Subaru Forrester Runs Rough



My 02 subaru forrester runs rough under load. There is a CEL code P0447 which my mechanic says is related to the evaporative emissions system and has nothing to do with the engine problems. The dealer says that it needs a vent solenoid to the tune of $245.

My Mechanic can’t find anything eles wrong (coil, wires, plugs) and has replicated the symptoms with his computer plugged into my car.

The dealer assures me this will cure the problem. Should I go with the dealer recommendation?


The vent solenoid will do absolutey nothing for the rough running problem. If your mechanic is sure the ignition system (plugs, wires, etc.) is okay try unplugging the AFR (air fuel ratio) sensor and see if it runs better. The AFR sensor is screwed in the exaust pipe. Unplug the sensor upstream of the catalytic converter. If this helps, replace the sensor (not a bad idea to replace it anyway).


What does “can’t find anything wrong” really mean? Did they replace the iginition components or are they using the eyeball method?

Just my opinion, but any time an engine performance problem exists a compression test should be performed. One always weeds out a mechanical fault first because if this is not done then every single part on it can be replaced with no improvement.

If they had the spark plugs out (“can’t find anything wrong”) then a compression test should have been done at that time.

Just curious, but is the engine rough at idle only, at all speeds, any bucking and jerking, etc.?


good questions … the mechanic removed and cleaned all of the spark plugs, sprayed water around the coil and wires. The computer was not showing any misfires so he was reluctant to swapping out ignition parts.

Regarding the compression test … I don’t think he did that but I will confirm.

It runs rough at all speeds, especially under load (accelerating). There is noticable jerking.

Thanks for yor reply


These are symptoms of bucking and rough running are symptoms of a dirty fuel filter (among other causes). It is easy to change the fuel filter as it is in the engine compartment. Then, he needs to use a fuel pressure gauge to measure the fuel pressure AS THE ENGINE IS RUNNING, especially with the engine under load. An engine is loaded by running it at 2000 rpm with the transmission in DRIVE. Tell us the results of this test. Then, if need be, we can go to other tests.


Cleaning spark plugs is an utter waste of time and falls under the eyeball method. This method could work (sort of) back in the old days of leaded gasoline and carburetors but on modern injected cars is an exercise in futility.

My feeling is that a compression test is in order and all readings should be up in the 170 PSI or higher range with all cylinder readings being close to each other.

f there is a compression problem on a cylinder due to tight valve lash then you’re going to more than likely face an expensive repair; a valve job.
If this turns out to be the case the valves should be adjusted and compression readings taken again. This may help on the short term (depending on how many miles the car has gone while running poorly) but over the longer haul it’s going to be a problem.