Fuel Tank Vacuum

[Spindrift]

Over the holiday weekend we took the truck on its first, long trip. During fuel stops I noticed one big difference between the new diesel and my old Burban which was a gasser. When I opened the fuel cap, I heard a whoosh of air as it was entering the fuel tank; very similar to the sound that you would hear when you first puncture a fresh can of coffee. This happened with the old gasser, but only on extremely hot days. Is this normal?

[ingringr]

Spindrift,

I would be interested in the answer to your question also. Couple of months after I bought my 94 GMC Suburban 6.5TD in Spring this year I ran into the stalling problem. One of the things I noticed when attempting to diagnose the problem was the apparently strong vacuum that had developed in the tank. Eventually replaced the FSD with extension through the firewall onto with 12 volt thermostat activated fan, which fixed the stalling problem beautifully.

Interestingly as I read your posting I realized that the vacuum in the fuel tank seems to have diminished also!!

[catmandoo]

you sure it wasn't pressure instead of vacuum?mine always has pressure,

[jlog]

I think catmando is right, mine always has pressure also. It would make it difficult on the fuel pumps if the tank was developing a vacuum. The danger would also exist that the tank could implode.
john

[Spindrift]

What special properties exist in this system that would result in increasing fuel tank pressure?

[ingringr]

Mine was definitely vacuum, but not since replacing the FSD.. I thought at the time there might be problem with the fuel cap not breathing properly but I am now at a loss to explain it. My lift pump is powered with the ingition after being rewired by previous owner who had problems with a sensor....

[Jim P]

Mine definitely has vacuum. I was having trouble with mine stalling right after I would start it in the morning. When I run the fuel cap a few turns loose. it don't do it.

I read about someone that had a tank implode. After the tank collapsed it would only hold something like 19 gals and I think it originally held 34gals.

[turbovair]

I took an Xacto knife and made a little v-notch in the rubber seal of the cap where it contacts the filler neck. Drove it for a day, and opened the cap. Since it still had vacuum (whooshed), I deepened the notch ever so slightly and repeated. The vacuum went away. Been running like that for almost year now. The notch is so small I'm not worried about rain/water getting in with the flap closed. The truck runs fine, and no chance of tank implosion.

[bobt]

Turbovair, Is that the cap you have tested when you go in for emissions testing?
Bob T

[kowsoc]

There is a little spring loaded valve inside the fuel cap. I disabled mine...very easy to do. Any tank vacuum makes the lift pump work harder for no reason. We don't have emmision testing up here yet! :D

[jlog]

if you have a vacuum your fuel cap is not working properly. the valve in the fuel cap is designed to allow air into the tank but not out.
It is very easy to check using JK's lip lock test.
john

[Spindrift]

o.k. I'm afraid to ask and I'm know I'm setting myself up, but I'll ask anyway. What is the "lip lock test"?

[turbovair]

BobT,
Being from Tucson, obviously you are familiar with emissions testing here in Phoenix.I plan on getting another cap for emissions purpose.I just thought that was easier than monkeying with the guts(which a lot of people do)

[moondoggie]

Good Day!

Try looking at Fuel cap causing staling after re-fueling? (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=004960#000002)

I found it using the "search" feature, the most under-used feature of the Page.

Blessings!

Brian Johnson, #5044

[BuffaloGuy]

I had vacuum and funky fuel pressure readings at startup. Took the smallest drill in my set and drilled from the outside into the check valve then flipped in over and drilled through the check valve. I can see a little light thru it now. No more problems. It's a pretty small hole so I'm not expecting any leaks but if I get some THEN I'll replace the cap.

[gmctd]

Spindrift and ingringr

Check out the post Fuel Cap Hiss - use the "SEARCH" function at top of page.
Should give you more options than destroying your fuel cap.
Some vacuum is normal in cold weather, some pressure is normal in hot weather - it's all based in physics.

[johnny2can]

I have experienced "vacuum in the tank" when I take the cap off. After reading this and other referenced posts, decided to verify that it was vacuum and to do the "lip lock test".

