My '94 Blazer 6.5 with 256,000 miles stalls, when I unscrew the top of the fuel filter at the engine I get a hiss of air escaping, I close it and it starts right up, then does it again about 1/8th of a mile down the road,
I took it to a shop thinking the lift pump was sucking air at the tank somewhere since I can't find a leak anywhere else, it was there a week and never did it, then after 2 days of local driving it did it 3 times, the last time it would not start back up, but just cranking it pressurizes the filter,
lift pump and oil pressure switch were replaced last summer, I read the sock thread but it doesn't explain the air getting in the system, I did a search for air pressure fuel line but nothing seemed to match, I called for a rollback and it's at the shop again.
Ideas ?, Manuel

if you got air at the filter then you should bleed filter with liftpump running till all air is gone, if it persist then you got a restriction in the line pre liftpump {plugged sock} or a pinhole somewhere in the line from the tank intake to the liftpump, there is a short hose between top of tank and line in frame rail, could have cracks in it due to age
also check filler cap check valve. there should never be any vacuum in the tank period.

I just went thru this same problem. My fuel lines on top of the tank were rusty and bad.
A word of advice, if this is your problem, purchase a new spectra fuel sending/pickup from a parts store $60. Use compression fittings and steel brake lines to eliminate the rubber hose and make it all hard lines. I tried to use rubber hose and hose clamps, I had the tank out three times and still ended up with the above mentioned fix. It would have been much less painful if I would have just done it that way to start with.

Make sure the filter is seated right. My experience has been that the filter gasket are not sealing if it is not seated right. Challenge is you have to do right first time otherwise the gasket is stretched and you need a new one which is unavailable unless you get a whole new filter.

Which filter gasket ?.

Fuel sock in tank, sounds like a starvation problem rather than sucking air. The hiss could be vacuum not pressure. My truck did that until I change the sending unit and sock, would have done just the sock, but I kinked the lines when I removed the tank. Word of advice: make sure tank is right empty and you have wire or rope supporting the tank when you loosen the tank straps. When you have the bleeder open, the fuel should almost squirt out the top when the lift pump is running. Just a thought.

Which filter gasket ?.

Gasket should attach to the fuel filter.

It's pressure for sure, but when I vent the bleeder on top of the filter I don't get fuel flowing unless I turn the engine over,

If the filter gasket were not seated wouldn't I get leaking ?,

It's pressure for sure, but when I vent the bleeder on top of the filter I don't get fuel flowing unless I turn the engine over,

If the filter gasket were not seated wouldn't I get leaking ?,
if the filter gasket was leaking you would have fuel leaking too.

my first post and radrick's are your best bet me thinks

If there is no leak on the fuel filter, then there is no leak there. I just want you to check there since that is the easiest to see. In my experience, that is another trouble spot. Also, make sure that the LP/OPS system are working. The socks are a lot more difficult to do but it is a possibilities. My thought is always looking for the obvious first before getting to a more difficult.

It's pressure for sure, but when I vent the bleeder on top of the filter I don't get fuel flowing unless I turn the engine over,


If you don't get fuel coming out vent with engine running, then your ops is bad again. You should be able to open the vent on a running engine and get fuel to come out, or air in your case. This wouldn't explain the air coming out, but just something to note. But My lines between tank and lift pump didn't look horrible, but was causing me the same problem. Napa carries the fittings for lift pump on a small section of tube, I used those and some rubber hose. Not best looking, but a cheap fix.

Disconnect the fuel line near the back of the truck and blow compressed air (low pressure) backwards toward the tank. Make sure the fuel cap is off. I'll bet your tank sock is clogged.

I had a 94 suburban the other day with the same problem. Come to find out the fuel cap was not venting. It collapsed the tank and crushed the sender. So at 1/2 tank the eng would run out of fuel..

I vote for fuel line leak between pickup sock and lift pump. The ONLY way you are going to get pressurized air to the top of the fuel filter is if air is drawn in under a vacuum. The only area that exists as such in a properly working fuel system is between lift pump and sock.

If the sock was plugged, and properly submerged in the tank, there is no opportunity for it to pickup air, even if plugged.

Also listed in FAQ's. Here:
http://www.dieselplace.com/forum/showthread.php?t=103965