High Idle

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.

 

The actual IAC could be bad. Another thing that causes this is the coolant
temp sensor. The one that mounts in the driver's side of the cyl. head. On
mine it was the IAC BTW. Before you replace anything though try one quick
thing. with the engine idling create a vacum leak (pull off a hose). Than
seal the leak with your finger than reopen than plug the hose back on. Than
see if the engine behaves itself.
Larry

 

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.

 

I don't know what the air temp sensor even does

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.

 

<snip>
What State are you in? Isn't this normal when it is cold out? Mine does this
all winter long, but only when it is cold. When I start it warm, there is no
problem.

Richard