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Idle Seems High


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#1 pwonder

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Posted 25 September 2007 - 07:43 PM

92 sc400, obviously when you first start a cool or cold car the idle is higher than normal opperating temps. After my SC is warm and i come to a stop it seems to be idling just under 1000. Anyone know what i can do or whats causing this???

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#2 JIBBBY

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Posted 25 September 2007 - 09:20 PM

If it does idle too high that probably means your engine heat sensor read is bad... The cars computer (ECU) still believes the motor is cold so it continues to idle the motor high...How is your gas mileage? Do you notice the car is using alot more gasoline?

When the SC400 warms up the ECU (computer) runs the motor very rich... Infact you can sometimes smell gas fumes coming out of the exhaust of these cars during the warm up process...If the car is running very rich with a high idle then your heat sensor or ECU is faulty maybe... It also could be your TPS (throttle position sensor)... Lastly try cleaning out the throttle body and change the air filter and see if that helps...

#3 smooth1

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Posted 27 September 2007 - 06:21 PM

If it does idle too high that probably means your engine heat sensor read is bad... The cars computer (ECU) still believes the motor is cold so it continues to idle the motor high...How is your gas mileage? Do you notice the car is using alot more gasoline?

When the SC400 warms up the ECU (computer) runs the motor very rich... Infact you can sometimes smell gas fumes coming out of the exhaust of these cars during the warm up process...If the car is running very rich with a high idle then your heat sensor or ECU is faulty maybe... It also could be your TPS (throttle position sensor)... Lastly try cleaning out the throttle body and change the air filter and see if that helps...

That my friends is as good a "forum" diagnostic as it just about can get. A quick ODB 2 scanner would tell right away as alot of them allow you to adjust the idle speed manually. But the causes are as Jibby has stated.

#4 JIBBBY

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Posted 28 September 2007 - 07:13 PM

Thank you Smoothe.... Messing with these cars for over ten years now you definetely learn a few things....

- A Bouncy idle is usually the result of a dirty throttle body or and a really dirty air filter..

- A low idle usually means you have a vacume leak...

- High idle could be several things as I mentioned above...

- If it starts and then immediately stalls out look for a seperation or break in the intake system...


If a check engine light flashes then you can get a positve read on a diagnostic computer check...No engine light then refer to above... That's the best advise I can give I would think...

#5 lexus082

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Posted 29 September 2007 - 06:35 PM

If you lift your foot off the brake and gas how fast do you coast in gear on a straight road? Make sure your throttle cable isn't adjusted too tight.

#6 lexus082

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Posted 29 September 2007 - 06:39 PM

I myself have a very low idle and have yet to figure it out what is causing it. So low the car begins to shudder/shake at stop lights. If I replace all the vacuum hoses is there still a chance I have a leak somewhere besides the hoses? Is there any way to test to see if I might have a vacuum leak. When I first start the car (cold) it idles were it should when warm/hot in park at around 700 rpms. It then drops even lower as it warms up. I hope its just a vacuum leak.

#7 smooth1

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Posted 29 September 2007 - 06:53 PM

If everything else seems fine, then you probably do just have a vacuume leak. But you may wanna clean the air intake out and make sure the filter is good too. The engine idle is set by the ECU. You might wanna check that setting to make sure it is set to the correct rpm.

#8 JIBBBY

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Posted 29 September 2007 - 08:56 PM

A break or air leak in the intake system and or snorkel tube from the MAF to the throttle body would also cause the idle to shutter or be poor.. Make sure all the components to your intake system is tight and intact and also clean from the air filter box all the way up to the top of the engine (TB)... Yes, you can change all the vacume lines and see it that helps...Vacume hoses are cheap and you can pick it up at any auto parts store in one long peice and then cut pieces to fit... However, Usually you can see if a vacume hose is bad and not sealed, it will be cracked at the ends or disconnected.. You can also spray soapy water over all the little hoses while the engine is running (carefully) and see if you get visible air bubbles.. Kinda like finding an air leak in your tire...

The idle is also influenced by the throttle position sensor (TPS) if that goes bad then that could be the culprit as well...

I think that covers it....

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