Weird air fuel issues, gets worse the more you give it gas. Lean AND rich, any and all help needed/appreciated. No codes. 2004 Outback 2.5L

2004 Outback 2.5L spikes low and high fuel trims . At idle they’re much more consistent, but as the car idles they slowly get worse. On highway at a constant speed the trims will go through a cycle. It will sit at 0% and then climb up to around 10-20% lean or rich and then pop back to zero. The car backfires after you accelerate hard once, for pretty much the rest of the drive. No smoke from what I’ve seen, no fuel leaks. It has no logged or pending codes . Can’t find any sort of vacuum leak. It gets about 20 MPG (measured at pump), BUT it has gotten 28 MPG highway before ! So I know it has the capability (was on a trip back from Tennessee first two fill-ups were 28MPG average), but as of recently it has only been getting 20 MPG highway. Nothing has changed since this occurrence other than me fixing random things !

I’ve taken it to the Subaru dealership twice , the first time they said CAT. I replaced the CAT and problem persisted. Second time they said timing or ECU. Timing is correct . I have no way to test ECU as you need to reflash one, but I’ve heard Gen 2 ECUs are extremely stout and not prone to failure.

Part replaced: Replaced front o2, CAT, injectors, fuel pressure regulator, fuel pump. fuel filter, MAP, brake booster, spark plugs, wires, main coil pack, air filter, redid the headgasket (other reasons), refinished the heads fully, new gaskets. Probably other things I can’t remember.

I have a log file , the first few seconds are idle. Then I drive in the city to the highway. I get off the highway, and then back on. At one point, I rev my engine to redline for about 2-3 seconds (someone was about to hit me/louder than horn because muffler fell off). After this point the trims get MUCH worse for the rest of the drive for whatever reason.


I have no specific ideas, but here are several questions that might help lead you to the cause:

  • How many miles on the engine?
  • How much experience do you have with the car (owned since new, just bought it, owned for a few years, etc?
  • Is this a new problem or has something like it occurred previously, perhaps intermittently?
  • Did the problem appear suddenly or gradually over time?
  • Can you associate the problem’s onset with anything else - repairs or service, running out of gas, connecting jumper cables backwards or battery cable disconnected while running, an accident, long period of non-usage (bad fuel, rodents chewing wiring or hoses, spider nest in vacuum hose, a sticky control), someone else driving (and potentially abusing) it?
  • You mentioned checking for vacuum leaks, but original lines on a car this age might be loose or have leaks not easily detected, or be clogged - have you tried replacing them?
  • Has anyone looked in the fuel tank for foreign objects? Sometimes gunk or foreign objects can intermittently block the pickup causing odd problems.
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104k miles

I bought it around six months ago, previous owner did NOT take care of it well other than oilchanges. Lots of shitty repairs.

It was intermittent in terms of fuel consumption but now it’s constant. Not sure about fuel trims though, I got a reader a few months back and it since then the AFR has always been wacky. Backfires started after the exuast fell off (resonator + muffler). Made me think the headers could’ve gotten shaken or something but it’s properly connected without leaks.

It appeared the first time I ran it, it terms of fuel consumption issues. 14mpg :grimacing:

Other than the previous owner, no.

I haven’t replaced them, not sure where I’d find all the vacuum hoses.

I had to change the fuel pump a week or so ago because one of the high pressure fuel lines broke off when I was trying to change the in tank filter. I didn’t see anything foreign but it was 3/4 filled so I didn’t see much anyways.

Am I correct to understand that you put all these parts into it without first replacing the exhaust system?

Did this start to occur immediately after something else happened, like immediately after you refinished the cylinder heads?

If not, if it seemed to come on for no apparent reason

  • Check for exhaust system leaks. A 2004 could have a rusty exhaust system, especially if it was used for mostly short trips. Any small holes in the exhaust system will confuse the O2 sensor and make it impossible to get the fuel mixture correct. .Holes can widen or narrow as the exhuast system warms and cools which would cause weird symptoms like this.

  • Have you tried the propane wand technique? As the engine is mis-behaving (at idle, in the driveway) direct propane to various spots in the engine compartment. If the rpm speeds up, good chance there’s a leak allowing unmetered air into the engine at that point. (Possibility of fires, explosions so take appropriate precautions.) A can of starter fluid is sometime used for this too.

  • Try replacing the pcv valve. Odds are against you, but part is usually cheap and fairly easy to do, and might work.

  • Another low probability idea, but easy to do, watch the engine compartment at night, completely dark (outside, not in the garage of course). Do you see any blue-ish lights flashing around? If so, that’s an indication of electrical leakage in the high voltage system.