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1999 Impreza, 2.2 flat spot


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Wanted to throw this out quickly.  Got a chance at this vehicle for $500.  He has had a bunch of sensors replaced but cant get rid of the flat spot just above acceleration.  The only thing that fixes it is if the battery is disconnected.  Its fine then....til it warms up.

 

Anyone want to shoot from the hip on this?

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Could be a number of things but the best way to find out is to have a mechanic run an analyzer and look at the readings and if you can fix the problem yourself great. Generally, flat spots come from bad tune-ups, leaking vacum lines, dirty maf sensors, dirty throttle plate, ect ect. Best thing to do is just take it to a reputable shop and get it fixed properly it will cost less in the long run. 500 for that car even with the throttle issues is a steal.

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I had a throttle position sensor doing this to me. I found it by checking the resistance of the sensor while opening the throttle. It's just a potentiometer, they can corrode or wear out the same as volume knobs on a radio. It does the same thing to the computer that you hear when you change the volume on a worn out radio. Don't just go throw parts at it, test the parts before you replace them.

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Is the flat spot just flat or somewhat surge-y? My 98 Impreza had a bad knock sensor that wasn't bad enough to throw a code but still affected performance. Cheap and easy to replace if it hasn't been covered already. I think surging is the hallmark of a knock sensor issue though. 

The other advice here about general tune-up things is always a good idea though. 

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