temp sensor/idle problem

I tried to search but couldn't find anything. The problem is the truck (90 K1500) surges when it idles and cuts off most of the time when idleing. Slight pressure on the gas it runs fine. If I unplug the temp sensor it will idle great, changed the sensor still want idle, unplugged it again and it idles fine. The haynes manual says that the senor goes straight to the ecm with nothing in between so does this mean my ecm is bad or could it be something else? (Just finished a complete rebuild of the engine.)

Thanks for looking hope someone can help

Get a scanner on it and see what the sensor is telling the pcm. Possible another issue is causing the problem and what you are doing is just a band aid for the symptoms, als check fuel pressure if the readings on the scanner are correct. Vacuum leaks at the TB base are common also.

When you diconnect the CTS, the ECM reads -30 degrees. The ECM is tricked into thinking the engine is cold and will richen the fuel mixture.
Check for low fuel pressure, vacum leaks, misadjusted timing and check the injector spray pattern

Okay I cleaned EGR valve, Put in a different IAC (not new) and rechecked timing. Timing was still good, the injector spray also looks good hard to see but both injectors look to be putting out the same amount and have good cone shape. Now the idle is high until the temp gets high. Its a little rough at cold but only for a min then it idles, but its a high idle. Then when the temp gets hot it stalls then want idle anymore.

And the scan is telling what info as suggested?

Try a coolant temp sensor from the gm dealer, I put one on my '94 from a local parts store and it didn't fix the problem, put on the one from the dealer and the problem was gone.

I don't have a scan tool nor do i know anyone that does. I think I'm going to buy a new IAC. I think both of the old ones I have may be bad.

Check the gasket under the TBI. If it is broken, it could cause a vacuum leak.

I would check some of the things the moderators said first before you do anything. Could save you some time and money.

Check your map sensor, had same problem change it problem fixed

Okay I finally got some fuel pressure readings. with the key in on it reads 0 with the engine running it reads 14. The hanes manual says that running should be 9-13 and that with the key on should be up to 5 higher than that, but dosen't give an amount. So now i have just as many questions as i did before I checked it. Do I have to install shutoff valves to ensure the injectors aren't leaking or can I just look at them and verifiy there not leaking? Is 14PSI to high do i need to change out the regulator? The guage is attached to a t fitting in place of the fuel filter if that makes any diff.

Given gage error I think you are ok here, it is a little high. Again as previously suggested have you checked the coolant temp sensor for proper readings when the engine is warm. If it is ok, you may think about replacing/overhauling the press reg in the throttle body.

thanks for the advice. I'm going to rebuild the TBI on the 15th. I was troubleshooting my fuel guage today and found that there is a short in the tank. So next weekend I'll try and fix that.

thanks for the advice. I'm going to rebuild the TBI on the 15th. I was troubleshooting my fuel guage today and found that there is a short in the tank. So next weekend I'll try and fix that.
I had the same problem and I had a short in the coolant sensor plug!

I just fixed a problem on my 90 K1500 with a surging idle and it turned out to be the vacuum hose between the MAP and TBI.

I finally found it when I used a vacuum pump on the hose and it would not hold. It turns out that the glue one of the elbows would get hot and not hold. I replaced the elbow with some vacuum hose and the truck runs great again.

The leak was causing the MAP reading to be off so the ECM would add fuel then it would run rich and so the cycle would continue.

Make sure the MAP can hold vacuum and the hose can also hold vacuum.

//2000CAYukon