I have a 97 SC2 and when I come to a stop the the engine wants to idle at around 2000 rpm, then it drops down to about 1500 for a short while eventually it will drop back down to normal. I just pulled the throttle body off and cleaned it, I took out the IAC valve and cleaned the carbon off the end of the pintle, reinstalled everything and the car still does the same thing.
Could I have a bad Throttle Position Sensor?
No codes by the way.
Good advice. I always look for a vacuum leak first and pinch hoses to check for any abnormal airflows while listening to the rpm. If RPM drops when the hose is pinched, that is a likely source. A full featured scan tool will do more than just read codes. On some of them, you can read the actual coolant temperature (or any real value from sensors) and compare this to the actual engine temperature. Somtimes this is a lot beter than just replacing sensos since it tests the entire signal run from the sensor to the computer.
Unfortunately, everything that I have seen in scan tools, short of the factory tools only does reads of the computers. They do not have the ablility to perform actuator tests. On another older vehicle, I had a diagnostic tool that could perform actuator tests like controlling valves and even the idle motor. I miss that ability on my lw300. A tech 2 tool would be nice but what shade tree mechanic can afford one of those? Oppie
I had this just recently - car would idle at ~~1500 or 2000. And you could tell the computer was trying to keep it there.
Coolant temp sensor.
On the older 4 bangers, you can swap the ATS and CTS cause they're the same. Quick and free. I don't know what the air temp sensor even does besides make the EPA happy...
The ATS (on most cars) is one sensor that will keep the ac from kicking on when its below 40deg to prevent you from destroying your compressor among doing other "good" things. This isnt 1978, emission control devices do little to rob power, if anything they help make it when they work correctly.
Induction air temperature sensing, along with throttle position sensor and manifold Absolute Pressure sensor are used to compute the Mass Air Flow of air entering the cylinder. This is a round-about way of not using a more accurate (and much more expensive) Mass Airflow Sensor. Ultimately this measures how much air is in the cylinder and then affects how much fuel to inject for a correct burn.
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