backfiring

[dltrial]

There is one fairly steep and long hill that I have to drive up on the interstate each week. Lately my subarau (91 Loyale wagon, 5sp manual, 330,000 miles) has been backfiring once every time I drive up it. It started as a little hiccup and developed into a full backfire.

 

Monday night I drove up the hill and shifted down into 3rd, travelling at 55mph, to see if that helped it get up without backfiring. However on schedule it backfired loudly and then lost power. Even when I managed to get up the hill onto the flat, it still continued to make a series of little backfires and to lose power slowly.

 

The temp gauge was showing a little hotter than usual, and the oil pressure seemed a litle lower, so I pulled over and stopped, and added oil. The temp usually varies somwhat in the lower third of the normal range; this time it was up to nearly halfway up the normal range. It leaks enough that it keeps catching me out on the oil and when I checked it, it was lower than it should be but still on the dipstick; I have had it not showing on the dipstick and still not affect the oil pressure or seemingly affect how the vehicle runs.

 

I started it again. After starting it did not start up and run strong but hesitated so I gave it a little gas and it idled ok. Then I drove it home with no more problems.

 

I don't know if the oil pressure was just lower because it was loosing power and had nothing else to do with it. There is a section of flexible tubing covering a gap in the exhaust pipe and that tubing has developed a hole and the exhaust is leaking and getting loud enough for me to know about it just the last few days and I have no idea if the loss of exhaust back pressure might have anything to do with it, other than that one of the last backfires probably blew out a bit of rusty metal to cause the visible hole.

 

So the question: what could be causing this? Could it be as simple as a timing issue or is the engine on its last legs?

 

Derek T.

[Dee2]

backfiring is typically a timing issue.  I'd check that first.

[old sub freak]

Then do a compression test.That will tell the story.I have had had backfireing so bad i thought it was going to blow a headgasket or something!!! sounds like gunfire !!.Yea,    check your timing and then do a compression check. that is why I like subs so much,   they WILL take this kind of stuff and keep going.But Hey, keep the oil level up or all bets are off.lack of oil will ALWAYS ruin your engine...It took me just one to learn my lesson.I buy oil by the case..its so cheap compared to a new engine!!!

[NorthWet]

Might help to define what you mean by "backfire", as it is commonly used for 2 different events.  Is it making a loud noise in the exhaust system, or is the noise in the intake system?  

[dltrial]

Might help to define what you mean by "backfire", as it is commonly used for 2 different events.  Is it making a loud noise in the exhaust system, or is the noise in the intake system?  

 

 

How could one tell? As far as I know its in the exhaust system. As mentioned, I am guessing that a backfire blew a hole in the rusty exhaust tubing. But if the noise actually originated under the hood, I might noty be able to say.

[NorthWet]

Intake "backfires" are usually a low-frequency "whump!" type noises, sounding like it comes from the top of the engine.  Exhaust backfire is a little more likely to be a sharp "bang" below your feet, or even behind you.

[dltrial]

Well, I'm pretty sure that it is an exhaust backfire. What are the possibilities? Either kind could be due to timing or valves, right? What else?

[NorthWet]

IMHO, most exhaust backfires are caused by too rich of a mixture finding some oxygen from an exhaust leak.  In and of itself, this does not make much sense combined with a power loss.

 

More likely scenario is that you get some cylinder(s) misfiring, and the exhaust providing an ignition source. I would look at the distributor/cap/rotor/wires/plugs.

[djellum]

backfires come out of the intake and afterfires come out of the exhaust.  most of the time theres enough fuel to ignite in your exhaust, its what the catalytic converter takes care of, so if you have an exhaust leak thats likely the culprit of your after fire.

 

there is (on my old carbed motor anyway) a couple of systems tied to the exhaust that can let air in.  search on the site for ASV system, and antiafterfire valve (i dont know if you have one on an fuel injection system though).

 

like said above, id check your ignition system out and spray for vacuum leaks.

[subarusaver]

Just my experience...My 82 gl used to have a backfire (ehaust only) that would come and go, pretty bad at times until ONE DAY .Finally it quit and would not start back..took it to my Subaru mechanic and there was basically a HOLE the size of a quarter in the intake....Where the a/c bolts on to the intake, (apparently it had been OVERTORQUED) ,the threads in the intake had broken about 1 1/4 "diameter hole and air was sucking around the whole perimeter of it....So now , if I get backfiring , I check vacuum first..Good Luck!!