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1992 Sc400 Idling Problems


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My 1992 Lexus sc400 started revving by itself about a week ago, it revs to about 1700 then drops to 1200 and keeps doing over and over. I also noticed when I'm driving it, it starts to pick up speed with out me pressing the accelerator. When I'm at 45 mph it feels likes it's on cruise control. When I pressed my cruise control button the light in the cluster didn't come on.

I've tried cleaning the IAC valve checked all the spark plugs and wires, cleaned my EGR valve and a new MAF sensor. There are no vacuum leaks, I checked every spot with carb cleaner and I'm completely out of ideas... Any suggestions!? I would appreciate it!

Oh and do I need to do anything special for a new mass air flow sensor since it's an OBD1 or should it just plug in and work?

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  • 7 months later...


I posted this on another thread, but thought it may help you...

Hey all,

So I had a problem that may be the same as a number of you. Others sound like they may have a different problem.

Intermittently, usually just after lifting off the gas pedal, the car would die. It was also idling rough, and low, and sometimes moving between 500 and 1200 rpms regularly like once per second (very rough idle).

Giving it gas almost always would stop it form dying, but i would ahve to sit at a light with left foot on the brake and right foot trying to help the idle stay up.

A number of theories proved incorrect: Failed power steering pump (yes I have rock hard arms now), failures in the electical systems (fixed but no effect), and others.

Then I thought about the fact that I almost always get gas at the same place. One day I went somewhere else, a more expensive gas station, and threw in an extra container of fuel injector cleaner and engine cleaner stuff that is supposedly in all these gasolines anyway. Think Chevron Techron, Shell V power, etc. Anyway, the problem hasn't recurred since that fillup and since I stopped going to the chevron by my house.

We all know, I assume, that the engine needs high octane (premium gas). In fact it calls for 93 where I can usually only find 91 and that's in San Francisco, not Santa Fe. I am suspecting that that chevron gas either was not the rating it claimed, or did not contain the techron. I had been going there for years. probably 19 in 20 filllups happened there. The car has 200k miles on it, and I think giving it lower quality gasoline especially without the cleaning additives doesn't manifest immediately, but slowly, and if the octane rating is a little low, it may just not give the engine the power it needs and probably confuses the idle computer, etc. And obviously if the detergents and other additives aren't in the gas, it's going to muck up slowly.

So my suggestion: Fill it with one of those Lucas fuel injector cleaner bottles and some nice premium gas from a nice gas station. I bet it stops happening.

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