Hi, new to forum, I have a 99 Ford Ranger with a 2.5l engine. When I start the truck in the morning I have a very high idle. When you put the truck in drive you would probably do around 10-15 mph without stepping on the gas. My main concern is that this causes the transmission to downshift hard when I come to a stop. Once the engine is warm everything is fine. I have changed the Idle Air Control Valve and cleaned the Mass Airflow Sensor but neither corrected the problem. Any ideas? Thanks

 

Could be a faulty Engine Coolant Temperature (ECT) sensor.

Test the ECT as follows:

Disconnect and test the resistance of the sensor as follows;

Cold (68° F) 35 - 40 k ohms
Warm (176° F) 3 - 4.5 k ohms

Test the supply voltage with the senseor disconnected 4.6 - 4.8 volts.

With the sensor connected test the voltage, wrt to battery -ve, on the brown/green lead. This may not be the correct colour code for your car, but one will probably be solid brown, it should be the other lead you need to test. The results should be about ( and this bit is from memory) as follows;

Cold 3.8 volts
Warm 1 volt.