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Discussion Starter · #1 ·
Hello all!

A month ago, I purchased a used 02 Outback wagon with 160,000 miles on it from a local used vehicle dealer. 2.5L, 4spd auto.

Shortly after buying it, we began to notice occasional hesitation and "misfiring", especially under moderate load climbing hills. Scanned for codes, none, none pending.

Checked the plugs (NGK Platinums, installed about 60K miles ago, new Bosch wires then too) and wires. Did water spray test, no arcing or missing noted during test. Pulled plug wires, both ends look very good, no leaking oil, no tracking, no corrosion.

When gas was at half tank, added a can of Seafoam to the tank to see if that would help. Nope. Still no codes, no CEL.

Next tank was Midgrade to see.... Nope. Problem has worsened over time. Now misfiring heavily on some hillclimbs. Tach is observed to "bounce" 200-300rpm when this happens, generally in the 2000-3000rpm band.

I took the car in to my local Subaru dealer who charged me 100 for no diagnosis. They suggested having the used vehicle dealer mech check timing, service fuel filter, and drop exhaust to check valve guides.

Used vehicle dealer took the car for a weekend for their mech to check out. He checked the timing (all is well, belt 60K miles old), checked plugs'n'wires. He changed fuel filter. He drained the gas tank, finding some water/sludge. He did not check exhaust guides. When I picked the vehicle up, it seemed to be significantly improved, though it still hesitated off-idle when launching on occasion.

We filled up with fresh gas again, adding gas dry to ensure any remaining water was taken care of, drove it that week. On the weekend, we took a camping vacation, about 100m each way. By the return trip, the misfire had returned. Filled up with supreme 3 days ago, continuing to add gas dry.

This week, the problem has worsened again, still no codes with none pending. On 15 mile drive home today, the car must have misfired 500 times.

I'm bringing the car back to the dealer again tomorrow. Except for having sold me this car, I cannot fault them, they have tried, but seem as stumped as I am. Any thoughts? We've put about 1500 miles on the car over the last month, and its generated NO codes, with NONE pending. HOW is this possible?

Thanks for looking.

J
 

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I'm not really sure what a misfire is or how you tell but I have bumps in rpm quite a bit... and its also tough to get up hills... gotta floor it... idk If that's normal as I just got mine also
 

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The absence of any codes, and, I presume no sign of the check engine light coming on, is unusual, especially over the time frame and number of "misfires" the problem has been experienced.

Do the check engine light (CEL) and the AT Oil Temp warning lights work? Do they come on when the key is turned to ON but the engine isn't started, and go off when the engine is started? When the key is turned to ON and after the engine is started, does either warning light stay on, or come back on, or flash?

I can't discount the possibility that there might be some types of engine misfire that won't trigger a code/CEL, although offhand I can't think of any. But that's if the issue is an engine misfire to begin with. I wonder if the symptoms you're feeling (hesitation) and seeing (tach changes 200+ rpm) could be something else.

Perhaps this is off-the-wall, but I'm thinking that the same sensory symptoms could be caused by a malfunctioning torque converter lock-up clutch. When the clutch engages, rpm usually drops about 250 rpm (in 4th gear), and it feels as if the transmission is shifting. This might well be felt as a hesitation or sudden change in thrust/velocity of the car and a change in engine rpm if it were repetitive. And, if this were the case, there very well might not be any OBDII code generated.

By the way, a Subaru dealer's Subaru Select Monitor (scanner/logger/code reader) can log a wide range of engine operating parameters, many of which would be affected by an engine misfire, so if this type of testing is done, and there is repeated engine misfires, I would think it would show in the data. RomRaider Logger is a free program that, with an adapter cable, can do a similar job with many Subaru model years. For more info, see: http://www.subaruoutback.org/forums...426-freessm-complete-access-your-ecm-tcu.html. Long thread, but perhaps worth the read and consideration.
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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next time it happens, try manually selecting a lower gear. Might help point to, or exonerate the transmission - not sure.

If the mechanic was actually able to discover the timing was OK, then I'd guess the knock sensor is OK.

maybe Throttle Position Sensor? seems like they usually fail at low-ish rpms but .....
 

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Discussion Starter · #5 ·
CEL and AT lights light and extinguish normally at start-up, or when key is in "run" with the engine not running. extinguishes normally approx. 1s after startup. I've got an OBDII scanner in the trunk, and have checked for codes a dozen times. 0. 0 pending. ARGH!

I don't *think* it's xmission caused, as it happens at idle both in and out of gear from time to time. When things decide to run well, the xmission shifts appropriately and normally, TC lockup is distinct and seems normal to me.

It happens in both D and 3, and I've noted stumbles/hesitation when in 1 pulling up a ferry ramp on the weekend.

The lack of codes is baffling me. I'll look into your links tomorrow when I'm not on dial-up... *sigh*.

I can sometimes be climbing a hill, mid throttle, and the engine will seem to simply lack power.... Adding more throttle yeilds no response, until suddenly... Vroom, off we go.

