i have an 86f150 with an 88 fuel injected 4.9l. my truck recently developed a high idle (around 1450 rpm). but sometimes it idles perficlty. as it warms up it seems to get slightly higher. At times it also has a dead spot as soon as i touch the throttle. i have repaced the iac and the tps sensors already in hopes of fixing it. yesterday i scanned it and i got code 21 coolent temp sensor out of range. but my temp gauge works just fine. cold this be the problem?

 

The engine coolant temperature sensor is not the gauge sensor. Two different components. The ECT will have two wires, and the gauge will have one wire. The ECT acts kind of like an automatic choke works on a carbed engine. The computer checks the ECT for engine temperature at start up to see if more fuel is needed if the engine is cold, or less fuel if the engine temp is hot. Depending on how the sensor fails, it can cause hard starting when cold, erratic idle, etc., but once the engine is warm the engine starts ok. jd