faulty fuel gauge and excessive pressure

[coxfmly]

Well it happened, i ran out of fuel for the first time in 20 yrs, drr!!
i was running at 1/4 tank "per the gauge" when the truck stumbled and stopped. coasted to the side of the freeway and started checking things out. Installed my backup PMD, checked my lift pump and gave it a once over, still dead. called AAA and waited 2 hrs for a tow!

Dropped it off at the shop and decided to double check everything for a final time. I had fuel flowing out of the pit-cock for a moment then the lift pump went quiet? hmm, that was strange so I opened the gas cap and an excessive amount of air blew out. when i checked the gauge again it was beyond the empty mark?

I always run 500 miles between fill-ups and usually put 30 to 32 gallons in. I loaned the truck out and evidently it wasn't topped off so my numbers were off, my bad!

Question: how much back pressure should the tank have when i pull the cap? It seems like the tank isn't venting which is causing a vapor lock condition. I was out of fuel this time but it has had excessive pressure before. is their a EVAP sensor or switch that can fail? looks like i will be using the odometer until I can get a new fuel gauge.

Thanks.....

[DmaxMaverick]

No evap sensors on the Diesels. Replace the cap (cheap, make sure it's a "Diesel Only" cap). It should be venting any "excessive" pressure, and vacuum. If you are using a gasser cap, it will cause fuel system issues, including incorrect pressure venting. However, 2 PSI (vent relief pressure) on an empty tank is a lot of volume to vent.

[greatwhite]

The cap should hold a small vacuum on the tank by design. Diesel doesn't really need a high vacuum like gassers do (gasoline vapor control) as it "offgasses" much less than gasoline.

You should get a small "shh" when you remove the cap if it's functioning properly.

If you get a big "SHHHHHHHH" or if you get the sound of a metal "kerkunk" following the shhh when the cap is removed then the vacuum is definitely excessive.

The cap vent may have failed, or as DMax mentioned, you may have the wrong cap on the filler neck.

[DmaxMaverick]

The Diesel cap vents both ways, and maintains a max of about 2 PSI pressure or 2" Hg vacuum. Gasser caps only vent in. Gasoline is highly volatile, Diesel is barely. A little vacuum, add a little heat, and gas won't stay liquid.

[coxfmly]

Its definitely a big "SHHHHHHH"...Looks like its time for another cap.
I will let you know how things turn out. Thanks..

[coxfmly]

installed a Napa cap that says "diesel fuel only". Not that it matters but it might say they were thinking about the specs specifically for a diesel. Yea, probably not but it looks fancy......drove about 30 minutes and checked the pressure, a slight shhhh, nothing more. Hopefully this fixes things...