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A Fuel Pump Primer


2/23/2011
By Larry Carley

A hard-working fuel pump moves thousands of gallons over its lifetime. When it doesn’t, there’s trouble.
 
Fuel pumps are something that have been used in vehicles since the 1920s. Most cars from this era didn’t need a fuel pump because their fuel tanks were mounted up under the seat or under the cowl. The relatively high location of the fuel tank allowed gravity to supply fuel to the carburetor. As technology progressed, fuel tanks were relocated to the back of the vehicle to improve safety. This required a mechanical fuel pump or low-pressure electric pump to pull the fuel from the tank and push it to the carburetor. When carburetors were replaced with electronic fuel injection in the 1980s, mechanical pumps were replaced with high-pressure electric pumps (usually mounted inside the fuel tank) to push gas from the fuel tank to the engine’s fuel supply rail and through the fuel injectors.

Electric fuel pumps in late model vehicles work hard and spin continuously as long as the engine is running. Over their lifespan, they will circulate thousands of gallons of fuel. In most fuel injection systems, fuel that is not sprayed through the injectors is re-routed back to the fuel tank. The fuel pressure regulator maintains a certain amount of pressure in the injector supply rail, and routes excess fuel back to the tank through a return line. At idle and low load, therefore, much of the fuel that is pumped to the engine is not needed and is re-routed back to the fuel tank.

This fuel pump’s best days are definitely behind it.
One of the drawbacks of this setup (besides pumping a LOT of fuel unnecessarily) is that the fuel picks up heat as it circulated through the fuel rail on top of the engine. The heat is carried back to the fuel tank, where it may increase the operating temperature of the pump if the fuel level inside the tank is relatively low (say, less than a quarter full). Because of this, the life of the pump can be cut short if a motorist is always driving around with a low tank. Not only does it cause the pump to run hot, but it may also starve the pump for fuel when cornering or braking hard. That’s not good because a high-speed electric fuel pump requires a steady flow of fuel through it for both cooling and lubrication. Running out of gas is even worse because it runs the pump dry. This can damage the pump or cause it to fail.

In newer “returnless” EFI systems, the fuel pressure regulator has been moved from the engine to inside the fuel tank where it is part of the fuel pump assembly. This eliminates the need for a return line from the supply rail while helping the pump run cooler. When combined with a variable speed control, it can also greatly reduce the volume of fuel the pump has to move during its lifespan for reduced wear and better reliability.

FUEL PUMP FAILURES
Something else that can kill a fuel pump very quickly is rust, dirt or debris inside the fuel tank. Rust is a common problem with older, high-mileage vehicles that have steel fuel tanks. Plastic tanks in newer vehicles won’t rust, but after eight to 10 years of service, some plastics start to break down and may shed flakes of debris into the fuel.

The fuel filter offers minimal protection to the pump in a return-style EFI system because it filters the fuel after it passes through the pump. It will remove debris that could recirculate back to the pump through the return line, but only after the debris has passed through the pump at least once. In a returnless EFI system, the filter offers no protection whatsoever for the pump.

The pump’s only protection is the filter sock or inlet screen at the pump inlet. The screen can block large chunks of rust or debris from being sucked into the pump, but it can’t stop the micron-sized particles that can wear out a pump over time.

When a fuel pump is worn, it may get louder (more buzzing or whining sounds) and it will usually spin at a slower RPM. This, in turn, can cause a decrease in both fuel pressure and flow that can cause engine starting, driveability and performance problems.

A weak fuel pump may still be capable of producing enough pressure to meet specifications at idle. But when the engine is accelerating or working hard under load, a weak pump may not be capable of delivering enough fuel flow (volume) to keep up with the engine’s demands. This can cause fuel starvation, lean misfiring and a loss of power.
If a fuel pump is really weak or has failed altogether, the engine won’t start or run because it isn’t getting any fuel (or not enough to start).

One of the first diagnostic steps in checking a fuel pump is to check fuel pressure when the ignition is turned on. The pump should run for a few seconds and bring pressure up to a certain level, which can be measured by attaching a fuel pressure gauge to the service fitting on the fuel injector supply rail. If the engine starts, fuel pressure can then be checked with the engine idling. As before, it must meet the vehicle manufacturer’s specifications. Temporarily disconnecting the fuel pressure regulator vacuum line should cause the pressure reading to increase slightly (usually 4 to 6 lbs.).

Unfortunately, some technicians and do-it-yourselfers stop at this and go no further. If the pump fails to meet the pressure specs, they assume it is bad and replace it. If the pump does meet the pressure specs, they assume it is good. Both of these assumptions can often turn out to be dead wrong!

A good pump may fail to produce enough pressure if the voltage supply to the pump is low. This will make the pump run slower than normal, and produce less pressure. The problem is not the pump but the wiring or voltage supply circuit. Likewise, a pump may generate enough pressure at idle to meet specifications, but not flow enough fuel to meet volume specifications. If flow is not tested, a pump that is assumed to be good may in fact be weak and incapable of delivering enough fuel to the engine at higher speeds and loads.

