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Ask Ted G. Your Own Question
Ted G.
Ted G., ASE Certified Technician
Category: Car
Satisfied Customers: 6153
Experience:  20 years auto repair experience, ASE Master Tech, Mechanical Failure consultant, Expert Witness
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2001 Forester: stop sign..shudder and hesitate..clunking..shifter)

Customer Question

My 2001 Forester has suddenly begun to run roughly. At low rpms and when in the lower gears especially (e.g. starting at a stop sign) it will shudder and hesitate. There is often clunking in the drivetrain associated with this (I can hear it under the vehicle between the front seats, and can feel it vibrating in the shifter) and it is worse when the engine is cold. If I increase the rpms (e.g. 3000+) and/or stomp on the gas, this 'lagging' will stop and often there is actually a noticeable surge (beyond the normal acceleration). Finally, sometimes while just idling, the rpms will drop and the vehicle will almost stall out. None of these problems are apparent at high rpms, such as during highway driving. This all happened pretty much over night, prior to that the car was running fine with no history of major issues. Knock sensor was done about 25K ago.
I used methyl hydrate to pick up any water in the fuel and have tried upgrading to higher octane gas with no effect. The mechanic has c
Submitted: 7 years ago via
Category: Car
Expert:  Ted G. replied 7 years ago.
On your engine there is a part called an idle air control valve. It is designed to open and close at idle on a signal from the computer to maintain a smooth and consistent idle speed. I suspect yours has failed. It is not an uncommon failure on older vehicles. graphic

Unfortunately it is relatively expensive on that vehicle, around $300 for the part alone.