fuel tank vacume

[2Don2]

1994 3/4 Suburban
How much is normal?
what would be to much?

I have a lot of sucking noise when I open my fuel cap.

My friend has a 6.5 as well and his sucks smile.gif evan more than mine. He has a problem that he runs out of fuel with the fuel guage reading 1/2 full and the tank is half full. We know this because it only takes half a tank to fill it and then he can drive on. This is getting progressivly worse and I do not want the same Problem.

any comments would be nice.

Don

[catmandoo]

it should have pressure not a vacuum,replace the cap.the pressure helps the lift pump.

[gmctd]

Warm fuel exposed to cold ambient will contract, creating some vacuum in the tank(s).

Catmandoo is correct - the fuel cap is vented for pressure and vacuum, with vacuum set to vent at less than 1" wc.

As the fuel contracts, increasing vacuum will draw in whatever is on the cap vent - snow, mud, whatever - which eventually plugs the vacuum relief.

Replace the cap regularly under extreme enviromental conditions.
And check that the replacement is for Diesel service.

[patrick m.]

Don,
If your friend runs out of fuel at half tank, then he has a tank unit/pick-up problem.
the tube that extends from the top of the tank to the bottom to pick up the fuel must have a leak, allowing the fuel sys to draw in air instead of fuel.
If this is the problem, i would think he would have a difficult time getting it started after re-fueling, because that is a lot of air to purge from the system.

[captainvector]

There is another possible cause for your friend with the half-tank problem. If the fuel cap vent is plugged, the resulting vacuum in the fuel tank can actually cause the tank to collapse. When this happened on my truck, it would quit from fuel starvation when the gauge read 'half'. When I replaced the tank, I found that the baffles inside the tank had mangled the pickup tube, requiring its replacement as well. I had similar symptoms: There was a strong hissing noise when I opened the fuel cap, and it became difficult to remove. The truck would quit when the fuel gauge read about half, but it would only take about 11 or 12 gallons to fill (34 gallon tank). I ended up also having to replace the fuel lift pump. The quick and dirty fix is to remove the large rubber o-ring (gasket) on the fuel cap. This should keep the tank from developing a vacuum.

Good luck!

[Polar Diesel]

my 93 always has some positive pressure when you open it up.

[norm]

My tank almost always has slight vacuum when opened. Standard replacement fuel caps seem to be same application for gas or diesel. When new, before exposed to fuel, I tested the venting of the cap and it does work to vent in, so I think unless it malfunctions, the tank will live. It is interesting to me that some folks have positive pressure in the tank - where is it coming from, except just increase in ambient temp? (as diesel is not as volatile as gas)

[gmctd]

Diesel fuel foams when warm. The injection system returns excess fuel warmed by the engine, to the tank.
Foam is compressible, solid fuel is not.
Foam also reacts more quickly to warmer ambient temps.

Result - tank pressure. Not much, as the cap also vents above a certain pressure.
And, even minimal tank pressure aids the lift pump in it's task.

Large volume at low pressure can seem like much higher pressure, since it takes a while to vent that volume.

Science 101, right? ;)

[DBlake]

Well it is not the Diesel that is expanding and contracting but the air in the tank. Also, as you use Diesel, you do need to replace the volume with something or the volume reduces (collapsed tank). This vacuum/pressure thing can become a major concernd out west where one can drive a few miles and drop or raise several thousand feet :eek: I had a minor problem and replaced the fuel cap with one from Pep Boys. The one I have now you back off a quarter turn after tightning to put it in it's vent mode. When I am not going to drive the truck for a week or two I leave it in the non vent mode to prevent tank breathing with weather changes and moisture build-up in the tank.

[gmctd]

Correct - and which directly relates to fuel temperature and quantity of fuel in tank volume.

Guess I'll need to check those new fuel caps out. Remember hearing about them, but never got around to looking at one.

Related an incident, where I had an absolutely pristine fuel tank for a project. Detailed it a little, closed it with a non-venting cap, placed it out of harm's way.
Had one of those winter weather excursions here, where temperatures can drop 40deg in an hour - 70's to 30's, that time.
Heard a strange noise similar to a paint can contracting, but wierder, went to investigate, and that fuel tank was but a caricature of it's former self.

Talk about the slings and arrows of outrageous fortune.