I bought a used 99 Forester S and the check engine light has been on and off (mostly on) for over a year..it always reads the same code P0303 misfire on cyllinder 3. The light always flashes before staying on indicating it is detecting a misfire. I usually clear the light by disconecting the negative terminal. I have had the dealer where i bought it (not a subaru dealer) and the local subaru dealership try to deal with it..I have had 2 new sets of plugs,and wires, 2 new coils.. but the problem is still there..it seems to run fine, plenty of acceleration and very smooth running, except when idlling, you can occasionally hear the put put of a misfire from the tail pipe.. any ideas anyone...

Sounds like it may be injector or intake valve deposit related. Have you run any injector cleaner through it? If there is a clogged injector or really bad valve deposits it may take an aggressive regimen of cleaners or a professional cleaning to improve things. I suppose you could move the #3 injector to another cylinder to see if the miss follows. Make sure the connector is secure and clean on the #3 injector. Has the fuel filter been changed recently?

thanks for the input..I will try some injector cleaner and see what happens....do you think a compression test on #3 might be wise??

Sounds like it may be injector or intake valve deposit related. Have you run any injector cleaner through it? If there is a clogged injector or really bad valve deposits it may take an aggressive regimen of cleaners or a professional cleaning to improve things. I suppose you could move the #3 injector to another cylinder to see if the miss follows. Make sure the connector is secure and clean on the #3 injector. Has the fuel filter been changed recently?

A compression test is not complete unless all four cylinders are checked. You need to know the individual compression and comparison between cylinders as well as checking for a blown head gasket between cylinders (not that I suspect that a head gasket is your problem).

Let us know the results of the comression test. I'm having similar trouble on a 2000 OBW, and the compression is low in the offending cylinder. I'm hoping for a stuck valve myself.

Update:....had the compression test done today..180 on # 1,2,4 120 on #3, thank god for extended warranties..they are replacing the engine...

Let us know the results of the comression test. I'm having similar trouble on a 2000 OBW, and the compression is low in the offending cylinder. I'm hoping for a stuck valve myself.

New update...haven't got the new engine yet...the dealer (not a subaru) is tearing the engine down next week (thurs) to determine exactly what is wrong in # 3 cyllander...any ideas out there....the dealer seems to think that a new engine is what might be needed but can they just do the one cyllander and if they do should i consider getting the others done at the same time although compression was good on 1,2,4 180 and only 120 on 3. I have all the service records of oil changes etc. so i don't think the warranty company can deny coverage but if they do any ideas of minumum to max cost?? Ps.. just goes to show what a great engine it is...except for hearing the engine missing when idling it has tremendous power and acceleration at all times...would doing nothing also be an option...already had the cat converter replaced last year (under warranty)..thanks

The misfire could have been caused by the valve lash adjuster being to tight, which in turn may have burnt the valve .Causing the low compression

 

Running the car for a year with a misfire probably didn't help and would have an negative effect on the catalytic converter as well

 

If the insurance company is going to pay for the one side only (which they may)I would suggest you do both sides (valve grind , head gaskets ,ect)

 

 

SEA#3

I was using the search engine using P0303 and came across some interesting posts.. the one on misfire 1 +3, buddythedog suggested to check for a faulty exhaust valve using the dollar bill test (unfortunately here in canada bills start in the $5 amount)..anyway i tried it an lo and behold the bill was alternatly sucked in then blown out of the tailpipe...so when i take it in next thursday for the tear down i will tell the mechanic about the test and ask him to check for a defective exhaust valve..will keep everyone posted..

Update: Just heard from the mechanic..burnt exhaust valve in # 3 cyllander..looks like that bill test at the tailpipe actually works...they are doing that side under the extended warranty and since they have it torn town I will get them to do the other side although i will be picking up the tab for that.Would it be a good idea to get the clutch checked/done at the same time since they will have access to it???? thanks

I was using the search engine using P0303 and came across some interesting posts.. the one on misfire 1 +3, buddythedog suggested to check for a faulty exhaust valve using the dollar bill test (unfortunately here in canada bills start in the $5 amount)..anyway i tried it an lo and behold the bill was alternatly sucked in then blown out of the tailpipe...so when i take it in next thursday for the tear down i will tell the mechanic about the test and ask him to check for a defective exhaust valve..will keep everyone posted..

On my 99 Forester I had the clutch upgraded, the oil slinger replaced, the tensioner, water pump and all the seals done while it was apart.

 

My hope is not to take it apart again for 90,000 miles.

My 99 Forester had the same check engine code problem. Changed plugs, filters, EGR valve, injector cleaner, Sea Foam and even did compressor check on all cylinders which were all OK. We even switched injectors from cylinder 3 to cylinder 1 and code kept throwing cylinder 3 misfire. At this point, I suspected electrical problem. In a dark garage w/ wife reving engine I could see light blue sparks coming from wires. Replace all wires and problem has gone away.

 

 

Good luck to those chasing similiar problems.