Mystery problem when warm on long drive

I’m having a really tough problem with my 2002 Outback 2.5L automatic (82,000 miles). The local mechanics haven’t been able to diagnose it. Here are the details:

Car starts great, idles great when cool/warm.

Runs great around town on short trips.

No CEL, no codes ever.

Problem does not seem to be weather/water related and happens even in -10F temps.

Problem has been occurring for about 6 months & is getting worse.

On a long trip (> 30 mins), the car often skips/stumbles/misses. The happens most often when driven 1 hour down a mountain and the then halfway back up. Hot, and under load on the return trip, the car suddenly develops a bad stumble/gallop behavior as though several cylinders are mis-firing. If you pull over and idle in park at this point, the car will usually stall. If you keep going, you can often push through the trouble by using a very heavy foot on the accelerator.

I’ve run a couple of cans of gas dry through the tank, but haven’t tried anything else. I can’t really think what to swap first since the nice behavior around town would seem to rule out most of the usual culprits. Any ideas?



Try removing or loosening the gas cap. Kind of sounds like your fuel supply being restricted, perhaps by a vacuum in the tank? Just a wild thought.

Sounds like a fuel pressure issue. Try changing the fuel filter and see if that helps, but you may also be looking at a fuel pump.

My daughters Toyota started doing that the other day, even after the spark plugs were replaced. We did a bunch of things trying to figure it out until I got a can of caburator cleaner and hosed down the throttle valve assembly. We let it set for a while and the porblem is gone. It is a cheap try.

Intermittent ignition breakdown when heat soaked. (Sound like)

It certainly behaves like a fuel issue (I’ll try taking off the cap next time), but wouldn’t a clogged filter or a bad pump show themselves with normal driving? If I take it out cold right now, I can zoom around at highway speeds/accelerations no problem… You have to drive hard for a long time to see it happen.

It certainly also behaves like ignition. If it was doing this at cold idle, I’d be checking the ignition leads. What part of the ignition is breaking down under hard, long load and then getting better?

Be cautious of spraying carb cleaner in some plastic intakes. Some of them have a coating that can be dissolved by the wrong solvent. Throttle body cleaner might be a better choice.

New info – replaced the coil pack with no improvement. Then I got a CEL and code P0172 = fuel trim system too rich. O2 sensor next?

If the fuel cap release didn’t work, try disconnecting the O2 sensor. The engine light will come on and it will run a default “get you home” fuel map (if it runs at all). If that fixes the issue, replace the O2 sensor, they do go bad from time to time.


waste of cash!

its just fine and letting you know,its got a prob.with the trims.


“Fuel trim”? What’s that? This article will explain:
Have you done all the routine maintenance, such as air and fuel filters, etc? Are you running lower octane gasoline than called for by Subaru?
Clean the MAF (Mass Air Flow) sensor with MAF Cleaner, and clean the throttle body (internally), and the throttle plate, and the idle air control valve and passages with Throttle Body Cleaner. It’s cheap, an’ if it don’t do nothin’ good, it don’t do nothin’ bad, either.