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Ask Away! with Jeff Smith: When Troubleshooting Fuel Systems, Keep it Simple

holley sniper efi fuel tank


Jeff Smith: Instead of the usual question from a reader, I thought the following scenario that a friend and I recently suffered through might be of help to car guys working on their machines.

My friend (who shall remain nameless because this could happen to anybody) recently ran into a series of drivability problems on a car he had just completed from the ground up.

Over the course of several phone calls discussing this issue, he eventually worked out the solution but it demanded a more than a couple of weeks of his time and was very frustrating. So I thought that a run-through of what we both learned would be helpful to others who might be caught in a similar situation.

These issues began when my buddy fired up the car and drove it for the first of many, many test drives. The engine was a late model LS engine with EFI controlled by a popular and reliable aftermarket EFI system.

At first, the car ran great. But then, very quickly the car stalled and appeared to be out of fuel. They towed the car back to his shop to discover that the tank still had plenty of fuel. They suspected the EFI system was having some issues but later identified the in-tank fuel pump as the culprit because fuel pressure was dropping to near zero—but only at certain times.

They drained the fuel and removed the tank and replaced the fuel pump with a slightly higher capacity pump.

Once everything was re-assembled, they took the car back out for another test drive only to have the same issue crop up again. What made it even more frustrating was that if the car was allowed to sit for about a half hour or so and cool down, the engine would restart and run fine for about 10 minutes before eventually stumbling and quitting again.

At first, they suspected all kinds of heat-related issues such as vapor lock or perhaps that the inlet tube to the pump was somehow collapsing. The eventual solution came, as it sometimes does, completely by accident. After the engine died again out on the road, they were near a gas station and dead-stick landed at a gas station just to get the car off the road. My buddy decided to add fuel just to be safe and was greeted by a giant whoosh of air when he removed the gas cap.

When he called me to report what had happened, it was immediately obvious to both of us that the tank was not vented. I asked him why he had not vented the tank. His response was that he thought he had by virtue of choosing what he thought was a vented gas cap. The reality was he had instead chosen a screw-on cap for a late model car—as he called it—“a clicker type cap”. These are used for late model cars that employ a sealed fuel system and are not vented.

The solution was easy enough. I only half in jest suggested drilling a half-inch hole through the middle of the cap would solve the problem. He elected instead to find a different cap but also installed a separate vent in the tank to make sure the tank did not suffer from this in the future.

This wasn’t the end of his troubles however. After completing the vent solution, he again ran into trouble with a series of leaking fuel injectors due to debris clogging the injectors. Over the course of several more conversations, I remember asking if he had installed a new fuel filter in the line in between the in-tank fuel pump and the injectors. He had done this but wasn’t exactly sure which filter he had installed. I offered that this is where it can be easy to make a mistake since for gasoline there are two different filter elements that externally appear exactly the same.

All the high performance fuel pump companies like Aeromotive, Holley, Walbro, FUELAB, and others insist that the inlet side of the pump be filtered with a 100 micron filter while downstream, after the pump that a much smaller, more restrictive 10 micron filter be used. A micron is one millionth of a meter—or in decimal form is equal to 0.000039-inch. So a 100 micron filter will allow 10 times larger debris or dirt to flow past compared to a 10 micron filter.

The 100-micron filter is designed to prevent large debris from disabling the fuel pump but the finer, 10-micron filter is designed to filter out dirt or material that might clog up the much smaller flow orifices in the fuel injectors.

The best and only way to tell these filters apart is to look on the end of the filter cap for a marking that should say either 10 or 100. My buddy wasn’t paying attention and installed the 100-micron filter in the downstream portion of the system. He did this because, in his words, “100 is better than 10.” This might be the case in terms of dollars and doughnuts but for filtering the fuel system to minimize the size of the junk that might find its way to the fuel injectors, a 10-micron filter is the way to go.

Once he installed a 10-micron filter in the fuel system between the fuel pump and the injectors, this solved the problem of the injectors becoming dirty and leaking into the engine. It was a long and arduous process to get all of these problems sorted out and looking back at all of the potential problems that we thought might have been the cause, it turned out that, at least in many cases, the simpler solutions turned out to be the best.

Through hard-learned lessons we’ve learned that when diagnosing problems its best to attack the simplest solutions first.

There’s nothing like deciding that the fuel pump needs replacing only to discover that the fuel level indicator was really at fault and the tank was empty of fuel. This happened on a different car, but the result was much the same. Some lessons are learned much quicker than others.

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