Short in 5V wire to temp sensor???

[anthony464]

My 1988 S-10 2.8 TBI Blazer will turn over and fire fine when it is ice cold however it will run very rough at a very low idle for about 30 seconds (like the IAC isn't choking air flow at all) and then shoot up to about 3,000 RPM and stay there. The only way to get the engine to idle where it should (around 1,000 RPM) is to let it get warm, shut it off, and restart it.

There is no code from the ECM however when the engine is ice cold the scanner registers that the water temprature from the manifold sensor is in exces of 400 degrees F. I have replaced the Temprature sensor in the manifold and now the ECM with no avail. Whats next? A prom? or is there a short to the sensor. I'm getting about 3.5v on the yellow wire. And I have ruled out the IAC as I had an extra one and put that one in and still have the same problem.

[ricksza]

The yellow wire should read 5 volts. That's provided by the ECM. Have seen this before, could be a bad new ECM. Was it reading high temp with the old ECM? While you have the scanner hooked up, try giving the ECM a few slight taps with a screw driver handle and see if the sensor readings change. Otherwise, check the circuit for a short to ground or high resistance between the ECM and the Coolant Temp Sensor.

[BlazerLT]

Originally Posted by anthony464

My 1988 S-10 2.8 TBI Blazer will turn over and fire fine when it is ice cold however it will run very rough at a very low idle for about 30 seconds (like the IAC isn't choking air flow at all) and then shoot up to about 3,000 RPM and stay there. The only way to get the engine to idle where it should (around 1,000 RPM) is to let it get warm, shut it off, and restart it.

There is no code from the ECM however when the engine is ice cold the scanner registers that the water temprature from the manifold sensor is in exces of 400 degrees F. I have replaced the Temprature sensor in the manifold and now the ECM with no avail. Whats next? A prom? or is there a short to the sensor. I'm getting about 3.5v on the yellow wire. And I have ruled out the IAC as I had an extra one and put that one in and still have the same problem.

First note, there is two temperature senders, one for the computer and one for the temp gauge.

Which one did you replace?

Also remove, clean and adjust the IAC valve to 1.10 inches and clean it thoroughly and the hole it installs into and make sure there is no carbon.

[anthony464]

Thank you for the advice. I will check for high resistance between the ECM and Temp sensor. I believe it is a short somewhere at this point. I changed the temp sensor in the intake manifold as the sensor in the head is for the guage. The truck is doing the same thing with both ECM's, Both Temp sensors and both IAC valves (good thing I work for a part store because I would be broke trying to diagnose this thing) I will check for a short. Thanks again

[anthony464]

Since that sensor only has a yellow and a black wire where does it ground to? Does it ground to the manafold when the threads enguage? I ask because I used teflon tape on the threads, could that inhibit a ground?