Fuel Pump Failure in a Mercruiser 4.3

Fuel pump failure, whether in a Mercruiser engine equipped with a carburetor or in one with fuel injection, is almost a certainty at some point. You should pressurize your fuel system with the primer bulb and look for leaks every time you go boating -- minimizing one cause of fuel pump failure. This simple act, though, won't eliminate the problem. Regardless of your inspection, other reasons for fuel pump failure may lie in places you wouldn't expect.

  1. Hoses

    • The fuel pickup hose has to extend to within 1 inch of the bottom of the tank, but no further. If it extends further into the tank, it can pick up trash. Even if the fuel hose is not too long and it doesn't pick up trash, the fuel lines can leak at every point where they join to a fitting. The different types of connectors, including clamp-over-nipple, the quick-connect used on some fuel fittings and the wrap-around-and-screw-tight hose clamps are neither foolproof nor leak-proof. Even though the fuel lines are U.S. Coast Guard approved 1A gasoline lines, they will eventually leak. This results in a reduction of fuel flowing to the injectors or carburetor, and a serious fire hazard. If the leaks are generous enough, the fuel pump, whether a low-pressure vacuum-diaphragm pump or a combination system that incorporates a vacuum pump and an electric pump, will fail.

    Talking Trash

    • Mercruiser's Shop Manual No.31, with its service instructions points to the fuel pickup hose as the first potential problem. The Manual says that the hose must extend to within 1 inch of the bottom of the tank, but no farther. If the extends farther into the tank, it can pick up trash from the bottom of the fuel tank. "Trash," in this context, can mean nearly anything. If you were cutting your grass last season, trash -- in the form of grass clippings -- may lie in the bottom of your gas tank. The varnishes that come from gasoline left too long in the tank, known as stale gas, can hide on the undisturbed bottom of the tank, to clog the high-pressure fuel pump and ensure its failure.

    The Vacuum Pump

    • Engines equipped with a low-pressure, vacuum-diaphragm pump aren't prone to the same failures as engines with low- and high-pressure systems. Low-pressure-only systems, like those found on the 4.3 L Mercruiser, are more likely to suffer the same kinds of failure found in Mercury outboards. They clog, often with trash picked up from the fuel tank, or the vacuum line to the manifold is cracked or loose. Like all engines, they may suffer from leaks, another common reason for fuel pump failure.

    The Electric Pump

    • The electric pump raises the fuel pressure for the injectors. The fuel must enter the injectors fast enough, and in a large enough quantity for service. It is dependent on a steady flow of electricity not only to itself, but to its control devices, the throttle position sensor and the electronic control module. Should any of those "outside" systems fail, the pump will fail straightaway.

    The Fuel Pump Complex

    • Fuel-injected Mercruiser engines are equipped with two pumps. The first is a vacuum pump used to move fuel from the tank to the electric pump. The second is, of course, the electric pump. While it's unlikely trash will make it past the vacuum pump, it may become trapped in and clog the electric pump. Or the engine's electronics will fail. In either case, the fuel pump system will fail, in part because failure is expected in a complex system and in part because something got in that shouldn't have, or something that should have gotten in, didn't.