Thoughts on a rough running Subaru?

2002 Impreza TS, EJ251 5spd, 201k miles, 100% original underhood save for wear items and a knock sensor 30k miles ago, new Genuine Subaru air filter/fuel filter/plugs/wires within the last 1k miles. Dead stock with the exception of an axleback straight pipe.

I'll start with: "no CEL." Yes!

This car is giving me fits. It's been a fantastic runner for the last year but has slowly taken on the bad habit of intermittently cutting out on a cylinder or two. Under load, idling, hot or cold ambient or engine temps, all throughout the rev range. Seems to clean up a bit if I rev the crap out of it, though it's worst after a spirited run under load for anything more than a few seconds. Hot and cold idle revs are normal save for the stumbling, and a very occasional 100-200rpm hunt.

I had a B13 SE-R with a bad MAF sensor years ago, the stumble under load feels pretty similar to that, though I have no MAF to clean or replace, so I'm back to square one.

I've done all the normal tuneup goodies, and I'd assume it's not timing related if it runs perfectly normal on occasion. Any ideas? Crank/cam sensors, bad grounds, etc? This thing is older than the hills, I don't even know where to begin.

Once again, no codes!!

Check the plugs and wires. My '93 did something similar (although it went to full limp mode without a CEL), and it was a bad plug wire. Found the offending wire, replaced it with the 17 year old stock one, fired right up and ran fine.

Morbid wrote: Check the plugs and wires. My '93 did something similar (although it went to full limp mode without a CEL), and it was a bad plug wire. Found the offending wire, replaced it with the 17 year old stock one, fired right up and ran fine.

Brandy new OE properly gapped plugs and wires, the car performed exactly the same on the old OE junk that was in there before. Good suggestion though, that was my first thought!

Coil pack (or coil packs, I forget for that year).

Sounds like you need to do some old fashioned diagnosis. Its got to be one of three things causing the misfire: spark, wacky air/fuel, or compression. Sounds like you did the easy stuff like checking wires and junk. So if it was me I would read my plugs and then do a compression or leak down test.

I'm basically a pessimist so you dont want to know what I think the problem is when the misfire is intermittent with no codes and 200k miles

Ckosacranoid had a rough running subie - he was running on 3 cylinders - traced it to a bad coil - although IIRC it threw a code - but basic diagnostics...

Your '02 probably has the separate coil up on the intake. They are known to fracture. Pull the wires one at a time and look for a 'tracer'. Unbolt the coil so you can look at the underside. I've not had it happen to me but have read enough about it to call that as "step 2" after doing the plugs and wires.

I agree, sounds like a coil flaking out...tough one to diagnose without more info.

Bryce

Ah, I'd forgotten about the crap '00-02 coils. I'll dig into the current relic this weekend if it lasts that long.

Update!

In the interim, the car has been running much worse. It's definitely a full on failure in the works. Ever run a car out of gas, that last few hard coughs before it runs dry? That's how mine runs 80% of the time.

Did some poking around on the car this weekend. Hauled out the new plugs to find that they're accumulating an ashy white/yellow coating. Running lean...

Yanked the coil, no arcing, hot spark to all four leads, swapped with a known good unit(I'd never considered the wife's Forester a parts car...), no change.

Jetting about on Google just now, I ran across: http://www.ultimatesubaru.org/forum/showthread.php?t=67123

That chap does a fine job of describing what's been up with my car, though the current state is a bit worse. For those who don't want to click the link, it ended up being his MAP sensor.

The wiring for my MAP was vampire tapped when new for aftermarket cruise, I'll resolder that before installing a new MAP.

I'll post an update if I get this pile running well again before it has a meltdown.