By on January 25, 2012


TTAC Commentator NICKNICK writes:


I can’t believe it’s been two years since I asked you to post a problem with my 1999 Subaru Legacy 2.5 GT.  It may have been fixed with just a new gas cap.

I recently got a CEL for evaporative emissions control.  I don’t know if you’re familiar with this problem, but Subarus have a plastic cover over the fuel filler neck that traps dirt and salt and causes them to rust.  Once they perforate, you get evaporative emissions warnings.  I pulled off the cover, but there was no rust.  I checked the gas cap, and the O-ring was somewhat brittle.

I replaced the gas cap, and I haven’t had a CEL or my original hesitation problem since!  I can’t say for sure that was the problem, but it certainly correlates.

My theory is that I had a small enough leak to lose the fuel vapors stored in the canister that get burned, but the leak wasn’t bad enough to set off the check engine light.  I’m guessing that the 20 year old technology in my car isn’t smart enough to know how much fuel vapor gets caught in the canister.  I bet that shortly after ignition it switches over to burn from that canister no matter what.  Normally there is enough trapped vapor to burn for a second or two, so the ECU tells the injectors to not add extra fuel.  In my case, the vapor wasn’t there because it escaped out the gas cap and caused the hesitation because there wasn’t anything to burn.

I freely admit that i don’t really know how that vapor recapture/reburn system works, so I’m grasping at straws to try to explain my observation with the gas cap.

Anyway, I don’t know if it will be useful information to you or not, but maybe someday you’ll run across a similar problem and it might be worth your while to try a $7 gas cap.

Thanks for featuring my car in Piston Slap and getting it out there in front of the Best and Brightest–I appreciate the help!

Sajeev answers:

This is one time when I wish I had an electronic database of componentry for all vehicles…I’m still waiting for you to contact me, ALLDATA!  Or not, because I can put it into one sentence. And hope for mercy from the Best and Brightest.

No matter how a modern fuel system is designed, they are always pressurized and if there’s a drop in said pressure, the computer throws a warning code.

Odds are the brittle O-ring was dry/flat enough to make a weak seal, lowering the pressure in the system (when running) and triggering the warning light. And it is entirely possible that extended use of rubber-munching E-10 fuel did a number on that O-ring. Ya never know!

Bonus!  A Piston Slap Nugget of Wisdom:

I’ve hammered on the fact that rubber parts on a 10+ year old vehicle go bad, no matter how pristine the vehicle is to the naked eye.  Tires, belts, hoses and…WAIT FOR IT…O-rings.  In your A/C, power steering, fuel systems and many, many other locations. O-rings go bad with age, and believe it or not, anything rubber is your car’s worst enemy.

Send your queries to [email protected] . Spare no details and ask for a speedy resolution if you’re in a hurry.


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17 Comments on “Piston Slap: O-rings are the Enemy Within?...”

  • avatar

    Light, sure, but what about the hesitation?

  • avatar

    Be sure to check the condition of your fuel filler neck. There’s a little plastic cover that traps debris between the neck and the cover which can cause it to rust out. This can be exacerbated by living in the rust belt like I do.

    Mine had a crack in it, which caused a loss in pressure, which eventually led to the neck leaking out fuel. I patched it up with a bicycle inner tube (seriously, I did) until the replacement neck I ordered from my local Subaru dealer came in.

    I sprayed the replacement filler neck down with two coats of primer and three coats of rustolem paint. Install wasn’t too bad, but an extra set of hands makes it a lot easier because it’ll require a bit of wiggling to get it into the tank and lined up with the filler door.

    EDIT: Mine is a ’96 Legacy, which has the same style filler neck as your ’99

    I clean out the protective guard every few months now too.

    • 0 avatar

      Same thing happened to me, all that salt and corrosive soil built up behind the cover of the filler neck and caused a pinhole. My check engine light came on. I found a good powder coated filler pipe at for much less than the dealer. Needless to say i didn’t put the fuel filler neck cover back on.

  • avatar

    The D3 had a cousin of this issue for years, at least since OBD I has been out. Leaving a gas cap loose would triger a CEL that that would “magically” cure itself after the next fill-up where the gas cap was tightened down properly. This had the effect of frightening owners into unscheduled dealer visits for years due to fears of impending catastrophic damage to an otherwise trouble-free automobile. I think they finally started placing a decal near the gas cap telling people to tighen the cap securely “or till it clicks” because of all the complaints from owners (and service managers).

    • 0 avatar

      Funny, my OBD I equipped 92 Ford Ranger never threw a CEL (and yes, it works) when I left the gas cap OFF and thus loosing it when I put gas in the tank and drove off with it still on top of the vehicle.