FUEL PUMP BENCH TESTING
Most auto parts stores have an alternator/starter bench tester and battery test equipment. But few have a fuel pump bench tester. The only company we know who makes an electric fuel pump bench tester is US Motor Works LLC (www.usmotorworks.com). They have a unit that sells for around $500. The USMW fuel pump bench tester uses a non-flammable or low-flammable test liquid to test both the pressure and volume output of any electric fuel pump.
Warning: Gasoline should NEVER be used to bench test or wet test a fuel pump because of the obvious fire risk.
Here’s another caution: Never test a fuel pump dry. Fuel pumps require gasoline or a test fluid for lubrication. Spinning the pump dry can damage it or ruin it very quickly. Also, a dry test does not tell you very much. It can tell you if the pump spins or not when voltage is applied, but it can’t tell you how much pressure or volume the pump is capable of producing. Without that information, you have no way of knowing if a pump is good or bad.

Bench testing a fuel pump with a test liquid flowing through it allows you to measure the pump’s pressure and flow outputs. The numbers will tell you if the pump meets specifications or not — and if not, that the pump needs to be replaced. Pressure and flow specifications can be looked up in a manual that comes with the bench tester, or found in a factory or aftermarket service manual, on the vehicle manufacturer’s service information website or an aftermarket service information database.

If a pump meets BOTH pressure and volume specifications, the pump is good. There is no need to replace it. So if a customer is having a fuel-related performance problem or his engine won’t start, and the fuel pump tests good, the problem is something else. Other possibilities include wiring problems in the fuel pump circuit (loose, corroded or broken wires, bad grounds), a defective fuel pump relay or blown fuse, an obstruction in the fuel pump inlet filter or fuel line, a plugged fuel filter, or leaky or defective fuel pressure regulator. If the pump is a variable speed unit, the PCM or module that controls the voltage to the pump could be defective.

Auto parts stores should be performing fuel pump bench tests for the same reasons they now test alternators, starters and batteries — to remove the uncertainly that goes with diagnosing these parts. If a pump tests bad, you know your customer needs a new one. If a pump tests good, you know the problem is not the pump but something else. You can then offer suggestions as to what else they should be looking at as a possible cause of their problem.

Bench testing can also reduce warranty returns on new pumps. If a brand new pump tests good out of the box (which it should), then you and your customer both know they are getting a good pump that meets specifications. Fuel pump manufacturers tell us that most of the pumps they get back under warranty have no fault found. The pumps are returned because somebody misdiagnosed their fuel delivery problem. Testing a pump before it goes out the door can reduce comebacks and warranty returns.

PUMP REPLACEMENT
Looking up a replacement pump requires knowing the year, make and model of the vehicle, and sometimes the engine size. On pickup trucks with dual tanks, you may also have to know if the pump is a transfer pump or a main pump.
Some replacement pumps may not look exactly the same as the original. That’s because some fuel pump suppliers have replaced older designs with newer, more efficient designs to consolidate SKUs. The appearance doesn’t matter as long as the replacement pump meets the vehicle manufacturers pressure and flow specifications.

In-tank electric fuel pumps can usually be replaced separate from the fuel sending unit. But on some late-model applications with returnless EFI systems, it’s all one assembly and has to be replaced as a unit. Also most of these units have a “lifetime” fuel filter as part of the assembly. Not replacing this filter may lead to problems down the road.

It’s obviously cheaper to replace the pump only, but it’s more convenient to replace the whole fuel pump and sending unit assembly. On older high mileage vehicles, it’s a good idea to replace the whole assembly because the plated metal electrical contacts on the fuel sender unit lever arm are often worn or corroded, affecting the unit’s ability to send an accurate fuel level signal.

The inside of the fuel tank should always be inspected for rust, dirt or debris when a fuel pump is replaced. If the tank is dirty, it needs to be cleaned. If it is rusty or deteriorating, the tank needs to be replaced. Nothing will kill a new fuel pump faster than installing it inside a dirty tank.

A new fuel filter is always recommended when replacing a fuel pump, along with a new fuel pump inlet screen or sock.
The fuel hoses and lines on older high-mileage vehicles should also be inspected for signs of deterioration, damage or leaks. Rubber fuel lines more than 10 years old are a leak waiting to happen. Use only high-pressure replacement hose that is rated for fuel injection applications, not low pressure carburetor hose or any other type of rubber hose.

FUEL PUMP RESOURCES
If you are looking for additional information about fuel pumps, a good source is www.fuelpumpinfo.org, a website sponsored by the Fuel Pump Manufacturers Association. The website has training videos, safety procedures, fuel system diagnostic and testing procedures, fuel disposal procedures, and fuel tank cleaning procedures.














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