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?
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?
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