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6.5L Diesel Engine Discuss the 6.5 GM diesel engine & associated components. Automatic transmission questions & problems belong in the 4L80/85 - 4L60E - 6L90 Transmission Forum

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Old 08-26-2005, 10:15 PM   #1 (permalink)
knkreb
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Reference Material: Fuel Cap

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.

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Old 11-25-2005, 09:39 PM   #2 (permalink)
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where is a goodplace to find thew proper fuel cap?

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Old 11-25-2005, 10:27 PM   #3 (permalink)
knkreb
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NAPA sells some, dealership may have them too. Just a few ideas.
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Old 11-28-2005, 06:38 AM   #4 (permalink)
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Old 02-06-2006, 08:50 PM   #5 (permalink)
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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?
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Old 02-06-2006, 08:58 PM   #6 (permalink)
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Quote:
Originally Posted by knkreb View Post
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.
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Old 02-06-2006, 09:16 PM   #7 (permalink)
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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.
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Old 02-06-2006, 09:25 PM   #8 (permalink)
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think i could just take the spring out of the new cap?
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Old 02-06-2006, 09:29 PM   #9 (permalink)
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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.
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Old 02-06-2006, 09:30 PM   #10 (permalink)
ChevyDave
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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.

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