Vacum in fuel tank/hard to start

[ronale]

When I opens the fuel tank cap, I have to use "power" to pull cap out due to much vacum in the fuel tank. Is this normal? I have replaced the cap with a new one, but no change. Could this vacum suck the fuel lines empty? My problem is that I always have to crank more than normal to start(cold or warm). The lift pump is not pressure checked but delivers 0,5l/15secs.
Injection pump was replaced (gm warranty)two years ago but no improvements. Any suggestions?

[Hubert]

Did you use a GM diesel cap? The OE cap has a valve in it to allow a little pressure or little vaccum. Something like 2psi pressure and a couple inches Hg vaccum. Should not require a lot of force to open it. An incorrect fuel cap can cause fuel delivery problems.

Need more info on starting. Its been long cranking for 2 yrs? Miles on engine, injectors, glowplugs, health of batteries, starter, timing chain and timing setting etc. - grounds and voltage at ECM and FSD; all contribute to starting could be anything or nothing.

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[gmctd]

Diesel fuel cap is vented for 1psi and 2"hg vacuum - 2"hg is equivalent to 1psi.

Full tank has about 1gal of volume above the fuel level, empty tank has about 25gal volume above the fuel level.

The volume of the pressure or vacuum changes with fuel level, but not the numerical value, if fuel cap is functional.

Diesel fuel caps are not the same as Gas caps - make sure your fuel cap is for Diesel service.

Winter temps - warm fuel out of the ground into a cold tank will contract, creating vacuum in the tank.

Summer temps - cool fuel out of the ground into a hot tank expands, creating pressure in the tank.

Warm fuel from the return line exacerbates the situation.

[ronale]

To be more precise about cranking: I think normal cranking is a couple of secs, I use 4-5 secs to start engine (warm or cold). My former 6,2L started by a touch of the key while warm.
It has been long cranking for 8 years.The cap is GM diesel. Kilometres on engine is 163K (100K miles), injectors have never been tested, glowplugs are original and has been tested for continuity, all ok, but all will be replaced soon. Filters have been replaced at service intervals. Batteries are in good condition, starter is original. I have read somewhere that some original starters has low rpm, and by replacing starter - no more starting problems.

gmctd-
There is most vacuum at low fuel level, no change in summer or winter.

Thanks.

[gmctd]

If you still have the original fuel cap, install a hose barb or schrader valve in it, attach a vacuum gage, and check the actual reduction in pressure - should not be greater than 2"hg, full or empty.

If Baro is 14.7psia, in-tank absolute pressure would be 13.7psia, or 2"hg vacuum

What about the battery cable terminals at the batteries, and the opposite ends at the engine grounds and starter solenoid.

Turn the key ON, measure the voltage on the ESO solenoid, there on top of the Inj Pump - should be at or above +12.6vdc, referenced to the engine block ground, and at the negative batt terminal