      Odd as it never even hiccuped or anything, I never knew the issue until I went to fill up again or realized as I’m driving down the road what I’d just done, by then, the cap was long gone. Done it twice since I bought the truck used in 2006.

      • 0 avatar

        A ’92 is not OBD II, which is the first version to start monitoring the EVAP (evaporative emissions) system. OBD II was not mandated until the ’96 model year.

      • 0 avatar


        If you read my comment correctly, I said OBD I, not OBD II as I’ve spent part of Saturday trying to obtain the codes from my OBD I system in my truck using the CEL light to no avail (never got the codes right).

        Turned out the issue I was having was related to the Idle Air Controller so I could not rev beyond 3K or so rpm, if I was lucky to go that high. Had to have Midas diagnose the truck on Monday as I thought the problem solved itself but nope and I still didn’t get the CEL light when it acted up.

  • avatar
    DC Bruce

    Amen to your last italicized comment, Sajeev! After spending the better part of a Sunday two weeks ago replacing all of the cooling system parts in my cooling system failure-prone ’01 Z3 3.0 (hoses, radiator, thermostat + housing, water pump, expansion tank, serpentine belt), I was feeling pretty smug and proud of myself. Then, yesterday, the CEL started throwing a code indicating a bad exhaust camshaft position sensor. Unfortunately, a little Internet research reveals that the likely cause is not a bad $60 sensor, but a failure in the oil-actuated variable valve timing system. The source of that failure — wait for it! — buna rubber o-rings that seal the little pistons that actuate the system. This is an age-based, not mileage based, failure (only 64K miles on my car).

    On site says repair takes about 4 hours for a professional, 6 for an amateur. Just to replace 4 lousy o-rings, the size of a silver dollar!


    • 0 avatar

      Try Besian systems for upgraded O rings as your replacement. They also have very detailed instructions on their website for the not too difficult replacement. You can also contact Dr. Vanos to do a complete exchange for a rebuilt vanos, with the upgraded seals already installed. There is a core charge until you return yours.

      • 0 avatar

        +1 to the Beisan kit – instructions are excellent. I rebuilt my VANOS unit to get rid of a noisy valvetrain (worked). I found the rebuild process fun, so don’t be afraid of tackling it. The hard work is getting the unit out of the car, not the actual replacement of the wear items.

        Also, to the original topic of this post, BMWs are well-known for throwing an evaporative emissions code for a slightly loose or old gas cap. Of course there is also a poorly designed evap canister behind the rear passenger footwell that breaks apart also causing that fault code. That was the case on my car (wish it had just been the cap!).

  • avatar

    In Texas, one of the things I had to fix to get out of smog control inspection hell was to replace my gas cap. It must be a pretty common problem if they inspect for it routinely.

    I am no expert (who is?). Prior to this 91 S10 all my cars have been new enough that it never popped up or old enough to be exempt from smog inspections. I’ll probably put myself there again as soon as I can.

    • 0 avatar

      Yep, even in Washington St. That’s the first thing they do when one does an emissions test, check the gas cap. Fortunately, mine have been good. The only reason I’ve replaced them is loosing them due to the tether on the original one being broken and recently lost the first replacement.

  • avatar

    I’ve been told that this issue led Ford to develop their cap-less fuel system. It purportedly eliminated the warranty costs and customer dissatisfaction associated with errant check engine lights. As an added bonus it makes fueling the car more convenient and less gas residue on the hands.

    Everytime I fill up my ’03 Focus I find my self wishing it had the cap-less system our new Explorer has.

  • avatar
    Educator(of teachers)Dan

    My mother has had the “I didn’t tighten the cap enough” issue before. With my 2004 F150 the original cap went bad in 2009. The cheap Pep Boys replacement is still going strong at this point.

  • avatar

    I had a hose clamp on my Escort rust through and break over the filler tube. A 2 dollar part that should have been of better quality. It failed tank pressure testing at emissions testing.

  • avatar

    These issues are not always due to rubber based parts. Anybody with a GM W body that lives in salt country may be surprised to find a rusty fuel filler neck and the related vapor line next to it. When either of these items rusts enough, a tiny vapor leak forms and sets the code for a small evaporative leak. There is another code for larger leaks. Some carmakers, Ford for example, have a separate light for this small leak and it is marked for checking the gas cap. Any small leak will trigger the gas cap light even if the cap is fine. This was done to prevent those trips to the dealer that kill satisfaction ratings. If you have to sell a car with an evap leak that you can’t find, make sure the tank is totally full or nearly empty. Under these conditions, the evap test does not run.

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