Hi Everyone,

Today with a warm engine, while driving my 1992 245 (LH 2.4) for 15+ minutes, it instantly started rough running/missing at idle, and
while accelerating slightly above idle. At 1000rpm + it runs great as always.

When the engine is cold, it idles and runs fine. After it warms, it idles very rough when driving. When it's sitting in park, the idle is fairly smooth with only slight intermittent roughness.

The throttle body is clean, TPS is working and properly adjusted, AMM (cleaned) and ECU are 6 months old. Anything 1000rpm+ runs strong and normal, regardless of temperature. I spent a couple hours checking for vacuum leaks by spraying down every hose and connection. Almost all have been replaced fairly recently and all seem fine after spraying them. I disconnected and plugged the vacuum to the inside so no problems there. Vacuum gauge shows 19-20 lbs, but dips slightly when the engine misses/rough. When I rev to 1000rpm, I have a solid 21lb of vacuum and it's smooth.

The engine goes instantly from smooth to rough once a certain temperature is reached. I let it cool, start it again, same thing. I ran codes and got an all good 1-1-1.



I checked the ect resistance and voltage, works perfectly through its temperature range. I check the IAC resistance and voltage, working. Maybe I should try cleaning it?

I went over everything and there's no vacuum leaks anywhere. Checked vacuum gauge again, 19-20, and solid smooth 21lb at 1000rpm.

I have a Bosch white label ECU 0-280-000-951 in the car. Tried my backup remanufactured Bosch unit and got the same results.

I check all ignition components and it checks good, cap, rotor, wires, coil. The Sparkplugs look good, consistent light tan color.

I'm out of ideas.



Any ideas are very appreciated. Thanks, Jim

 

Coolant temp. sensor may be bad, staying "cold', dumping too much fuel after the motor is warm. Should set off a code but OBD1 is not perfect... MAF sensor can also be fruity--it can be erratic like that.

 

Lev, I check the resistance and voltage from the temp sensor to the ECU and it tests good through its temperature range. I guess it could be the MAF. I'll try a backup to test.

Thanks, Jim

 

You have gone through pretty much everything... Also check the Power Stage, (Ignition amplifier)... I think it's on the inside fender wall half of foot from the driver side h/light. Don't know how to test it, I swap them...