Cold start system causing Rough Idle (archive)

Certainly, your cold start system is causing your delima. It starts and runs better early because not including the O2 in the motronic loop causes the engine to run rich. This helps at start because overriding the O2 sensor acts like the cold start system is operational. There are a couple of things this will hurt. #1 is the fuel economy. Not including the signal forn the O2 sensor gives you horrible fuel economy. #2 is that you will eventually clog your converter with carbon and soot rendering it useless. They are very expensive. It only takes a few weeks for this to happen.

My suggestion to you is to check the engine temp sensor for the proper resistance across the ternimals. Next check that the cold start relay is activating upon having a cold condition and finally if both of those items check ok would be to make sure the Cold start solenoid is activating. All three of these parts are inexpensive and are wear items on your car. Check them out before you have to spend $400 for a new converter. Good luck!

I have an 86' 535i and on occasion when I start it the idle is real low. The car almost stalls but catches itself and eventually stalls. It will not respond to the throttle when this happens. If I let it sit for a while and then start it again and play with the throttle it eventually smooths itself out and idles normally. The other day when the car was doing this low idle/stall thing I unplugged my oxygen sensor and it the idle returned to normal. I have been driving for a couple days with the oxygen sensor unplugged and the car has been starting and running fine. I was wondering if anyone knows why this happened and what I need to fix the problem. Thank You in advance.

Mauro

86' 535i

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