LB7 fuel line collapse

A few months ago I posted a thread whereby I discussed a problem I was having with my 2003 GMC shutting off intermittantly. I took my truck to the shop where the diesel mechanic said my truck was experiencing fuel line collapse that was causing vacuum build up in the supply to the fuel filter. This was evident by noticing that after the truck would act up and shut off, the primer piston would be sucked down due to vacuum build up in the line.

So, the GMC garage replaced all the rubber lines on my truck and off I went. I was ok for about a thousand miles and now I am experiencing the same problem. The truck has shut off several times on me now and each time it is shutting off in a very predictible way. I can run on the freeway for an hour or so at average freeway speeds with no issue. Then, when I exit the freeway, I can go maybe a mile at most----then the truck will shut off. It's done this five or six times now----each time after having run on the freeway for an hour and shutting down following my exiting the freeway. It's beyond frustrating.

When the truck shuts down I had noticed that one of the freshly replaced fuel lines (work performed by a certified GMC garage) on the drivers side of the engine seemed to be flattened out. I would disconnect one end of this line, break the vacuum, reprime the filter and the truck would be off and running for some period of time. However, once I would run on the freeway again for an hour or so, the same problem would happen again following my getting off of the freeway. I would get the truck started again by repeating the process I described above. No fun when the truck shuts down in an area when you cannot pull off to the side of the road.

The line that seems to be the one that is collapsing was a rubber hose that the dealer connected with hose clamps. It had a pretty tight bend in it that caused it to flatten out somewhat (egg shaped inside rather than being round) when it was in use. I replaced this line yesterday morning with a section of braided line that is somewhat stronger and more resistant to collapse. I have not run the truck since I did this replacement.

The GMC garage said that there was no restriction in the line anywhere and that the problem I was having was due to collapsed lines(the truck is a 2003, so the rubber lines were old). However, their replacing the rubber sections did not solve the problem.

Does anybody thing that perhaps the problem was due to the line I replaced with the braided line being just kinked too much due to improper installation? I am half afraid to drive the truck anywhere at this point.

I know, I know, if I put a lift pump on this shouldn't be an issue, but it really shouldn't be an issue anyways with new lines, right? I am fairly disgusted at this point.


Also, does anyone know where I can purchase braided steel lines that I can use to replace ALL of the rubber fuel lines on the vehicle? It seems to me that these would be much less susceptible to collapse.

I am just looking for input regarding this reoccuring problem. It's been a real pain for me as my truck now is totally unreliable insofar as I am concerned. I live in the Los Angeles area and the truck shutting down in the middle of an 8 lane freeway is a hazard that I don't want to deal with anymore. Any thoughts on the matter would be most sincerely appreciated. Thanks.

I looked back thru some of your old posts. Did you ever check the fuel cap or try running with it loose to see if that was the problem? And have you ever replaced the sock in the tank and possible got hold of a gas sock not a diesel sock?

I know what you say about the traffic there. I used to live in Riverside and commuted all over, Mission Viejo, Palm Desert, Torrance, High Desert. I got out of there in 77 and hate when I have to go back.

Yeah, the diesel mechanic at the shop checked the inside of the tank to make sure that there wasn't something unauthorized going on in there----he said it was all okie dokie. Funny you mention the fuel cap---when the truck shut off on me the first time this weekend I had just exited the 10 freeway in Cathedral City. The first thing I did was pull the fuel cap to see if there was any sign of vapor lockage back there, but it was ok..I then checked the little rubber elbow under the driver's door that connects the lengthy piece of steel line from back by the tank to the steel line that runs up to the engine area. It was fine. Then I popped the hood and checked the supply line on the drivers side of the engine...This line had been replaced a few months back by the GMC garage with a regular piece of 1/2 in fuel line (parker hannifin?). That line was sucked flat. I loosened the one hose clamp I could reach with my screw driver and broke the vacuum. Then primed the filter (when I broke the vacuum the primer valve returned to its normal "up" position--it was sucked in due to the vacuum build up) and started the truck like nothing had happened. On my return trip home, I exited the 215 freeway to get on the 74 in Perris when the truck died on me again....Same problem, same fix. It's starting to get a bit old.

Boy, I don't know what to tell you. Other than I don't trust others to work on my vehicles, I do my own work so I know it was done and not just "good enough".

The line you said collapsed, is it diesel rated line or just fuel line?

Maybe Kennedy's fuel pump isn't such a bad idea. . . . .

If the fuel sock is not a problem, the only other item that could cause excessive vacuum is the fuel tank cap...

You must use a "diesel" fuel tank cap. Gas caps are not vented. Diesel caps are vented to keep internal fuel tank pressure between 2-psi positive pressure and 1" vacuum.

Run with the tank cap loose for a couple days to see whether that has an effect on the stalling. If the problem goes away, buy a new GM diesel fuel cap. They're inexpensive.

That OE fuel hose, while not too spectacular really isn't that bad. I routinely remove the first section from the tank outlet when installing lift pumps. It is quite stiff. There is no way a guy with normal hand strength could pinch it, BUT if you do pinch it or kink it there is somewhat of a memory.

The LB7's have a vent hose that is a hard plastic tube exiting the tank at the top towards the rear. This line runs over behind the heat shield where it squiggles and terminates. behind the heat shield. This could be plugged I guess.

Additionally, you can do a simple mouth test with the cap. It should breathe in easily yet resist blow through.

There is no "sock" in the tank, but there is a screen over the bottom of the pickup.

I have seen cases where a "glop of ****" got into the ficm and restricted flow. There are also a pr of butterfly type valves in the filter head that could be giving grief.

Make sure the fuel filter is pushing the plastic ball up into the filter head or better yet take the ball and spring out altogether.

Also a lift pump might not be a bad idea. ;)