How do I check and diagnose my GM vehicle fuel pressure?

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Answered by: Stephen, An Expert in the Cars - General Category
In late-model GM vehicles with electronic fuel injection, insufficient fuel pressure is a frequently overlooked cause of a number of mysterious malfunctions including poor performance, lack of power and hard starting. Often fuel pressure is the last thing checked (and only after all manner of ignition suspects have been ruled out) when it should be among the first.



Fuel pressure gauges which connect to the common, Schrader-style test valve on the fuel line in the engine compartment are widely available at a price less than a mechanic will charge to check it for you. Specifications for GM vehicle fuel pressure for your specific model and engine, and the procedure to test it, can be found in any of the mainstream aftermarket repair manuals available at parts stores. Connect the pressure gauge to the test valve, start the engine and observe the reading at both idle RPM and a higher, cruising RPM. If the fuel pressure is below listed specs anywhere in the range of acceleration, you're facing a limited number of possibilities.

First, the easy one: The inline fuel filter may simply be clogged. Usually located somewhere on the stretch of metal fuel line underneath GM vehicles, fuel filters are generally good for around 30,000 miles or 24 months before replacement. However, this figure assumes the gasoline you're buying is free of dirt and impurities. This may not always be the case, particularly in times of high fuel costs when many people shop by price, not quality. Most fuel filters in GM vehicles are user-replaceable with a pair of end wrenches and some rags to absorb the gasoline that leaks out of the open fuel line.



The replacement procedure is normally outlined in the Routine Maintenance section of repair manuals. If you decide to do it yourself, you'll want to observe all recommended safety procedures for working around gasoline.

While you're under the car, visually check the integrity of the fuel line extending from the tank to the engine compartment, looking for leaks or dents which might reduce or restrict fuel pressure. If a simple inline filter change doesn't boost the GM vehicle fuel pressure, and there's nothing obviously amiss with the condition of the fuel line, you're probably looking at a failing fuel pump. This is more likely to be the case if your GM vehicle has over 100, 000 miles on it. If you couldn't find your fuel pump under the hood, it's because fuel pumps in fuel-injected GM vehicles are inconveniently located inside the fuel tank itself. These pumps are electrically-powered and controlled by a fuel pump relay inside the engine compartment.

When the ignition switch is turned on, the fuel pump runs for several seconds to pressurize the system for starting then stops. Once the engine is running, the fuel pump maintains continuous fuel pressure to the injectors. Electric fuel pumps may fail suddenly or slowly, quietly or with warning. In addition to producing a low pressure reading on the fuel pressure test gauge, a faltering electric fuel pump may also emit a noticeable whining noise — particularly conspicuous when the fuel level in the tank is low. Once the low fuel pressure is traced to the fuel pump, a new pump is the only option and usually something you'll want to leave to a professional mechanic. Electric fuel pump replacement in GM vehicles entails dropping the fuel tank out of the truck from below, calling for special jacks and expertise.

Once the pump is replaced, be conscientious about making future fuel filter changes according to manufacturer's recommendations for your GM vehicle. One more tip: Some mechanics advise owners of vehicles with in-tank electric fuel pumps to maintain the fuel tank at least one-quarter full when possible. This keeps the pump submerged in fuel, cooling it and preventing premature failure from overheating.

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