You say to yourself "why in the world did someone devote a thread to a fuel cap." Well, I'm just the nut to do it.

There seems to be some misconceptions about fuel caps. Notice I said FUEL, not a GAS cap. They are different. Although they may fit in place of each other, they will not function in place of each other.

Gasoline engines today use all sorts of emissions equipment. Gas caps keep tank in some what of a vacuum state. The charcoal canister and other parts are not found on diesels. (hey, this is a diesel forum, go figure out your gasser's somewhere else ok? ;) )

Gas and diesel are two very different fuels. Gasoline will evaporate. Don't believe it? Leave the cap off your gas can for your lawn mower in the shed and come back next week. Next thing you know your pumping another $10.00 in fuel in that little tank, saying "I'll at least put some electric tape on it this time."

Diesel won't evaporate like gas. It will stay there, and stay there, and stay there some more. It will start to do some evaporation, but not the disappearing act you'll see out of gasoline.

So why do I bring up this fuel cap thing? Because, it may be causing issues with your fuel system.

If your fuel cap is not allowing air in to your diesel tank, it will create a vacuum. A vacuum is not something that you want to build. Vacuum will make your fuel system pull harder to get the same amount of fuel out. This could lead to power loss, and other strange fuel related problems.

There is an actual spec on the fuel cap and the amount of vacuum that is allowed before the vent opens in it. The spec is measured in inches of water column. What does that translate too? You build that much vacuum in a straw for your drink. So, we are not talking much here.

How can you tell if you have a bad fuel cap? Most easily it is detected when you have a full fuel tank. This will give you the most fuel, and the least amount of air space. These conditions will tell on you the best.

Say, after fueling go down the road, and then pull off after a few miles. Unscrew your fuel cap. Do you hear a good 'hhhuuushhh'? Chances are you've got a fuel cap issue.

Tanks that are only 1/2 full or less may not be as easily to tell either. The vent in the cap maybe allowing little amounts of air in, but not enough to keep up with the fuel demand.

Where can I get a fuel cap? Some auto parts stores. Discount auto parts stores usually don't. Becareful, they may sell you a gas cap. It needs to say 'Diesel' on it.

Can I fix mine? Cost prohibitive. It's only about a $5.00 usd item. Unless you're paying yourself less than minimum wage, you're not going to make any headway really.

where is a goodplace to find thew proper fuel cap?

NAPA sells some, dealership may have them too. Just a few ideas.

Advanced auto. Cheap, it locks, and it's the correct one.

i purchased a vented cap from o rileys that has a little red spring loaded button in the middle he said it was vented but i still get a woosh sound when i open it. is the spring to tight?

Say, after fueling go down the road, and then pull off after a few miles. Unscrew your fuel cap. Do you hear a good 'hhhuuushhh'? Chances are you've got a fuel cap issue.


uh-oh. From a full tank to empty (or nearly) the whooshing doesn't stop until after I've put the cap on the bumper.

but it runs like a champ.


:shrug:



I'll look for a new cap next time I'm at the parts place. Also need a new 710 cap under the hood. thing just jiggles around, but it stays in place.

I bought the same fuel cap from O'Reilly's and had the exact same problem. Still whoooshed like the old factory unit. So I simply took my dremel and cut out the plastic center of the factory cap and then used some needle nose pliers and removed the valve and spring. I then used a round dremel sander and cleaned up the center portion where I removed the plastic and used compressed air to make sure nothing was left in the cap. Have not had any more issues with whoosh sound full or half full. Took me all of ten minutes to do the job. Nice, clean, easy to do.:cool2:

think i could just take the spring out of the new cap?

Yep....you can do the exact same thing, gut the new one. The reason I did the factory unit is because it draws air from under the cap and I don't have to worry about contaminants getting into the lid, whereas the O'Reilly cap has the button on top that will probably come out if you gut it, leaving a hole where the air should come in. Although I am guessing at this. I guess I could go out to the truck and get the newer cap and take a look at it and know for sure.

lol....OK going out to get it and take a look at and will respond back in a bit.

I experienced lots of whoosh and even replaced the fuel tank because the vacuum got so bad it collapsed the tank. I then replaced the cap and still had lots of whoosh, so I took the old cap and drilled a 1/16 hole in top and now no more whoosh only slight hiss when opening cap.

I just took some pics of both caps and just now gutted the after market cap showing pics of that too. Will post them as soon as they download on my desktop. Just pry out the red center with a screwdriver. The aftermarket cap vents underneath also and the button stays where it is. No problems.

