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?