You can put 5-10 psi in the fuel tank and you should be able to see where you have a leak. Some common problems are the WIF sensor they crack and may never drip, also the filter housing can spring a leak. I have had both problems, if you use the correct wrench on the WIF sensor and you probably not have that problem again, at least that is what I have found.

 

You can put 5-10 psi in the fuel tank and you should be able to see where you have a leak. Some common problems are the WIF sensor they crack and may never drip, also the filter housing can spring a leak. I have had both problems, if you use the correct wrench on the WIF sensor and you probably not have that problem again, at least that is what I have found.

Great thread and great info... thanks everyone. Can anyone elaborate on how to do this? Can I (or have someone else) just put the air hose into the tank with a rag stuffed around it? Is this safe? Seems harmless but you can never be too cautious around fuel

 

Most likely-
Cracked/Loose WIF Sensor
Cracked/Loose Bleeder
Cracked Filter Head
Loose Filter
Old O-Ring left on filter head
Leaking Heater
Leaking Primer

Could Happen-
Rotted Fuel Lines Under Truck
Rotted/Collapsing Rubber Lines
Pick-up in tank cracked
Leaking Filter Body (Very rare, but I have heard of this, especially with plastic WIX filters)

Remember, you DO NOT have a fuel pump in the tank or under the truck, the fuel system is under a vacuum from the pick-up in the tank to the CP3 injection pump on the front of the motor. This means you will most likely not see fuel leak out at the source of the leak unless the fuel system is pressurized. A trick to pressurizing the system is to take an old fuel cap, tap it with an air line connection, and put an air hose on it. Others have used a tennis ball with a hole drilled in it or even a shop vac blowing into the tank (with a sock/rag on the end to catch debris) to put pressure on the system

 

This morning I checked the fuel filter assembly over well with light, mirror, etc., and could not see/find any obvious cracks or leaks, but did notice a damp spot at the base of the slip on stud for the filter wire located on top of the filter head.

I pressed the manual pump a few times and finally saw a trail of bubbles and then some fuel emerging from around the base of the black insulator surrounding the connector stud where it protrudes out of the top of the filter head. I am assuming this may be where I am loosing some or all vacume which (I think) would allow fuel to drain back.

If this is my problem, is this repairable or would the filter head assembly need to be replaced?

Thanks, Dave

 

FYI; I picked up a new fuel filter assembly from my local GMC dealer ($126)(new filter included) and had it installed and reprimed in an hour or so. Haven't had any trouble since then. Thanks to all for the info and help.

Dave

 

Start then die

This was my problem for almost a year and many trips to the dealer. They even replaced the injection pump (twice) two injectors a fuel rail and then wanted to replace the turbo. They claimed the computer was shutting down the engine due to the turbo not running optimally. I finally replaced the bleeder screw with an aluminum one and the problem was solved.

 

This was my problem for almost a year and many trips to the dealer. They even replaced the injection pump (twice) two injectors a fuel rail and then wanted to replace the turbo. They claimed the computer was shutting down the engine due to the turbo not running optimally. I finally replaced the bleeder screw with an aluminum one and the problem was solved.

I hope all that was under warranty

 

Not under warranty. When I finally picked up the truck (after being assured it was done) and started to drive it, it started with a loud fuel knock .I drove it back to the dealers and they replaced another injector. It had no problems with injectors before the stall situation. I asked for my check back and told him I would see how it ran on the way home (450 miles). It ran fine, but stalled after start a few days later. It would seem that it would only happen when the fuel was hot ( shut off with less than a 1/4 tank and running hard). I replaced the bleeder and saved myself $4200. The clue was the second injector pump didn't solve the problem and they still had trouble with the third.......

 

You can put a air fitting in an old fuel cap. Normally you can just pump the primer and watch and you will see some seepage.

 

My leak was around the base of the small black plastic insulator, left of the manual pump plunger. There is a push on type wire connector that connects to the copper stud sticking up inside that black insulator. Not sure I described that very well, but hope you get the idea.

I pressurized the system by pumping the manual pump until it got hard and then watched closely. Using a headlight, I finally saw the base of the black insulator getting wet and then saw a tiny and barely detectable trail of bubbles forming around the base.

I replaced the entire fuel filter head assembly ($126 at GMC dealer), reprimed/purged the system with bleeder valve and manual pump, and haven't had any problem since.

Hope this helps.