Piston Slap: O-rings Are the Enemy Within?

by Sajeev Mehta

Updated: May 16th, 2014 Published: January 25th, 2012

piston slap o rings are the enemy within

TTAC Commentator NICKNICK writes:

Sajeev–

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 sajeev@thetruthaboutcars.com . Spare no details and ask for a speedy resolution if you’re in a hurry.

#CEL #CheckEngineLight #E-10 #Ethanol #GasCap #O-ring #Rubber #Subaru

Sajeev Mehta

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2 of 17 comments

GS650G on Jan 25, 2012

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.

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Golden2husky on Jan 25, 2012

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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