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Your Auto Inspector
Your Auto Inspector, ASE Certified Manager
Category: Saturn
Satisfied Customers: 874
Experience:  ASE Certified Parts and Service
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1999 Saturn: the throttle position sensor..idles high..idle down..Rad

Customer Question

I changed the throttle position sensor on my 1999 Saturn SC1. It still intermittingly idles high on start up and when driving long distances it will not idle down properly. Rad fan doesn't seem to work properley either. I don't hear it come on. What's the deal?
Submitted: 7 years ago.
Category: Saturn
Expert:  Your Auto Inspector replied 7 years ago.

Thanks for using Just Answer. I will be glad to assist you.


The idle on your engine is controlled by the idle air control valve, not the throttle position sensor. It sounds like the idle air control valve is failing.


Here is some information about the idle air control valve:




The PCM controls engine idle speed by controlling current to two stepper coils contained within the idle air control valve assembly.

The PCM controls a two-coil stepper motor, which controls the bypass air intake into the throttle body, to decrease or increase engine idle speed. The motor allows air to bypass the throttle plate and enter the intake manifold. The determination of how much air is allowed to enter the engine is based on current engine speed, A/C compressor operation (On/Off), coolant temperature, throttle position or if the transaxle controller requests greater engine torque during power-off downshifts (automatic trans axle vehicles only). The PCM/TC will also use the Idle Air Control (IAC) Motor to control coast down shifts.

The purpose of the idle air control system is to control engine idle speed, while compensating for changes in engine load.

The idle air control (IAC) valve, mounted in the throttle body, controls bypass air around the throttle plate. By moving a conical valve, called a pintle, in towards the seat will decrease airflow. If the pintle is moved outwards from the seat a controlled amount of air is allowed to bypass the throttle plate. If the PCM determines that rpm is too low, more air is allowed to bypass the throttle to increase rpm. If rpm is too high, less air is allowed to bypass the throttle plate decreasing rpm.

During idle, the proper position of the IAC valve is calculated by the PCM, based on battery voltage, coolant temperature, engine load and engine rpm. If the rpm drops below specification and the throttle valve is closed, the PCM moves the IAC valve to control speed.

Once the PCM has sought a desired idle it can retard or advance spark timing to smooth the idle out in slight changes. This operation keeps the IAC valve from continuously moving in and out. The PCM will learn the IAC position for both A/C and non-A/C operations. These values can be displayed using a Scan tool.

An unstable TP sensor at idle, varying more than four counts (TP counts read on Scan tool), will disrupt closed loop engine idle control.

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Customer: replied 7 years ago.
I just bought this car the other day. It was an as is you know what that means. Pay your money and take your chances. I had a suspicion it was a simple fix. To the experts of course. It was worth the 11 dollars. I'm so greatfull I stumbled on your web site. I'll get a IAC tomorrow and put it in. Thanks a TON man. Regards
Expert:  Your Auto Inspector replied 7 years ago.

Not a problem at all.


The idle air control valves are pretty common on those.


Please let me know if there is anything else I can do to assist you.

Customer: replied 7 years ago.
Just wanted to let you know that I took your advice and purchased an IAC valve today and installed it. The car works great! Thank you for your saved me a chuck of change. It was Sooooooooo........worth the 11bucks. I'll be definitly back on this web site if I need your assistance again! Best Regards XXXXX XXXXX Ontario. Canada
Expert:  Your Auto Inspector replied 7 years ago.

Youre very welcome.


Please let me know if there is anything else I can do to help you.


In the future please feel free to request me to answer any questions you have.


Thanks again,


Your Auto Inspector

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