vented or non-vented fuel cap?

[Perry]

I went to our local autoparts store today to buy a locking fuel cap for the Suburban. Noticed that they had two models for our GM diesel trucks, a vented and non-vented cap.

Which is correct?

[Barry Nave]

The OE cap has a spring that will allow the tank to hold some vaccum. The cap won't vent to the out side air but will let air in. Some have posted that they have had a psi build up in there tank and have drilled a hole.
When you remove your cap do you notice a small amount of vaccum? My system has always had a vaccum.
I'd say a non-vent but it must be able to allow air to inter tank.

[Hubert]

The diesel cap has a checkvalve in it. It should NOT ever build any real vaccum. The woosh sound is positive pressure form the warm diesel recirculating and heating the cold fuel and air in the tank (warm it expands). Positive pressure in the tank is good it helps the LP get fuel to the IP. A vaccum will be an added strain on a weak designed LP. You want as much throughput fuel to IP to recirculate on a DS style pump as you can get with stock components.


You should get the vented cap vs the non vented.

on edit:
I guess it depends on what they mean by "vented". Like above post it has to be able to vent inward to tank. It does not have to vent out to atmosphere. I don't think venting out to atmosphere is necessarily bad for the truck maybe for the atmosphere though.

[ 01-20-2006, 05:35 AM: Message edited by: Hubert ]

[Dvldog 8793]

Howdy
I have had 2 gassers that would not run properly due to fualty gas caps. These were aftermarket caps that were listed in the book for my application. My advice would be to stick with a OEM cap and put a locking fuel tank door on the truck. As a side note, if someone really wants to steel your fuel they will punch a hole in the bottom and drain the tank into boat cans. ;)
L8r
Conley

[More Power]

These vented diesel fuel caps are supposed to limit pressure & vacuum to no more than 2 psi & 2".

Aside from the affects on the fuel injection system, not venting both pressure & vacuum have resulted in either ruptured seams/hoses/fuel leaks or collapsed tanks.

Gasoline systems have been running non-vented caps for quite a while, due to the emissions requirements. In these systems, the tank incorporates a third vent line that is routed to a cannister inside the engine compartment that is designed to capture and recycle gases formed in the fuel system.

Jim

[Perry]

My Subruban has never had a whooshing sound when opening the fuel cap until last week when I was stranded at the ski slopes. As part of trying to keep the rig running, I opened the fuel cap, and a there was a big whoosh...but even with the cap barely on, it still wouldn't run, until the next day after it was towed to the dealer who claimed I had the failing PMD...

Since its still currently running with the cap loose, I figured I could spend $12 on a new cap and see if that solves my problem.

I would prefer an OEM cap, but living in an urban area and parking outside, I need a locked cap to keep the honest people honest, and keep the bored kids from doing something stupid.

[jspringator]

Mine always has the whoosh sound. I believe it is pressure, rather than vacuum. I can feel air coming out of the tank with my hand.

[Hubert]

Dang, while I am at I guess I better rethink my positive pressure is good - NOT . I forgot about positive pressure in tank would also push on fuel return line too. So no gain in circulation.

I know if you suck and blow on the tank side of the cap it acts like a scuba regulator with a plugged exhaust. I guess I can't blow 2 psi but can pull 2" vaccum. IIRC 2 psi roughly equal to 55.4" of water column. So it favors a little positive pressure but but not a full air tight checkvalve.

[Cowracer]

I simply never screw my cap down tight.

Seeing how its behind a door, I just tread it in a turn or so.

Tim

[Barry Nave]

My whoosh is for sure a little vaccum. Runing a full tank down to a 1/4,removing cap will suck it to the filler untill whoosh is done. This has always been like this sence 98.This subject was also talked about years ago.
The test is as above, I can suck air through cap but can not blow air through it.

[DmaxMaverick]

If your fuel cap allows any significant vacuum, it's the wrong cap. You should be having pressure, if anything.

Vacuum will cause issues with the lift and injection pump over time. It will seek out any small leak you may have and cause starting issues, even air lock in the extreme. A small leak will suck in air even if it won't leak fuel out.

My '85 since new would always have considerable pressure in the tank. I spin the cap with one hand, and catch it at arms length with the other hand (looks like a circus act). My fuel system always seemed happy with that.

[Barry Nave]

So after 9 yrs. with truck having vaccum I should be looking into a newer cap. Don't have to catch my cap. I have to pull it off.
Maybe I'll just get the drill out ;)

[ToddMeister]

My 95 had a vacuum when removing the filler cap, I just drilled a small 1/16" hole in it. No more vacuum.

[More Power]

Diesel fuel expands and contracts with temperature variations. A full tank of warm fuel that cools overnight will pull a vacuum. More fuel and with a wider temperature differential will pull the most vacuum.

Likewise, warming a cold tank of fuel will create pressure. Spring & fall produce the largest swings in temperature.

I had a fuel cap on a 1994 6.5TD once that would cause the injector fuel return hoses to leak fuel about mid afternoon - after sitting in the garage all day.

Jim

[JohnC]

Originally posted by More Power:
Diesel fuel expands and contracts with temperature variations.The air in the tank expands and contracts with temperature also, and the coefficient of expansion of air is greater than fuel. This is why the tank can build pressure while the engine is running. Even though fuel is being drawn out, the air is getting heated by the return fuel and expands more than enough to make up the difference.

[garre1tt]

My 97 had a collapsed fuel tank when I bought it or in the drive home from Texas to Montana. Lived most its life in Colorado. Replaced the fuel cap and expanded the tank and have not had any problem since.
Michael D