So my '88 IROC isn't passing smog. Its high on HC's by 10 ppm's. I hooked it up to the scanner today and it measured 246 degrees after about 2-3mins of running......not likely. The temp gauge on the dash read about 120. I'm thinking the temp gauge on the dash is right but the ECM one is wrong. To test my theory I unplugged the temp sensor by the manifold and checked the scanner again with it unplugged. This time it read 113 degrees, how is that possible! With the sensor unplugged its an open circuit, it should read zero. Also i'm told by a friend of mine that if the ECM sees that the engine is too hot the ECM dump fuel in it. The short term fuel trim (intergrator) was switching from 128 to about 131 then back down. I'm thinking thats why its not passing smog? Now the problem is figuring out why its doing that whick brings me back to the ECT. Any ideas, tips, hints? Would a bad ECT be it? How in the hell is it possible to read 113 degrees when you have the sensor unplugged!! Thanks for any help you guys may have.

 

Here is a couple diagnostic charts for the coolant temp sensor. They should get you going in the right direction. From what you stated, sounds like a wiring problem or a bad ecm. Hope these help you out.

 

when u unplug the sensor it should read close to 0 and when you ground the 2 plugs together u should get a readin of 300 or above if im not mistaken u could have a bad wire somewhere

 

Thanks alot for the flow charts. They say that it could be a short to ground if the voltage was 268 for at least 3 secs. First off the voltage didn't go up to 268 degrees. Next when I disconnected the sensor, the voltage changed. If it was shorted to ground i don't think the voltage would change because the short would be b4 the ECM obviously. What you guys think?