Why is my Subaru stuttering after a fillup?
March 11, 2008 11:13 AM   Subscribe

Does anyone have any ideas about why my 2003 Subaru Outback stutters on acceleration for the ten or fifteen minutes after I fill it up with gas?

For the past few months, I've noticed that in the short period after I fill up my car with gas, it stutters on acceleration -- not terribly, and not so much that I feel like I'm in danger of stalling, but definitely a noticeable stuttering that wasn't there before. Like I said, this only ever happens for the ten or fifteen minutes after I fill up, and then never again until the next fill-up. I don't have any stuttering or idling problems in the cold, immediately after starting my car normally, or any of that; my check engine light isn't on (and doesn't come on during these periods), my gas cap is nice and tight, and I was last serviced about 6K miles ago.

A few details: the car is a 2003 Subaru Outback Limited Edition with a manual transmission, I fill it up with 87 octane gas (as I always have), no major driving pattern changes in a few years. The car has less than 50K miles on it to date.

I'm not the most trusting person when it comes to my local Subaru dealer (long story), so I was looking for a little advice as to what might be causing the problem before I bring it into someone to take a look. Any auto mechanics out there who have a few ideas to toss into the ring?
posted by delfuego to Travel & Transportation (9 answers total)
Maybe a gas cap venting issue? Gas caps have vents, and sometimes thy fail. As an experiment, next time you fill it up leave the gas cap off for a few miles and see if the problem goes away. If it does, get a new gas cap. If it doesn't, no problem.

Also, don't run the gas down to almost empty. Whatever deposits you have in the tank are highly concentrated in the remaining fuel, and if there are a lot it is possible you are stirring up a lot of sediment when you fiill the tank, and this takes some time to settle back out. (It's speculation, but if this is the case, it's also time to change the fuel filter.)
posted by mosk at 11:24 AM on March 11, 2008

Seconding what mosk said, never run your car below 1/4 tank. That low fuel light is there as a "You. Gas. Now." light not as an "I'm getting kinda thirsty" light. I'm not sure what the exact recommendation on fuel filter changes these days are, but I generally replace mine every 12k miles, a habit taught me by my grandfather from the carb days. They're not very expensive and I think they fall into the preventative maintenance category.

Also, don't overfill (i.e. top off) either. I had fun with my evap system and canister on my relatively new Honda because of my bad habit of doing that. Your car needs that little bit of extra room in the tank to return unused fuel from the fuel rail to the tank. Without that ability, strange things can happen.
posted by moitz at 11:42 AM on March 11, 2008

I think a lot of Subaru's of that age range specify in the owner's manual to use at least 89 octane fuel, to prevent engine knocking. Check the owner's manual. if that's the case, give the higher octane fuel a try.
posted by jrishel at 11:52 AM on March 11, 2008

mosk et al, lately I've been filling up before I get down to 1/4, and it hasn't really affected anything; I've been thinking that maybe the stirring-stuff-up theory is correct, so I'll definitely ask my service folks about that. I don't overfill, though, ever -- some time ago, I learned about what moitz mentioned, so I always just let the pump automatically stop filling, and then leave it at that.

I'm not so sure about leaving the gas cap off; on my car, that triggers the "check engine light", so I assume that there's a sensor looking for proper pressure in the system. (I also don't remember if that means I need to get the dealer to reset the light.) For the scientist in me, what does leaving the gas cap off do?
posted by delfuego at 12:12 PM on March 11, 2008

See if you can get codes pulled. Here are the OBD-II codes for Subaru. I bet a dollar you'll get P1423, EVAP Vent control high input.
posted by jet_silver at 2:23 PM on March 11, 2008

Your fuel system is moderately complicated (scroll down to the bottom of the page for an illuminating illustration, assuming yours is like the picture).

I would guess (very tentatively) you might have a weak main fuel pump in your gas tank. When the tank is full right after filling up, I think the slosh caused by acceleration could cause the fuel cut valves at the top of the tank to kick in, sealing it off from replacement air and vapor from the charcoal canister, and causing the pump to have to pull against a partial intermittent vacuum as air/vapor bubbles through the fuel cut valves. I suppose the same thing could happen if the tube from the charcoal canister to the atmosphere (the tube that goes straight down from the canister in the diagram) is at all obstructed, and they often are.

Try filling it to a gallon or more below its capacity sometime when you refill it. If the same problem happens, this theory is disproved.
posted by jamjam at 2:34 PM on March 11, 2008

What does leaving off the gas cap do?"

Hmmm, after a bit more reading, I think I was off-base when I suggested driving with the cap off. My old school knowledge doesn't always take into account OBDII sensors.

A gas cap needs to seal your tank to prevent gas from escaping and to close the EVAP system to the atmosphere. My thought was that the vent may have been blocked and you were seeing the results of a vacuum buildup in the tank. If you leave the cap off, drive for a few miles, and compare the performance, you could easily tell if this was the case. But yes, OBDII vehicles will throw a code if the EVAP system isn't sealed.

A bit more Googling (e.g., this thread on Ford F150 trucks) seems to point towards the fuel filter as the cause of a similar problem in those vehicles, so that may be someplace to look. Ford != Subaru, but the situations sound similar, so it may be worth a look.
posted by mosk at 2:34 PM on March 11, 2008

I know nothing about cars so I asked my dad, he says:
1.) Try a gas treatment
2.) Check the fuel filters
3.) Then check the throttle control body and see if it needs cleaning.

You may just have water in the fuel, get a fuel treatment you can just pour into the gas tank. OLD SCHOOL- if you don't want to spend the money for the fuel treatment you can just put a bottle of rubbing alcohol in the tank *** Ask around about that ** i have never tried it myself but have heard it works!
posted by CreativeJuices at 3:45 PM on March 11, 2008

There is no logic to the idea of never running your tank below 1/4. The fuel pump always draws fuel from the bottom of the tank whether it is full or near empty so don't worry about it.

Are you in the habit of manually topping off the tank after the automatic cutoff on the pump handle? This could be forcing raw gas into the vapor canister which is then pulled into the intake manifold causing the engine to run rich until the canister is emptied. You should avoid topping off a fuel tank.
posted by JackFlash at 5:22 PM on March 11, 2008

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