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Registered
2001 Forester S 5MT
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10 Posts
Discussion Starter #1
hey all, I have a 2001 subaru forester that has recently (last month or so) seemed to have lost a lot of power. It is anemic accelerating, and has difficulty maintaining 70 in 5th on some (relatively modest) hills on the highway. I've noticed a corresponding gas mileage drop from high 20s (occasionally even 30) to low 20s on the past few tanks. Sometimes when I park the car after driving it hard (uphill, accelerating quickly, etc.), I notice some whitish smoke coming from under the hood.

Some background on existing issues: last summer I nailed the rear cat pretty hard on a rock while driving on a rough mountain road in CO. The cat itself is visibly dented. Shortly after, I got the dreaded P0420 CEL, presumably because there was either a small exhaust leak or the cat wasn't doing its job. Anyway, late last winter, the car developed a more pronounced exhaust leak (diagnosed by the engine noise dramatically increasing). however, with both of these issues, the drivability didn't seem much affected - the gas mileage remained relatively constant and the power seemed fine. It wasn't until this spring that I noticed a sharp decline in power.

I know the exhaust needs some work, i'll probably need to get a new cat and have some leak(s) welded/replaced. But given that the problem existed before the power loss, I'm wondering if anyone has other explanations. I haven't replaced the fuel/air filters or plugs recently, would any of these cause this sort of issue? I'm also worried about HG issues, given the age, but i've never seen it overheat, and although there might be a slight coolant loss over time, it certainly does not seem to be dramatic.

any ideas?

thanks!!!
 

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Registered
04 Forester X, MT
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1,039 Posts
I presume still no other codes other than P0420? Did you get it checked again (will do it free at many parts stores).

Going from winter to summer fuel can make maybe 3 less mpg if ethanol content goes up a lot. Just depends on exactly where you are.

You could consider replacing front O2 sensor even if no code, especially if over 100,000 miles old. But they can be hard to remove. You should replace it if you replace the cats and it's old.

There is a "cheap fix" way using spark plug anti-foulers to try and get rid of the P0420 code, see sticky on this forum. It may put off an expensive cat repair.

If you have exhaust leaks upstream of the front O2 sensor that could really effect things, since that is how the ECU determines A/F ratio.

I would go ahead and replace fuel filter, they are cheap.

Plugs would be unlikely to cause a sudden drop in performance but if they are really old they might. I believe interval is 30,000 miles.

If you had enough of a misfire or sensor issue (knock, crank, cam for example) you would likely also be getting a code.

As for any smoke, does it smell? You could be getting a little engine oil or CV grease on exhaust from leaks.

Tests can be done for exhaust gas in coolant or engine oil analysis for coolant in oil if you are worried about that. If HG leak was bad enough to cause the drop in power you should be noticing coolant disappearing or coolant in oil on dipstick (kinda like chocolate milk if bad).
 

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Registered
2001 Forester S 5MT
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10 Posts
Discussion Starter #3
thanks woody!
I've scanned the ECU recently, no other codes than the P0420. Does that mean that it's unlikely the plugs are causing this? they're definitely over 30,000 (maybe a lot over), so maybe it'd be a good idea to replace them regardless...

I've never noticed residue on the dipstick other than oil, and although the overflow reservoir isn't quite at the 'high level', it hasn't dropped much (I'd say ~1/3 of the way down when cold...)

I've got a little over 130,000 on the car, as far as I know the O2 sensors have never been replaced... sounds like that could be a likely culprit? or a possible exhaust leak ahead of the front O2 sensor?
 

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The Modfather
2019 Impreza 5dr Sport - Manual
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8,052 Posts
@ 130K I think it's a good idea to replace the front sensor. It can only help with MPG's as it is helps set the air fuel mixture. the 420 can easily be from a leak. Plugs and wires (OEM are good wire replacements and not all that costly) I bet need changing too. At the very least check the spark plug wires up at the coil pack. They may have gotten corroded, it's not uncommon. If they are shine them up with some emery cloth or similar.
 

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04 Forester X, MT
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1,039 Posts
I generally replace plug wires somewhere between 100 and 120,000 just to be doing. Cheap insurance and usually makes finding problems easier. I use OEM for plug wires too. I use a light coating of dielectric grease where it goes on the plug and also where it fits snug in the valve covers. Makes it easier to get them out when next replacing the spark plugs.

Anyone have any recommendation on the coil pack, if it is good P/M to replace at some point? I have seen it said more than once the Subarus tend to go a little earlier than others.

If the plugs are quite old you are likely to mess up 1 of the boots anyway. I usually try gently twisting the boot a little to "break" the bond with plug and/or valve cover before pulling to remove.

I really like the NGK standard plugs without all the platinum, iridium etc. crap. Advance has them. (Newer Subarus come with different plugs. Basically I would go with OEM design).


Also, you could check the compression and the timing. Compression test will help rule out mechanical problems. Timing check will show if OK at idle and if spark advances with throttle (basically responding normally). Come to think of it you could also look at mechanical timing to be sure camshafts and crankshaft are all still correctly aligned (i.e. at 130,000 miles, the timing belt and components were serviced, weren't they?)
 
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