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Where is the temp sensor for the dash gauge? (archive)

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Posted by Jeff J. on March 01, 2001 at 21:44:26:

In Reply to: lunging car and erratic coolant needle posted by Jerry in D.C. on March 01, 2001 at 00:47:19:

Update on an earlier post: my 89 M3 w/110k miles was lurching really bad. Also, when I declutched the engine speed wouldn't come down below 2500 rpm, and sometimes it would even surge back up to 3000 rpm. It was like the idle had gotten set way too high. This was going on for a couple of days. During this same time period, the coolant temperature gauge needle was bouncing around the gauge like it was possessed. Applying Occam' razor, I figured the engine problems and the coolant needle problem were related. My theory was this: the engine managment system must monitor the coolant temperature because it would want to know if the car was in start mode or normal running mode. If the coolant is cold, then typically that means the car has just been started. Accordingly, the engine management system would increase the idle speed, the timing of the fuel injection, the richness of the fuel mixture, etc. Thus, if the coolant temperature sensor were malfunctioning, it would cause both the coolant temperature needle to flucuate wildly and the engine to act up. Nice and neat theory, right?

Problem #1: the engine problems mysteriously cleared up entirely, but the coolant needle still contiued to behave strangely--though not as bad as before. It quit bouncing around, but it tended to stay in the lower temperature range, even though I could tell from the heater that the engine was quite hot.

Problem #2: there are actually 2 coolant temperature sensors. One reports to the engine managment system, and the other reports to the gauge in the dash. As far as I can tell, they're completely independent of each other.

Jiggling the sensor that reports to the gauge has gotten the coolant needle to behave normally. Given that and given that the engine problems had already cleared up on their own, I don't currently have a problem. Still, I'm stumped. Any thoughts would be appreciated.

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