Hope this helps.

http://pg.photos.yahoo.com/ph/dwayne_3966/detail?.dir=/74d1&.dnm=bcb6re2.jpg&.src=ph

http://pg.photos.yahoo.com/ph/dwayne_3966/detail?.dir=/74d1&.dnm=d787re2.jpg&.src=ph

Well bugger, is my first time trying to load full size pics from a URL. Can anyone see them?

Nope, unless they are little boxes with red X's in them.... do it under "manage attachments" in "Go Advanced" posting.

Nice red X's you have there??

kewl....love red x's......lol.......ok will try again. Thanks guys

Let's try this again!!!

I had the vacum issue and just took the O ring off of the cap

thanks ill try that tomorow

Gotta be careful. I called the dealer and asked if they had a cap in stock for my 95 6.5 DIESEL suburban. I was emphatic about the diesel part. They said yeah, so I had my son pick one up on the way home from school- 12 miles away.
Yup, you guessed it . they gave him a GAS cap.

yea im starting to not trust anyone else to work on my truck. I went to a tranny place the other day to ask where the pressure plug is on my 4l80e so i could put a tranny temp sender in and he looked at me like i was nuts.

I just went out and checked my truck. It's been sitting in the driveway since yesterday with half a tank. As I unscrewed the cap it still had plenty of "whoosh" :eek: , so I gutted the cap.;)

sometimes the woosh is caused by air escaping from the tank as the system builds pressure. cap is designed to allow air in, which should relieve vacuum build up.

A new - or functional used - cap should be venting at 1" hg vacuum or 1psi pressure.


Actual volume which requires venting will depend on fuel level in tank.


Warm fuel from in-ground tank will contract when pumped into frigid vehicle tank - winter - creating vacuum - 1" hg max.


Cool fuel from in-ground tank will expand when pumped into warm vehicle tank - summer - creating pressure - 1psi max.


Word up.....

my truck has always had a big whoooosh.

Might 'in be time fer a new cap, eh what?

Hey, wrote this stuff way back when, guess what, I just the other day replaced mine finally. Finally back into the "I can tighten my cap all the way crowd" again.

when i got my truck i got a big whoosh. after i saw this post i drilled a 1/16" hole in the inside valve of the cap and no whoosh anymore.

So if the cap is working properly it won't allow presure to build either?I thought that pressure was OK.So a gutted cap won't hurt the pump's by NOT allowing any presure to build?

my nold cap caused mucho vacuum. In fact it collapsed the center of my tank by 6". I installed a new tank wasn't thinking and used same cap and stupid me did it again. I then bought a new cap but still had lots of vacuum so I just drilled 1/16 hole in center of old cap and now no more vacuum. I don't trust the caps even new one had lots of vacuum.

I tested new and used Diesel fuel caps -

Max vacuum before venting is 2"hg = 1psi equiv, normal is 1" = 1/2psi equiv

Max pressure before venting is 1psi

No matter how much the 'whoosh' a functional cap is vented either way at ~1psi

26 gallons is a lot of volume to vent when the cap is removed.

And, yes, a faulty vented cap can result in a collapsed tank - that's part of the reason state inspections require checking the cap yearly.

Punch a hole in the cap, defeat or remove the vented seal, and you're just inviting fuel-thriving bacteria into your tank each time the fuel cools and contracts - which is each time you shut the engine off.

The more humid the weather, the bigger the party.

Word up, folks..............

Not to mention that dirt/dust is introduced into the fuel system, I've seen Army trucks "suck" cup fulls of sand into the tank because the cap was not tight or had a faulty rubber seal

I tested new and used Diesel fuel caps -

Max vacuum before venting is 2"hg = 1psi equiv, normal is 1" = 1/2psi equiv

Max pressure before venting is 1psi

No matter how much the 'whoosh' a functional cap is vented either way at ~1psi

26 gallons is a lot of volume to vent when the cap is removed.

And, yes, a faulty vented cap can result in a collapsed tank - that's part of the reason state inspections require checking the cap yearly.

Punch a hole in the cap, defeat or remove the vented seal, and you're just inviting fuel-thriving bacteria into your tank each time the fuel cools and contracts - which is each time you shut the engine off.

The more humid the weather, the bigger the party.

Word up, folks..............

So a 1-2psi whoooosh is ok? Just not a 3 psi whooosh. Am I reading this correctly?

2"hg = 1psi, so 2" vacuum or 1psi pressure - it is the volume of an empty tank that confuses the issue.

You can check the cap with an automotive pressure\vacuum hand pump.

Yes, some 'whoosh' is acceptable - Winter it will be inward, Summer it will be outward.

:clap: