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I have a 1999 Subaru Legacy Outback with a 2.5L motor. Mileage on the car is about 150k, but the engine has another 40k on it or so (the old one threw a rod).

I can drive the car for almost exactly 10 minutes (very consistent: +/- 1 minute), and I will then lose two cylinders (the two on the driver's side -- 2 and 4, I believe). It does seem to be a gradual loss -- for a minute or so, they'll fire for most throttle ranges, unless I'm trying to accelerate too much. Eventually though, it will get to the point that they will rarely fire at all. There are no backfires.

Restarting the engine does not fix the problem. The 10 minute timer seems to start from the first start of the day, or if I've left it for at least 4 or 5 hours.

  • It is likely not a fuel problem -- it smells like it's running very rich. I did replace the fuel filter just in case though.

  • It's not a compression problem. I checked the compression on both cylinders, and both checked out.

  • I replaced the spark plugs, distributor (I know it's not a distributor actually, but I can't remember what it's called), and spark plug wires. One of the wires had shorted, but replacing it didn't fix the problem.

  • During the first 10 minutes of drive time, I have all the power in the world. In fact, just this morning I drove it rather aggressively (as much as can be done in traffic), and had no problems whatsoever, until between 10 and 11 minutes.

Any ideas what I should check next? I'm thinking I'll probably get two injectors, a MAP sensor, and a temp sensor, just to throw things at it. What doesn't make sense to me is why it waits 10 minutes to become a problem. It could have to do with engine temperature, but the engine warms up to normal temperature a few minutes before the 10 minute mark (maybe 7?), and I would think that a correlation would happen sooner than that.

I read my OBD codes on the way home yesterday. Here's what I have:

  • Misfires on all four cylinders (P030x) (yes, I detected that too ;))

  • P0325: Knock sensor. After it's done raining, I'll have to check this again. I replaced it somewhat recently, but there's always the chance the wires broke or the bolt came loose

  • P0341: Camshaft Position Sensor "A" Circuit Range/Performance (Bank 1 or Single Sensor). This one I'm not too sure about. From some research, it sounds like this could be the wiring, maybe the sensor, or the timing belt. I did replace the engine (and the timing belt) about two years ago, and that was my first time doing that, so maybe I should check that again. Ouch.

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  • Have you read your OBD codes? If not that's the first place to start. If you have, what codes did you get?
    – GdD
    Commented May 22, 2019 at 7:11
  • Thanks for the suggestion. I thought of that again on the way home last night, and edited them into the question just now.
    – Cullub
    Commented May 22, 2019 at 13:46

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My guess would be electrical connection loss due to heat. Remember that the "engine temperature" you're reading is the coolant temperature, and something farther away from the engine that isn't directly in contact with the block will take longer to heat up. Perhaps some form of wiring is breaking or otherwise losing contact due to thermal expansion, but makes sufficient contact when cold. If you're positive you're only losing cylinders 2 and 4 (left-hand side of the car), then I'd start by checking any ignition-related wiring on that side of the car, or even the camshaft sensors (since you're getting a P0341).

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