My 94 gmc k1500 starts and runs good until it
Being your truck is a 1994, that means it's the OBD I operating or computer system. Now, if you know of someone that has a scanner that can read OBD I that would be great. What you are looking for is a scanner which can read live data as the engine is running, NOT simply a code reader.
You didn't say that the check engine or service engine light was on, but what I suspect is wrong won't "throw a code" so it doesn't matter if the light is on or not.
I highly suspect the coolant temperature sensor, NOT the coolant temperature sender, is bad.
I ran across 2 vehicles back to back that had the same basic symptoms you have described. One was an OBD I system, the other was OBD II. Both vehicles showed during a 'live scan' that the coolant temperature sensor was telling the computer the engine was -40°!!! Yup, MINUS 40°. This caused the engine to be getting fuel dumped into it like it was 40° below 0, even after the engine was warmed up.
The coolant temperature sensor tells the computer that the engine is cold, getting warm, or is completely warmed up.
As the 'puter is getting this info, it adjusts the air/fuel ratio, timing, etc. to make the engine run the most efficient.
The computer can only go by the info being fed by the sensors, oxygen, coolant temp. manifold air pressure, throttle position, etc.
If any of these sensors go bad and start 'lying' to the computer, the 'puter don't know the difference and adjusts according to the info received. Some of the sensors, when bad, WILL throw a code.
The coolant temperature sensor will NOT throw a code but it will cause a poor running vehicle.
Change the sensor. It should cost less than $10.
Hope this helps.
