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I tested the thermostat by putting it in boiling water, it opened properly. I then put the thermostat back in the car. I cleaned up the connection at the temp sending unit. I also removed the toggle switch and put the wiring back to stock on the sensor that is on the thermostat housing. STILL NO CHANGE. The gauge reads operating temp within 1 minute of initial start. The car drives normally and when cruising down the road at 40 mph the temp guage will read as normal. As soon as i start to slow down the gauge will shoot up to "H" within 30 seconds of idle. I am out of ideas to try. My only two guesses would be that there is a clog somewhere preventing coolant to circulate properly, or a short in the wiring somewhere for the temp gauge. The needle works and the sending unit is brand new, so that only leaves the wiring for the gauge.

Also both radiatior hoses get hot so i know for a fact that coolant is circulating. And the heat works great so i doubt there is an air pocket somewhere.

Nate

The conditions and sequence you describe of overheating within 30 seconds of idle points to a likely malfunction with the radiator cooling fan not coming on when it should. At roadspeed, all is good because your forward motion forces cool air thru the rad fins but at a stop, the fan turns on to pull cool air thru the rad fins (or should). IIRC you posted that the previous owner had the fan on a bypassed manual overide switch (weird?) and you put it back to stock wiring. Have you since started the car, let it warm up, and observed whether the fan is coming on when the car is idling a while and the gauge is showing increasing coolant temp?

The fan is not coming on at idle, im assuming why the previous owner wired the fan to a toggle switch. However, i feel the problem lies deeper than the fan not coming on. I believe there to be a problem with the gauge because it reads operating temp within 1 minute of initial start in sub freezing weather.

Nate

You say that your RS is a project car. Does that mean that you've done an engine swap. I know that most people who have swapped in the BP have this issue. They rectify the problem by using the thermosensors from the Mx-3.

Also, you say that you replaced one sensor. Not sure about the SOHC B6, but in the DOHC B6 there are at least two coolant temp sensors.

Its still the stock 1.6 sohc in there. I replaced the coolant temp sending unit. I guess the coolant temp sensor on the thermostat housing is next.

Nate

oh, I figured you replaced the sensor. That might be the issue. Did you test your current one? A resistance of around .40 will be the ticket, if you decide to test it.

but who cares? they're like a couple bucks or so at the store.

I'm actually having an overheating problem as well. I thought I'd start a new post but I decided might as well throw it in here.

It's pretty much doing the exact thing his cars doing minus a few quirks. First, the idle, just flipped out on me this morning. It would hit about 2X RPMs then rev up to 3X then rinse and repeat. I figured maybe the accelerator got stuck or something but it wasn't. I looked at the needle and it was overheating. I pulled off and turned it off for awhile.
When I turned her back on, she was at normal running temp, and was idling at about 1X RPMS. Got her down the street before it over heated again.

The kicker was, I waited about 5 or so minutes after that, started her up and took her in 2nd gear, moving pretty slowly all the way back, the needle never went above running temp. At idle it didn't rev up to 3X RPMS again like it did that first time, but I didn't drive it for too long.

I'm not too great with cars but I can do general maintenance and so on. Any ideas where I should start?