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Questions and Answers

Mitsubishi Eclipse Idle Speed

Q. Hello, I have a 1991 Mitsubishi Eclipse, 2.0 liter DOHC non-turbo, 133,000 miles, 5-speed. I’ve owned this car for about a year and a half, and ever since I bought it, it’s done some odd things that haven’t really been a problem until recently. Usually when I turned the car on, it would idle a bit high, about 3,000 rpm, for a few seconds, then start rapidly speeding up and slowing down (between about 1,300 and 1,500 rpm, making a "VRoom... VRoom... VRoom..." sort of noise.

Mitsubishi Eclipse Idle Speed

As soon as the engine started to warm up, this slowed down and eventually stopped altogether, idling around 800 rpm. However, recently it’s gotten a bit worse. The engine doesn’t slow down at all once warmed up. The rapid cycling just continues constantly.

Also, whenever I give the gas the slightest tap the engine jumps up to about 3,000 rpm for a few seconds. Needless to say, this is a real pain in the ass when trying to drive slowly. Now, a friend of mine had a similar problem and said to replace the O2 sensor. However, the only thing replacing the sensor did was make the exhaust smell worse. And I have absolutely no idea what else to try.

Someone else suggested that the EGR valve might be to blame. Another said I should check for vacuum leaks. Another said the fuel pump might be the culprit. I really don’t know what to try, or how to go about fixing this. I can’t really afford to buy parts and keep switching things until I find out what’s broken, nor can I afford to pay a mechanic.

The only other relevant information I can give you is that the car seems to have a few exhaust leaks, and that it seems to burn through a lot of oil. But there’s no blue smoke coming from the exhaust. Do you have any suggestions as to what parts I should check for replacement, or what might actually be the trouble? Any help or advice would be greatly appreciated.

Thanks very much,
Tim

A. The first thing I would look at is the Idle Speed Control (ISC) Servo.

The Idle Speed Control (ISC) servo, located on the throttle chamber regulates and controls the idle for the loads placed on the engine.

The ISC servo is a stepper motor that regulates the amount of air taken in through the bypass circuit of the intake manifold by opening and closing the servo valve located in the air passage. The ECU will supply voltage in one polarity direction, which will turn the motor in a particular rotation.

Mitsubishi Eclipse Idle Speed

The ECU uses several sensor inputs to determine the operating condition of the engine and the loads being placed on it. The ECU adjusts the idle for air conditioning and automatic transmission loads and controls fast idle for cold start. The ECU uses the idle speed control (ISC) servo to make all of the adjustments to the idle.

With this many miles on it, it is probably gummed up. I would try cleaning the throttle body and the ISC Servo. That could very well cure the problem.

Mitsubishi Eclipse Idle Speed

Additional Information provided courtesy of ALLDATA

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