Diagnosing air in fuel condition, plastic fuel filters, filter housing assemblies

Recently I had my LB7 MY 2003 engine experience air in the fuel. I live in Phoenix, AZ and the truck is 11 years old, so sealing issues are not a surprise. I talked to a friend who instructed me to perform a very simple test...one that I have not seen on the Diesel Page or elsewhere, so I am posting this for other's benefit.

The Pressure Test: pressurize the fuel system by removing the fuel filler cap and inserting an air line wrapped in a moistened shop rag (water) so that you bring the system up to a reasonable and low pressure...1 or 2 psi...Hint: work your way up by relieving the pressure and observe about a second or so of "Woosh" sound and air and fuel vapor relieving from the tank. This will pressurize the low pressure (vacuum) side of the fuel delivery system. Do this in a well-ventilated area, away from open flames, gas fired appliances, etc. The engine is OFF during this test. ( My specific test condition was using a dual-piston off-road type air compressor to very carefully apply 1 or 2 psi for 30 seconds. I repeated this three times. The leak I found at the bottom of the fuel filter was substantial and easy to spot. See more detail on this below. )

Then relieve the pressure and go under the truck and observe all the connections (you will not be able to see on top of the tank, however). If you see seepage at the connections, then you have found a seal problem. Check also at the fuel filter housing, the fuel filter top and bottom seals, and the few hoses you can see on the engine. You may be able to smell leaking fuel on the engine.

If this process shows no issues, use the GM-specified leak check procedure. Www.alldatadiy.com is a source for online subscription to access the OEM shop manuals along with the TSB's. If your low-side fuel hoses are hard and brittle and show cracks, I suggest you carefully replace them and flush the low side before reconnecting at the injection pump (Bosch CP3). Or have a qualified diesel mechanic perform this before you start removing hoses and performing the OEM vacuum leak checks. Contaminating the system is not an option!

Plastic fuel filters: I discovered a seal problem with the new-style molded plastic fuel filter at the bottom seal with the water in fuel sensor/drain valve. I had previously used NAPA plastic filters (I think Wix makes them) and had no issue. This time, the threads did not seem to match very well, I examined the threads and did not find a cross-thread condition. But, I had difficulty in knowing when to stop tightening the bottom assembly onto the filter. Here is where ONE of the leaks was in the system. I found it with the "pressure test". I tightened the fitting to see about 50% o-ring compression. Issue solved.

Second Note: I also had a leak at the filter housing assembly. This was evidenced by fuel leakage there under normal operating conditions followed by the engine being off for a day. I found a rebuild kit and an excellent video at www.merchant-automotive.com. I suggest anyone who cares to rebuild the unit carefully watch the video. Cleanliness is critical here. Do not over-tighten the two screws that hold the fuel heater to the housing. My leaks were due to two old o-rings that had compressed and took a set, and could not hold the pressure differential.

So by carefully pressurizing the fuel delivery system- that is normally under a vacuum- you can diagnose some leakage issues quite easily.

P. Curry, Mechanical Engineer
with special thanks to Automotive Engineer "Big Al" of Prescott, AZ

Thank you for the input.

This pressurized fuel tank leak test has been repeated here numerous times, but repeating certainly doesn't hurt. Many readers don't dig too far into the history.

However, I do not recommend using a "wet rag" to seal the filler. A much better option, as has been discussed here many times, is using an old (or new -- your choice) fuel cap, drilled with a tire valve stem installed. More simple is more better.... Low pressure, up to 5-10 PSI is acceptable if lower pressure isn't revealing. As you said, it is important to consider the location while doing this, as a leak can spill a lot of fuel in a short time. The fuel cap should be replaced as a regular service item, such as belts, hoses, etc. The vents do fail, and can easily cause misdiagnosed issues.

Also consider, a system that does not leak under pressure, may leak under a vacuum/suction condition. Many fuel system connections are designed for a bias integrity, meaning they are not designed to provide an adequate seal in an opposite environment, or in an extreme of their design. For example, a "normal", correctly operating system may leak with excessive vacuum, as such caused by a plugged fuel filter. While it may be an inconvenient condition, it is a positive indicator of needed service. Any weakness in the system, by design or not, will be exploited by any extreme or contrary condition. This makes system leak diagnosis inconclusive, and often difficult, at best.

In regards to the "plastic" fuel filters, they are hit and miss, regardless of the label. Some seal well, and some do not. I try to avoid them whenever possible. There also exists some reports of questionable quality with the filtering media (specifically the overall surface area), although I have not confirmed this myself.

Put a lift pump on it. The system is now pressurized whenever the engine is running. Your filter(s) will perform better and your fuel system will thank you.