Sucking Air/Fuel

[SteveMc]

The Suburban has been surging for the past several months and getting slowly worse. Have found only the smallest of bubbles with a clear line between the filter and the I/P. And then only occsionaly. There could be more bubbles that these old eyes are not seeing.

I am about to get some fuel line and change out all the flex lines in the steel fuel line from the tank to the manual pump. I think there are three places. I have looked them all over and see no signs of leakage. I would think if it sucks air it should leak fuel.

How many have been thru this before? Any tricks or sugestions?

Thanks---SteveMc

[BobND]

SLIGHTLY pressurize your fuel tank. When I was hunting down problems like this, I pried the top metal cap off of a spare fuel cap, gutted out what was left, and made a hole in the top of the plastic screw-in part with a Dremel that was the same size as in a tire rim, so I could pull a tubeless tire valvestem up from the inside of the threaded area.

You can then screw what's left of the cap into the filler neck, and CAREFULLY, SLIGHTLY preseurize the tank with a couple of reasonable bursts from an air chuck.

With slight presure in the tank, anything that leaks air INTO the suction line is gonna drizzle fuel OUT while it's pressurized.

In case you think this is a totally nutty idea, it's straight from a Caterpillar Diesel troubleshooting manual.

[SteveMc]

Not nutty at all. Ive done that on old tractor fuel tanks when finished repairing them. With all the purging to get the old fumes out for soldering you want to be sure the leak is fixed.

I have another fuel cap so may use the idea if changing the old rubber lines doesn.t work. My son's suburban had a leak in the steel line that got so bad it leaked onto the deck and made it easier. Not so Dad's!!

[SteveMc]

I put the clear tubing trick on the I/P outlet and with a loop in it was able to see air coming thru the system. Should have done that months ago. Pressurized the tank a little and went looking for fuel leaks. The only suspect spot so far was right at the fuel pump. Replacing that line did not help. Buying more 5/16 fuel line today and may eliminate some of the steel line to check that. Then its taking down the fuel tank. And I just filled up!

SteveMc

[britannic]

What kind of hose clamps are you using? I had always used the screw type for years, but finally switched to the OEM corbin clips, to stop a stubborn air leak on the suction side of my FP.

I found that the corbin clip resists vibration, and expansion/contraction, much, much better during the hot months in the western states.

[BobND]

I had weird fuel system problems with a 1984 6.2 Sub. I finally dropped the tank. It was CLEAN, and the "sock" was clean, and looked good, as well.

In desperation, I removed the sock, and put the tank back up. It made a world of difference. I think, over the years, the individual threads in the woven sock had swelled up, from continuous exposure to diesel fuel, making the plastic mesh less porous.

I'd trash the sock while I had the tank down, and either replace it, or put an external pre-filter ahead of the lift pump.

I'll be very interested to hear what you find out when you drop the tank.

[SteveMc]

Spend all of Tuesday on this air leak. Got rubber fuel line and put between manual pump and production break in fuel line half way up from the tank. Still had air. Took the tank down and checked the metal tubing, replaced the rubber hose. Then pressurized the fuel line with 25 psi air and bubble tested the line. No sign of leaks. Put tank back up and still had air. Installed electric fuel pump with filter and bypassed the manual pump and lines. Still have air. Tomorrow I plan to put a longer hose on the break in the fuel line right into a container of fuel to see if its in the fuel pickup where I couldn't see. Should have changed the pickup unit while I had it down. Wellll, its always eaiser the second time around.

What fun... SteveMc

[SteveMc]

I found it!!!! First I put a five gallon container of fuel and pumped from the OEM break in the fuel line to eliminate the fuel tank and rear line. Still had air. Then put the five gallon container up front to eliminate the front fuel line. Still had air.

NOw I had thought that sence the filter is pressurised by the lift pump that you would have a leak before you had air. But just to make sure, I put a clear hose between the OEM filter and the hose going to the I/P. (No air showed) coming out of the filter, but I still had air.

Could It be the I/P. I called the rebuild shop and they didn't think so. That left that little piece of hose going from the filter to the I/P pump and it was new. To make sure I ran it up on the filter real good and double clamped it and did the same thing on the I/P Air stopped.

Am going to replace that line with heavy duty line and the clips Brantic mentioned.

SteveMc