I have had a problem with cold running for some time now. The car is a 85 190E Euro (no cat.). Idle when started cold is about 1000 rpm (in Park), but if I leave the car idling, the idle speed will, after a minute or two, start dropping until it's about 700, and at this time its running quite rough. The idle will slowly rise as the engine warms up, and when reached operating temperature it idles smooth at about 1000 rpm. After previous post I was advised to replace the overvoltage relay, but it made no change.
This is also a problem when the car has been parked just so long that the engine temp has fallen to about 30-40°C. Then the idle will be very low (especially when in D or R) until driven a while. If the engine temp is lower than this, it will idle fine at about 1000 rmp (just as when started cold). I guess its at this temperature or lower the idle air valve sets in.
I am thinking about a faulty temp sensor, so I have tried to test it. First, to check if the circut was functioning, I removed the connectors with the engine at idle, and it almost stalled. Then I checked the resistanse at various temps by removing the two connectors and measuring the resistance between them. With cold engine (ambiente temperature about 15°C) it read about 8000 ohms, at 50°C about 3500 ohms, and with warm engine (80+°C) about 450 ohms. Anybody know if these readings are good or bad?
In addition, I always has to use the accelerator to keep it running the first seconds when starting, even when the engine is fully warm. It will start quickly, rev up, but then the revs will immediately falls down to about 500 rpm, and the car often stalls at this point.
Any help will be greatly appreciated!