Finding a Fuel or Air leak

A modified fuel cap can be used to easily pressurize a fuel tank and fuel supply system, looking for an air leak or fuel leak.

http://www.thedieselpageforums.com/p.../lyonsbcsm.jpg

Member Bob Lyons sent this photo to me a while back that illustrates how to modify an old diesel fuel cap with an air fitting, allowing an easy way to test the fuel tank and fuel supply system for either a fuel leak or an air leak.

It's important to not use more than 5 to 10 psi pressure. More pressure could pose a risk to the fuel tank, in that it may rupture when subjected to excessive pressure.

If the tank, fuel lines or fuel filter assembly has a leak, you'll see an oozing fuel leak at the leak site when the tank is under pressure. Take your time, allow a few minutes to pass without evedence of a leak before declaring the fuel system leak-free, if a leak cannot be located.

A test cap like this would be essential for a diesel shop or anyone who is perplexed by a stalling/hard to start problem.

Jim

Excellent tip! I'm putting this one in the "memory vault"... I just hope I can get it out when I need it... :rolleyes:

Am I correct in assuming that by pressurizing the tank if air is leaking then that is my problem? It sounds like there is air leaking from the top of the tank but I cannot see anything. It looks like I have some stains on the side of the tank that I assume is from fuel leaking when the tank is full. I am losing prime at the filter on a stock LB7 02 Sierra. Any input would be appreciated.

The earlier Dmax trucks had an actual vent tube that terminated in a pigtail behind the heat shield alongside the tank.

My preferred method is to add a lift pump as it will keep the system pressurized while you drive.

I was looking at the pump/filter combo on your web site. I might install this after I fix the immediate problem. I assume that this vent tube will keep me from pressurizing the filter assembly. Can you recommend a good place to break the fuel line for pressure testing?

I prefer to leave the pumps stand alone and run the original Mega under the air box tray. My point is that the lift pump will help you fix the immediate problem. It will put the system under constant light pressure and if there is a significant leak it will weep.

The OE sender is plastic and cannot corrode. The oring seats plastic to plastic so that leaves the lines at the tank. Since the nipples are plastic these too seldom corrode. Where the corrosion starts is the first joint to steel line immediately behind the fuel cooler.