Cts and mat read 89* in the garage at 58*. The engine is very difficult to start.
After about 2 minutes the temperature is at 200*. Mat sensor reads over100*.
Ther is no way it can be that hot that quickly. The fan is set to come on at 200*. In the past I would have to be stuck in traffic in 85* weather for the fan to come on. Can it be that both sensors are garbage. Or is there a problem with the computer.

 

You could unplug the sensors and measure their resistance on a cold engine. My FSM says that at 70F it should be 3400 ohms, and at 40F it should be 7500 ohms. Its resistance goes down as it gets hot.

Secondly measure the voltage in the sensor plug using a volt meter, not a light. With the key on and the engine not running, you should measure 5V between the connector terminals.

EDIT: At about 60F I'm thinking you should measure something around 4500 ohms on a temp sensor. Both sensors reading the same indicates a problem outside of the sensors I believe.

 

 

" outside the sensors " meaning the wiring or the computer?
The harness is 1 year old and all sensors and computer are new.
Looks like its time to replace my smashed multimeter..

 

If this suddenly came up after a year, perhaps something is shorting against a manifold or something somewhere. Definitely check for the 5V on the connector. If the reference voltage is off, then the ECM will read it wrong. Low voltage would make for higher than actual readings.

 

check your grounds

 

Thanks guys..I'll start with the connector voltage and then the sensor.
If they check out then I will check the ECM ground ground wire.

 

Last night I had 5 free minutes, so I was able to check the voltage at the connector for the cts. It was at 4.08, key on motor not running. That is almost 20% lower then the 5 volts afremont mentioned.
Could that be a bad ground on the ecm? Battery voltage is good.

 

Appears within range
See
http://www.chevythunder.com/cts_diagnostic_pg2.htm

http://www.chevythunder.com/cts_diagnostics.htm

 

So if the voltage at the connector is good..Does that mean that the ground is probably OK?
Electrical is not my strength (I'm a carpenter). I have a spare 165, I think I'll give it a try this weekend.
Is it normal for it to be very noticable when it goes into closed loop?? The engine kind of shudders for a moment, almost like when you put the a/c in a vehicle.

 

I have a 1990 FSM and it specifically says to check for 5V on the connector. I would be extremely wary of a 20% diff. I'll check mine later and post the info. I expect the voltage to be well within 5% of 5V, but we shall see. The FSM also indicates that you should go by actual scan tool temp readings and to test the sensor itself by measuring the resistance at different temps and making sure it's within range. Ice water and boiling hot water make good reference temps to check the actual sensor.

A blown head gasket can make for some really interesting temp indicator action. Engine indicates roasting hot within one minute of starting or constantly climbing to a high temp and then plummeting down.

 

Engine has about 5000 miles on it...Runs great. Temp is stable according to the temp gauge, which was always in line with the cts until recently.
Mat sensor always gave a reading when cold which was about 20* too high. Now they are both too high.

 

Replaced the computer with the spare that I have..I'll have to wait until the morning for a cold start. The fan functioned perfectly..I'll try to get a voltage reading off of the cts connector tomorow. It looks like the advance auto computer did not even last a year...I hope that solves the cold start issue.

 

5 volts at the cts connector..running good.