Blazer fuel tank pressure

[Ebrown]

1986 K-5 with 6.2: Here is the question I stopped to fuel up today and when I was taking the fuel cap off my truck I heard a Vac / Pressure as the cap came off. Should I hear this noise?

[Warren96]

Thats normal.Mine has done that since the day I bought it 12 years ago.

[Ebrown]

It seams that when the vac / pressure is gone my truck seams to run better!

[DmaxMaverick]

A little pressure is normal, but there should be no noticeable vacuum. Vacuum will cause a laboring fuel pump, and less fuel supply. If you have too much pressure, it can effect the injector timing, and likely fuel leaks at the return lines. If it in fact runs better when you relieve the pressure/vacuum, replace the cap, and make sure it is for "Diesel". The gasser caps will cause issues with tank vacuum on the 6.2/6.5 engines. Could the cap you have now be for a gasser?

[Ebrown]

I will check the fuel cap today. I'm running a 6.5 electric lift pump and no mechanical fuel pump.