Now that my full attention was on removing the cap, I discovered that there is actually about 1/3 to 1/2 a turn of the cap before I actually start unscrewing the threads. It's during this rotation that that the vacuum (now verified as such) is relieved, BEFORE the cap starts unthreading.

I'm in the habit of screwing on the cap until the ratchet starts making noise and leaving it there, but now I'm suspecting that the cap is designed to be "backed off" (without unscrewing the threads at all) after being tightened, to allow vacuum relief.

Looking for thoughts/experience/validation/wisdom... :confused:

[johnny2can]

oh, and because of my discovery, I opted out of the "lip lock test"... :D

[Barry Nave]

I also have had vac. when removing. Did the test and can draw air, though it seem to take a good suck :D
Can hear the vavle so the spring must be strong.
Better mod mine in some way also. Truck has been like this sence day one when I got it some 6 yrs. ago.
Cap seems to be design to allow some vac. to a given point unless my cap is not working right. :confused:
Always thought fluid flow better under a little Vacuum,also vacuum removes and prevents mosture.

[ 11-20-2004, 08:38 AM: Message edited by: Bnave95 ]

[DickWells]

Well, I did it again. Wrote most of a whole reply, and then hit a wrong key, lost it, and can't get it back!
Hope these both don't show up, posted.
This time I'm just going to make it in the form of bullets.
*Been running for 2-3 years with cap loose.
*Confirmed real vacuum, many times.
*Too lazy, cheap, old and forgetful to change caps.
*Cap valve confirmed to function by dis-assembly AND by lip-lock method.
*Agree that pressure, not vacuum should be present. All that hot by-pass fuel from the injector bleed offs returning to the tank.
*BUT. In a 42 gal. tank, running 5-6 hours non-stop,towing, in heat, the lift pump pulls out maybe 15-20+ gal. If your cap is sealed, that means vacuum.
*Vacuum will over work the lifp pump.
*I once sucked a Fiat tank down from 14? gallons to maybe 8 because the great factory people plumbed it backwards! I re-plumbed it and blew it back out with a compressor.
*I used to run with the cap loose on that one, too.
*I blast the heck out of the filler cap area when I wash the Sub. Yet, I get typically, just a few drops of water from the RayCor filter when I drain it. Conclude that my loose cap isn't hurting me too baddly.
*I may never know why my cap checks good, yet will let a vacuum build in my tank!
No wonder I can't sleep!
Good luck,guys.
Dick Wells

[norm]

Just a chime-in on this business of caps and vacuum:
I wondered why it seemed my tank had some vacuum, so I got a standard replacement Stant cap (listing included diesels). Still had some vacuum, so I ordered a GM part. It seemed difficult for the parts guy to figure it out due to so many caps listed for various set-ups, but he got me one and it seems to work the same as the Stant. The "suck" test requires about the same amount to break the vacuum hold in the cap mechanism. My guess is this is "within acceptable parameters" so to speak, but like many of you I am not too happy about requiring my lift pump to pull against any vacuum.
Unlike a tank filled with volatile gasoline, there is just no way to displace 30+ gallons of liquid with air without pulling a vacuum on the cap. If you fill up your tank before heading out for 350 miles there is no vapor (air) present to be warmed and expanded by the warming fuel. I don't think diesel can produce that kind of vapor, so it's got to be air from cap. Does anyone know a design parameter for this? Has anyone seen an imploded tank with a properly functioning fuel cap in place? Maybe the lift pumps are designed to pull against the max vacuum allowed by the caps.

[charliepeterson]

Has anyone ever tried running the truck off a bucket of fuel? This putting the feed and return in the same bucket. If you do you might see very small bubbles when you bring the RPM's up. Big bubbles is no good because your sucking air from some where in between. The bubbles are from the injection process? Hot fuel coming back to the tank from the injectors has to create some pressure?
I think it's pressure.

[norm]

The hot fuel returning would create pressure presumably by heating the air in the tank and expanding it. It seems to me that could happen say when tank is half full - fuel heats up, and expands the air. But as you use the fuel, the displaced volume becomes greater, and as soon as the fuel cools a bit you are heading into a vacuum situation, which is probably normal and expected due to the fact that the caps do release after some vacuum has developed.

??