Erratic Idle + Strange Compression Result

[emuguru]

I have a 1986 325e. When starting and idling (cold or hot) it stumbles at almost regular intervals. Feels like a shake every second or two. I have major hesitation when hitting the throttle. It also randomly idles at 1.2k and randomly idles at 750.

it has
new injectors
new spark plugs + wires
valve adjustment
new intake elbow
new afm
new timing belt
new vacuum lines
new coolant hoses
new speed and reference sensors as they did die.
new thermostat

I have cleaned the ICV and that helped a lot with idle smoothness but still idles high randomly and still has the shudder every second or two.

i did a compression test recently and came up with:
Cylinder 1: 180
Cylinder 2: 180
Cylinder 3: 179
Cylinder 4: 181
Cylinder 5: 180
Cylinder 6: 200

O_o

i have done a smoke vacuum test and solved all leaks except 2 possible.

1. I notice a very very small amount of leakage at the throttle body position sensor.

2. I notice an extremely small amount of leakage through the cooling system, coming out of the coolant expansion tank (would not have noticed had I not been checking the coolant just before and left the cap off)

so I'm afraid I've got a cracked head gasket, but would that cause the random issue with idle? Or would the leak at the tps cause it? Or should I be checking electrical?

[emuguru]

After some digging around the forum, I found a post that helped me out. The Coolant Temperature Switch turned out to be broken but still staying together when the engine was off. So I kept testing it and thinking it was ok. After reading this thread, I tried to test again and when I pulled off the wires, the sensor fell apart. I bypassed it with a jumper (a simple automotive fuse) the engine now idles so much better.

i still have hesitation from idle when accelerating and i still hear a quiet stutter like a misfire every few seconds but the idle is way smoother.

[82eye]

nice catch.

[emuguru]

This is my daily driver, and I put about 50-60 miles on her each day currently. After a day of driving, I am satisfied with the engine performance. Bypassing the temperature switch improved it a lot. But it has uncovered more odd behavior. Now, until the engine gets warm, shifting from park or neutral to reverse or drive causes the engine to die unless throttle is held open a bit. Once warm, it idles fine until I get where I am going and shift into park or neutral, at that point it idles high again. I tested pins 13 and 19 at the Motronic 1.0 ECU connector and the engine coolant temperature switch appears to be giving a somewhat correct resistance value. But I fear it may also be on the way out. Adjusting the ICV doesn't seem to have much affect at all since bypassing the temperature switch. Has anyone else encountered this?

[e30rik]

The blue temp sensor tells the ECU what temp the coolant is running and changes the amount of fuel depending. If it’s not working right you’re probably not getting enough fuel when the engine is cold. I would just get a new sensor, I think they’re less than 20 bucks and easy to change.

[emuguru]

I just changed that yesterday, it finally arrived. That makes the white, and blue sensors being replaced, and the brass one is currently bypassed until it also arrives, should be this week. That further smoothed out driving for sure. I still have the "almost stalled" hesitation on acceleration. I am now noticing a distinct smell of fuel both inside and out of the car. So I'm going to start diagnosing the fuel system next as I have not touched it at all since owning the car other than to fix the fuel sender as someone damaged it when they replaced the supply pump in the tank. I will report here as I go. Hopefully that's where the final issues lie.