SL2 sticky throttle problem

The throttle accelerator pedal on my '92 Saturn SL2 seemed to be
sticking so that when you stopped at a traffic light, the idle rpm might
be at 2-3K before slowly dropping back down to idle speed after 30 or 40
seconds. The pedal and cable linkage were fine. The 'Service Engine
Soon' light did not come on. Googling the problem suggested that the
the throttle body might be gummed so disassembled and cleaned it but the
problem was still the same. The throttle body had a little solenoid
valve and piston attached to it that opened and closed a little bypass
hole around the throttle body butterfly valve. Turns out that is the
'air idle control valve' or AICV and it adjusts to control the flow of
air through a little hole about 1/4" in diameter that provides the air
to the engine for idling when the throttle butterfly is closed. I
didn't think it could be bad because there was no fault code or SES
light but I decided to change it as there wasn't much else that could be
causing the problem. Et voila! The problem was solved.

What puzzles me is why there wasn't a fault code for the AICV function.
My guess is that the Saturn doesn't throw a fault code if the idle rpm
doesn't drop back down like it's supposed to perhaps because the
programming is set to maintain MINIMUM idle rpm and only issue a code
when the idle rpm is below that. At higher rpm, the throttle butterfly
is supposed to be open and the AICV closed but the computer has no way
of knowing that the AICV was failing to close. The computer should have
thrown a fault code when the throttle position was closed and the idle
speed was too fast but the car is a manual transmission so the rpms stay
up to correspond to the road speed even when you back off of the gas so
the computer didn't know the car was 'idling' rather that 'driving.'
The computers on newer OBD2 vehicles know what the vehicle speed is but
apparently the OBD1 Saturns don't report the vehicle speed to the
computer.

 

David, Thanks for posting this.

I have a very similar problem as I posted last month. (post copied below)

Mine is an OBD11 car with similar symptoms as yours. Shortly after my
posting the SES light came on, so I did a scan which returned a code
ISO9141, which nobody could identify??? On rescan I got a code PO147. I
cleared the PCM and SES light did not reappear. It was suggested to me that
I may have an IAC valve problem similar to yours and my reading and your
timely post seem to confirm that this is the area needing further inspection
and service.

Did you attempt to clean the IAC valve or just replace it?
Cost of new part?

The scan code ISO9141 was confusing but goggle returned
http://www.obd-codes.com/faq/obd-ii-protocols.php
I suspect this is important information when purchasing a code reader.

My error searches found - P0147=Heated Oxygen Sensor HO2S Heater Circuit
Malfunction (Bank 1 Sensor 3) M/T Only
I do not understand what this may indicate? The only O2 sensor I am aware
of is in the exhaust and it does not have a specific heater circuit. I had
suspected that this may indicate faulty CTS? but now think it more likely
that a malfunctioning IAC valve may be causing a false reading from the O2
sensor. Why would this be 'M/T Only'?

I will admit that I have been procrastinating the service somewhat as the
car is still very usable and I would prefer warmer weather for any repair,
and it is not uncommon for many problems to disappear in the spring.

I think that this is a good example of how a little research and DIY
mechanicing can eliminate the reliance on dealer service and save a lot of
$$ and aggravation and disappointment. It does require a bit of learning
effort but neither the work or the systems are all that complicated and IMHO
learning is a big part of life. I am grateful for this NG and all who
contribute here.
Happy trails to all.

Bob, Thanks for this suggestion, I had forgotten that replacing the CTS
seems to solve most similar problems reported here.

 

I just replaced it. I got one at Car Quest for $50. It took about 30
minutes to remove the throttle body, replace the valve, and reinstall
it. Youtube has a good video on removing the throttle body on the
Saturn S series.

It's always possible that there is more than one malfunction. The
overspeed 'sticky throttle' idle, though, is a dead giveaway for the IAC
valve if the throttle body butterfly valve is clean and working properly
since the only other way for air to get into the intake is through the
IAC port (assuming that there aren't any obvious vacuum leaks).

 

IAC & throttle body cleaned. (New valve price 80-105 C$) Problem solved,
total cost <10$ for carb cleaner and gasket.

Thanks for help and advice.