Another High Idle Thread...

[alexxander.foster]

Hello!

So I recently acquired an e36 318is from my father.  Unlike myself, my father absolutely hates cars and drives all his vehicles into the ground.  My poor 318is is in bad shape and need of some love.  Most of the problems are coming along smoothly, but I am totally stuck on a stubborn high idle issue.  Hopefully you guys can help me out here.

I have no idea how the high idle issue started but its been going on for at least 5 years.  When the engine is cold it idles just right, but when the car warms up and goes into closed loop, the idle gradually increases to about 1500 rpms.  Additionally, I've noticed that at around 1000 rpms, the car will stumble when revving the motor.    The stumbling occurs regardless of how gradually or rapidly the rpms increase.

Here are some issues that I've identified and repaired/replaced:
- Torn intake manifold gaskets -replaced
- Dry-rotted vacuum lines -replaced
- Cracked IAC supply hoses -replaced
- Cracked intake boot -replaced
- Clogged IAC -replaced
- Torn wires to IAC -repaired
- Blown exhaust manifold gaskets -replaced
- Cracked exhaust manifold (cylinder 3 & 4) -repaired (welded)
- Sluggish acceleration -replaced fuel filter
- Replaced engine coolant temp sensor
- Bypassed throttle body heating block

I've tested the following sensors per the Bentley manual:
- Throttle position sensor
- Cam position
- Crank position

These are issues I've not yet been able to tackle:
- Burned capacitors found on DME circuit board
(the burned capacitors were part of the IAC circuit and the DME relay circuit)
- Cracked housing on both knock sensors
- Oil change (I know I know, but sometimes it matters so I figured I'd put it down)
- Test for blown head gasket
- Check spark plugs
- Test plug wires
- Test DME relay
- Test fuel pressure
- Test fuel pressure regulator

The car doesn't appear to be eating coolant and the exhaust just smells like exhaust (i.e., no coolant smell).  I've disconnected the fuel pressure regulator from the throttle body and revved the engine but saw no change.

Any thoughts or ideas would be greatly appreciated.  Thanks guys!

[bmwman91]

My bet is on the burned up DME components that relate to the IAC motor.

[alexxander.foster]

thank you bmwman91.  I suppose that's probably the most appropriate next step.  Perhaps I can find someone with a good running M42 near me who would let me switch ECUs for 15 minutes just to rule out the issue.

Regarding the burned up DME components, I'm not quite sure what to make of them.  Another member send me pictures of his DME so I was able to figure out what their capacitance values were suppose to be.  I carefully removed each capacitor from the circuit board and tested them.  All of them were cooked on the outside, but came back well within spec -actually they came back above spec considering they're 20+ years old.  I'm planning on replacing them anyway since they're 19 cents per piece, but I have my doubts.