backfire

[jp1gt]

My 2001 forester 2.5 non turbo with manual transmission backfires when shifting from one gear to another. It does not do it often and is just a putt, not a big backfire. It does it on flat land, up hill and down hill. I am worried it will knock the inside of a cat loose. Any ideas?

Edited May 12, 2010 by jp1gt

[Fairtax4me]

You might have an exhaust leak. Fresh air getting into the exhaust system will create a lean enough condition for any left over fuel to ignite spontaneously.

 

Have you done any maintenance to the car recently?

[jp1gt]

No I have not done any maintenance other than oil changes etc. There are no exhaust leaks. It seems to me that the backfire is occurring under the car rather than out the tailpipe. Another weird thing happened in the past. The check engine light was coming on about every 500 miles. It said that there was a problem with one of the cats. I would use the scanner to shut off the light every 500 mi. What is even more weird is that it quit setting the code after I got a smog check( I live in CA) Before I got the smog check I bought a used exhaust system with the cats and O2 sensors, but have not put them on. Unfortunately I don't have the code written down. The check engine light and the slight backfire did not seem to be connected, just throwing it out there in case it helps.

Edited May 12, 2010 by jp1gt

[nickb21]

My '02 OBW has done this for as long as I can remember, just as you described. Usually in the 2-3 or 3-4 upshift I think. I assumed it was just unburnt fuel somewhere.. but why? I didn't have any cat/exhaust/HG problems until more recently (100k+).

 

Interested if you ever figure anything out.

[nipper]

No I have not done any maintenance other than oil changes etc. There are no exhaust leaks. It seems to me that the backfire is occurring under the car rather than out the tailpipe. Another weird thing happened in the past. The check engine light was coming on about every 500 miles. It said that there was a problem with one of the cats. I would use the scanner to shut off the light every 500 mi. What is even more weird is that it quit setting the code after I got a smog check( I live in CA) Before I got the smog check I bought a used exhaust system with the cats and O2 sensors, but have not put them on. Unfortunately I don't have the code written down. The check engine light and the slight backfire did not seem to be connected, just throwing it out there in case it helps.

 

 

Wooo slow down.

 

I see many things wrong with this, first how many miles on this car.

 

Have you EVER done a tuneup on this car?

plugs

wires

PCV vlaves

Fuel Filter

Ait Filter.

 

Why did you buy a used exhaust system and Cats. it sounde to me, since you passed CA emissions, you do not need cats, you most likely need a new O2 sensor.

[jp1gt]

It has about 85K on the car, low mileage. It has been tuned up 3 times including adjusting the valves once. I bought the used exhaust system before I got the smog check thinking that I would solve the mysterious random check engine light problem. I called the dealer and they said that the code that my car had would be caused by the converter or the O2 sensor, so if it comes on again, I have both. nickb21 you seem to have the same problem. I hope some else has had the same problem and found a fix. The situation for me is that the problem is not that bad, just annoying. WELL, I CALLED THE DEALER TODAY ON THE BACKFIRE PROBLEM AND HE SAID THAT THEY HAVE HAD SOME CRACKED Y PIPES JUST BEFORE THE CONVERTERS (UNDER THE HEAT SHIELD) When I get some time I will take the shield off and take a look.---Jack

Edited May 12, 2010 by jp1gt

[nipper]

no need to shout.

 

Exhaust leaks can be of two types. One that opens up when the car gets warm, or closes up when the car gets warm.

 

Ahev you noticed if the light comes on while the car is warming up, when it is cold, or after it is warmed?

 

Or just havent noticed (not a biggy).

[jp1gt]

Like I said, after I got the smog check, the check engine light never came on again. Really weird!!!! I live in a small town and the mechanic at the local Subaru dealer will talk to me. When I asked him about the light he said that they were replacing cats and Subaru said 80% were still good. So, if you were going to fix the problem, it gets a little complicated. You have 2 cats and 4- O2 sensors. Which one is causing the problem? Luckily it has not come on in 4K miles! I will post a note if I have a cracked Y pipe.---Jack

[Fairtax4me]

I'm pretty sure you only have 2 O2 sensors, but regardless, just because you can't hear it doesn't mean there might not be a small leak in the exhaust.

 

The pop that you hear under the car may even be coming from the area of the leak if there is one. Have a friend cram a rag into the tail pipe while you listen around under the car for a possible leak. It will become very evident if there is a leak because all of the exhaust will try to come out of that one tiny hole.

 

Another option is that for some reason or other the engine is running rich and excessive fuel in the cylinders is not being burned completely. The unburned fuel combined with decreased pressure in the exhaust system when the throttle closes mixed with a bit of heat creates an expansion of the gases causing them to ignite. So you could also be looking at a fuel or air delivery problem.

 

But either way the little Poof backfires aren't generally harmful. If you get a shotgun bang out the tail pipe then you can start to worry. How long has it been doing this?

[jp1gt]

Now I have the WHOLE story. It was my wife's car before we got married. She bought it new. She remembers it doing it from the beginning. I asked her why she did not take it in to get fixed and she thought it was normal, since it is not a backfire, just a putt. I got involved after the warranty ran out. You kind of wonder, if it was doing it from new, is it the exhaust? I will still check it. How come my problems are never simple!!!!

[Fairtax4me]

Then it's probably perfectly normal and has no ill effect whatsoever.