Extremely High Idle on 93 SL2

I have a 1993 SL2 which I bought new and now has 217K miles on it. It
is now idling at very high speeds under some conditions. When the
engine is warm (or hot) and the vehicle is moving, the engine will race
up to 3000 RPM when the clutch pedal is depressed. When the vihicle
comes to a complete stop, the engine slows to about 900 RPM. I have
replaced the engine coolant temperature sensor (twice). I have tested
the current sensor with an ohmeter and it seems to be fuctioning
correctly. The connector is also new ($40 at the dealer for a $2
part!). I cleaned the throttle body and replaced the Idle Air
Controller. I have also tested the Throttle Position Sensor with a
voltmeter and it seems to be functioning correctly. If I drive until
the engine is warm, stop the car's motion, wait for the engine to slow
down, and remove the connector from the IAC, the engine does not race.
This leads me to believe the controller is telling the IAC to draw it's
pintle back. My conclusion is that I have a sensor problem. The fact
that disabling the IAC eliminates the problem would seem to rule out a
vacuum leak or a malfunctioning EGR valve. I may be wrong. (If I had
the answer, I wouldn't be asking!) What other sensors besides the
coolant sensor and throttle position sensor are used to determine idle
speed. (Since the idle speed drops when the vehicle stops moving, I
suspect the Vehicle Speed Sensor is working correctly). Thanks for
your help. Terry

 

You've done all this work and replaced a bunch of stuff, but have you read
the codes from the PCM to see if it indicates what is wrong? Autozone will
do this for free, or you can do it yourself with a paperclip with the
instructions here:

http://www.differentracing.com/tech_articles/index.html
Refer to the OBD1 diagnostic code section.

I would suspect your IAC valve because it controls the idle. But read those
codes first to see if the PCM indicates anything else.

Lane [ lane (at) evilplastic.com ]

 

It sounds like you know more than the average person that comes through
here but just to make sure you do know your 93 has "two" ECT sensors
right? One for the dash gage and one for the ECM. The one to the ECM
would be the important one.

The speed sensors wont affect idle. Have you cleaned/checked the EGR
valve?

 

I did the paperclip trick. The PCM had no codes except the all clear
one, except the time I forgot to put the connector on the Coolent
Temperature Sensor. I pulled the EGR valve and cleaned it. It doesn't
close all the way - there is still an opening even when there is no
vacuum. Is this normal, or should it close all the way? Thanks!
Terry

I removed and cleaned the EGR valve.

 

In my previous reply, I forgot to respond to the part about 2 ECTs.
The temp gage in the instrument cluster has been working correctly (at
least it appears to) all along. The ECT I replaced was the one with 2
leads on it. I haven't messed with the single lead one above it.
Thanks again. Terry

 

I caved in and took the car to my mechanic. He has been able to solve
every problem on every car I have taken to him - until now. He saw no
codes to indicate why the idle is so high (up to 3000 RPM with clutch
pedal depressed). All the sensors appear to be functioning correctly.
The IAC reports being where it is commanded to be. He tells me that
given enough time at ($1 per minute) he can find and fix the problem.
His inclination is a problem in the Powertrain Control Module. Any
thought on this. Also, if this is the route to go, how do I get the
right module. I see some on Ebay where the seller says the numbers
must match. Is this true or do the two cars just have to have the same
engine, transmission, and perhaps some other items in common (for
example ABS vs no ABS). Since this is a rather long thread, here is
what has been done so far - 1) IAC replaced; 2) TPS checked and found
good; 3) ECTS and connector replaced; 4) O2 sensor checked and found
good; 5) throttle body removed and thorougly cleaned with throttle body
cleaner and tooth brush; 6) PCV valve and hose replaced (hose was
brittle); 7) Powertrain control module removed and replaced (having
been in the computer industry, I have seen a lot of problems cured by
removing and reseating connectors); 8) Vehicle Speed Sensor checked and
found good; 9) EGR valve replaced. I am pulling my hair out. Any
ideas besides changing out the PCM? Is it worthwhile ohming out the
sensor leads? Last clue (for what it is worth) - if I warm the engine
up, bring the car to a stop and wait for the engine speed to drop to
800-900 RPM and remove the IAC connector, the idle speed stays low on a
test drive under conditions that would normally cause the problem
leading me to believe the high idle speed is due to the computer
commanding the IAC to open the pintle valve.

Thanks!

Terry

 

Given what has already been replaced, I have two additional thoughts on
possible causes of high idle: a defective Coolant Temperature Sensor (CTS)
or a possible vacuum hose or intake manifold leak.

Bob