Also, in rush-hour traffic, creeping is often a case of "touch gas, nothing, squeeze in more, nothing, Oh, there it goes, BRAKE!" That was today's drive home. Not relaxing.

J
 

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Discussion Starter · #7 ·
It was suggested on another forum that the front o2 sensor could cause symptoms like this without giving a code... Is this plausible? I believe the sensor was replaced between 40 and 60k miles ago, on a "hesitation and CEL" complaint, IIRC.

J
 

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I've read posts along those same lines, but also ones that seem to suggest otherwise. In the ones I've seen where changing the sensor is suggested, I don't recall the outcome being reported, so we don't know if, in fact, the sensor turned out to be the cause.

The front a/f sensor plays a role in the fuel/ignition control system, so it does seem plausible that if the sensor is intermittently malfunctioning, the control system could be "misdirected", but at the same time, this begs the question as to why there's no code -- this is what the OBD should be looking for.

Either more analysis is needed, or if you're inclined, the sensor can be replaced and hope for the best. (In the latter case, posts here seem to suggest that the front sensor should be replaced only with a genuine Subaru part.)
 

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(formerly) 03 H6 OBW , (presently) 06 WRX Sportwagon & 2021 Honda CR-V
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If freezeframe data can be read with a code (even a purposely created code - like a loosened gas cap) it may reveal if the system is pulling or pushing fuel. The short and long term fuel trims should be small numbers. If they are double digit - negative for running lean or positive for running rich - that could point to an O2 sensor or MAF w'ever. If they look normal, then maybe TPS or some weirdness with the alternator/crank angle sensor/electrical ???
 

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Sounds like an intermittant or loose ground problem. I have experience this with a bad battery cable before (not in a Subaru). Something to check at least, make sure all your grounds are good.
 

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Just a thought - the lack of codes indicates that what is being felt is not a true "misfire" - that is, a plug that is not firing correctly. However, you can have similar performance issues if an injector is not operating correctly and thus not delivering fuel to the cylinder, but without the misfire codes. Something else to look into. Good luck.
 

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A faulty fuel injector will cause a misfire. Anytime the cylinder misfires it should throw a code. This can be due to fauly spark plugs, spark plug wires, fuel injectors burnt valves or timing issues.
 

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Discussion Starter · #13 ·
So, forgetting that I describe the performance issue I'm experiencing as a "misfire", what sorts of things can cause a performance issue felt as intermittent power loss, often cyclic on about a 1s period (makes power, doesn't, makes power, doesn't), occasional bucking under light-moderate throttle on moderate hill, but sometimes not cyclic, but sustained (occasionally :more throttle, no response for 2-5 seconds, then normal response)...

I use "misfire" only to describe the feeling, not to present as a diagnosis. It all-but-cannot be a true misfire, there are no codes.

Anyone want to comment on wether I should gamble the expense of dropping exhaust manifold to check valve guides (as suggested by subi dealership)? I'm leaning away from this as a cause, mostly based on the improvement seen after fuel filter change and tank draining.

I will disconnect the pos terminal tonight when I get home, to see if the computer is "learning wrong"... ie to see if things run well initially, and get worse again over a time. This would, in my mind, lead me towards sensor/EFI issues, rather than mechanical.

Thoughts?

J
 

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Mine had a similar misfire, and it did not set any codes. The CEL would begin to flash later as the misfire got worse, but it never threw a code. Turned out to be a plug wire shorting inside the plug tube. If the aftermarket wires have 60k on them, they could be to blame. That's about how much my aftermarkets had on them. I know you said you checked them, but I had sprayed mine down too, and since the problem was inside the plug tube, there was no visible arcing. Until, that is, I pulled it out and saw the gray mark on the boot and the black mark inside the tube. Who knows, just my most-recent misfire experience.
 

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Discussion Starter · #15 ·
Well, the problem's finally been identified and corrected, so I figured I'd post the solution here so others may benefit.

As I said in my last post, I disconnected the battery for the weekend to see if that had any effect on the symptoms, and lo and behold, it did. The car's performance improved for a time, slowly deteriorating again, thus pointing firmly at an EFI related problem. Disconnecting the battery basically set the computer to defaults, and then it slowly "learns" based on sensor inputs.

Armed with this info, I brought the car in to the local Subaru dealership, and told'em have at it. They insisted on dropping the manifolds to check the exhaust valve guides (their leading suspect, as they were resistant to the idea that there could be a sensor problem without a light or code). All was found mechanically fine.

Hooking the car up to their diagnostics computer, they eventually determined that the front O2 sensor was giving erroneous data, leading the computer to tweak the mixtures lean. Simply replacing the O2 sensor has got the car performing normally, much to my relief.

Slightly annoyed with Subaru's insistance on checking for a problem I ruled out (the exhaust valve guides), but that is tempered by knowing that the engine is mechanically in decent health.

Ultimately, the ordeal set me back about $500. So, all told, I've got about 3500 into this car (including purchase), so with luck I've got a decent thing going now.

It is nice when power output corresponds with throttle position.

Thanks for your input, fellows.

J
 
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