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View Full Version : Is vacuum in the fuel tank normal?


97-6.5TD-F
04-22-2008, 16:40
Mine vacs up pretty good, just wondering if this is by design or if my fuel cap should be venting?
DmaxMaverick
04-22-2008, 17:15
You shouldn't have any noticeable vacuum. A little pressure is normal, but any more than an extremely small amount of vacuum is not. Fuel caps are cheap. Make sure you get the Diesel model. The gasser cap won't vent properly.
Idle_Chatter
04-22-2008, 20:21
Bingo on the fuel cap. The diesel model vents inward only (to break vacuum) and they are prone to failure after a few years of use. They usually run $6 to $10, even at the stealer. Like Mav said, make sure you get the diesel model.
DmaxMaverick
04-22-2008, 20:48
Actually, the Diesel cap vents both ways. The gassers only vent one way.
ronniejoe
04-22-2008, 21:04
Are you sure it's vacuum? And are you sure that the Diesel cap vents both ways?...

I've found my tank pressurized on several occasions. It whooshes when you remove the cap...may be mistaken for vacuum...but if you crack a line to, say, replace a lift pump without loosening the cap, it will pressure feed fuel all over the place.

Been there, done that.
DmaxMaverick
04-22-2008, 21:14
The cap should vent at about 2" vacuum, or 2 PSI pressure. 2 PSI is a significant amount of air with a low tank level. My 85 will launch the cap out of my hand if it's near empty in the summer. Low pressure, high volume. The tank is 36 gallons. The caps rarely fail on the pressure vent, but a vacuum vent failure is common. Diesel fuel caps must vent both ways because of the fuel return, which gassers lack. The EPA does not allow gas to vent to atmosphere.
a5150nut
04-22-2008, 21:38
With current fuel prices I'd be thinking about a locking cap.................
DmaxMaverick
04-22-2008, 21:44
With current fuel prices I'd be thinking about a locking cap.................

No doubt!!
97-6.5TD-F
04-22-2008, 22:33
Yeah its for sure a vacuum, and is alot worse when I stop to fuel up on a long highway drive.

So the stock cap is designed to vent at 2"? Thats a fair amount considering the volume in the tank when low. When I crack it it sucks for about 3-4 seconds if only cracked very slightly.

Gas fuel caps are 100% sealed I believe, as the Evap system is designed to do all the venting to prevent hydrocarbon emissions (silly gas pots!haha).

As far as I know all GM gassers have fuel return lines, also.