My 2000 Forester hesitates and then backfires periodically. The problem seems to worsen as the gas level declines. THERE ARE NO CODES being generated.

 

I have an appointment with local dealer as my regular mechanic has given up.

 

There is 71K on the car. The fuel cap, fuel filter and air filter have been replaced but have had no effect on the problem. I tried various "fuel system cleaners" and that hasn't helped either.

 

I suggested to the service adviser about updating the ECU but he poo pooed that idea. The service adviser is already talking about 2 hrs of diagnostic time which he said would be about $200.

 

Does anyone have any suggestions?

 

Thanks in advance for your help.

 

~Howard

whenever this happens, you have to start with a tuneup.

This rules out plugs and wires and clogged filters.

 

Backfire could be a vac leak, a extra lean condition, leaking valve, leaking injector.

 

 

nipper

Ka-bang out the tail-pipe, or ka-thump out the intake?

 

Dave

Ka-bang out the tail-pipe, or ka-thump out the intake?

I believe we're talking afterfire (cat/muffler), not backfire (intake). There was a previous thread that should be referenced: http://www.ultimatesubaru.org/forum/showthread.php?t=98943

 

With respect to that thread, depending on exact conditions, the EVAP system can produce a slight vacuum in the fuel tank when the engine is running. However, if the engine is shut down (as for refueling) there shouldn't be a noticable vacuum when the gas cap is removed. Cougar gave some hints to diagnosing that condition in the other thread, and it makes sense to follow up on it. It's certainly possible for an EVAP/vent problem to develop that hasn't (as yet?) triggered a code.

My high mileage Subaru backfired on me once, and the problem was the coil pact.

 

A few years latter I had a cly. misfire code which also turned out to be the coil pact, but the second time they was no backfire. The coil pact is very easy to check with a meter and Haynes manual.

My high mileage Subaru backfired on me once, and the problem was the coil pact.

 

A few years latter I had a cly. misfire code which also turned out to be the coil pact, but the second time they was no backfire. The coil pact is very easy to check with a meter and Haynes manual.

 

 

Coil pack and plug wires were replaced last July.

 

~Howard

I took the Forester back to a local dealer. It seems I had 2 loose plugs and 4 fouled with oil. Plus one of the O2 sensors was bad (out of range error).

 

The plugs were replaced and the O2 sensor was replaced under warranty. The O2 sensors were replaced along with the cat back in Nov 2008. The parts warranty for Subaru is 1 year.

 

All is now OK and the Forester is running like new.

 

~Howard

All is now OK and the Forester is running like new.

 

~Howard

 

 

Congrats! Here's to years of happy driving for you. And thanks for letting us know how it resolved.

Thanks for the update Howard and glad you got it fixed. It seems kind of strange these problems weren't caught earlier.

Thanks for the update Howard and glad you got it fixed. It seems kind of strange these problems weren't caught earlier.

 

COUGAR:

 

I don't understand why the problem with the O2 sensor did not generate a code. I can't figure out how 2 plugs were loose.

But the car runs like new so I'm glad everything has been resolved.

 

~Howard

I would guess the plugs were not tightened enough when they were worked on last. As for the O2 sensor they can degrade over time and that will not cause a code to be set.

Lazy O2 snesors wont always throw codes, they are working just slow to respond.

 

Loose plugs, you are so lucky they were caught. usually a loose plug sint found untill a loud PING followed by a loss of power is discovered. The plugs come flying out and take the threads with them.

 

 

It is rare but it happens from time to time.

 

 

nipper