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2001 Forester Automatic
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19 Posts
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Hey guys, I have recently had a drop in fuel millage. I am at 16mpg right now with 50/50 city/highway (measured in distance not time). I know this is not right because I am getting around 260-300km to a tank of gas...(174 miles roughly), and I am not sure whats going on.

recently I have changed out these parts...

air filter
spark plugs
spark plug wires
knock sensor
maybe a few things I am forgetting.

I looked back on the previous owners history, and they have has a lot of stuff done such as the head gasket, timing belt/chain, full brake job, front O2 sensor less than 2 years ago (I thought this could be the problem but its so new that I am not sure it is), and various other things.

I have also had a check engine light on for my cat for a long time with no decrease in fuel efficiency, so I don't think it is that either.

I almost think that one of my cylinders isn't firing. Is it okay to remove a spark plug wire at a time while the car is running to check?

Any advice is well appreciated. I plan on trying to find the problem, and if I cant I am taking it to the dealer to tell me whats wrong so I can fix it myself.

Thanks,
 

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1999 A/T - 235,000 mi. WA state
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890 Posts
I have also had a check engine light on for my cat for a long time with no decrease in fuel efficiency, so I don't think it is that either.

I almost think that one of my cylinders isn't firing. Is it okay to remove a spark plug wire at a time while the car is running to check?
I've had a CEL light on for most of the last 50k miles. In earlier years some CEL's were eliminated by using a damp cloth to wipe the gas cap seal clean.

It won't hurt the engine mechanically, but see if you can do it safely while it's running and the belts moving.
 

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2015 Forester 2.5i CVT
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936 Posts
...I have also had a check engine light on for my cat for a long time with no decrease in fuel efficiency, so I don't think it is that either.

I almost think that one of my cylinders isn't firing. Is it okay to remove a spark plug wire at a time while the car is running to check?
Letting engine run rich (also cylinder not firing properly) for a long time can significantly shorten the life of the catalytic converter, I'd get that fixed ASAP.

Could be, easy test, carefully pull one plug wire at a time while engines running. Just be careful of moving belts and fingers. The cylinder that doesn't change idle or very slight change is probably culprit. Bad coil pack, spark plug or wire are most likely root causes of a cylinder misfire.
 
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