11
Andy.B
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Quote:
Originally Posted by squidmaster View Post
Glad you got it fixed! Did your ever take apart your ECU to find out which part failed?
I didn't open it up yet, but I will. I'll post pictures of I actually see anything noteworthy with my untrained eye.

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