fuel tank vacuum

[maverick_935]

98 Gmc 2500 6.5 turbo

Excessive vacuum in fuel tank when cap removed. If cap left tight then truck will randomly shut down. If left loose seems to be better. New cap installed and still continues.

Any thoughts

[DmaxMaverick]

Make sure you replaced the cap with a "Diesel Only" cap. A gasser cap will cause excessive tank vacuum (which is expected on a gasser). If you have the correct cap, and it works properly, it will manage tank vacuum/pressure to a barely noticeable (if at all) level. I suggest trying another cap.

[maverick_935]

cap is an only diesel only cap and still stalling. trying to find a spare pmd

[DmaxMaverick]

A bad new cap out of the box wouldn't be out of the question. It happens from time to time, like with any part. You shouldn't still have excessive tank vacuum with a good cap, under any condition.

Also check your fuel return line for inclusion, from the IP to the tank. If it's blocked, it can cause excessive tank vacuum (if the cap isn't venting), and cause stalling once the return pressure rises.

[arveetek]

I'm experiencing similar problems. New 'diesel fuel only' green cap, still have vacuum in tank, periodic stumbling and stalling due to air getting in the fuel filter. I can bleed the fuel filter and everything is fine, but air appears in the fuel filter once again.

I still haven't figured my issue out yet.

Would a return line restriction cause air to appear in the fuel filter?

Casey

[Kennedy]

It's really easy to test your cap. A bit crappy tasting, but simple. Just wipe it and give it a "lip lock" then gently suck and blow. You should find that it allows air in freely, but resists pressure.

[arveetek]

Thanks for the tip, John! I actually did that test BEFORE installing the cap when I bought it!! I'm glad I was doing it right!

Casey

[DmaxMaverick]

If the fuel cap is good, what's holding the vacuum in the tank? There should be none to speak of, regardless of other problems that may exist. If you are seeing air building in the filter, you have a leak, and it's between the filter and lift pump (and possibly the tank) if the lift pump is not keeping up. It's between the lift pump and tank if the lift pump is working properly. These are facts.