Auto, Car, Truck Fuel Pump Repair, diagnosis, symptoms,advice

What a fuel pump does: gasoline engines

Fuel pumps supply gasoline to run a gasoline engine. The pump sucks gas from the gas tank and pumps it to the carburetor or fuel injection system.

There are two basic types of fuel pumps: mechanical and electric.
Mechanical pumps are mounted on the engine and are driven by the engine directly. (via an eccentric lobe on the camshaft)
Electric pumps are driven by the vehicle's battery and charging system (the alternator).

All modern fuel injected cars use electric fuel pumps, as well as some carbureted cars. Many carbureted cars use mechanical pumps.

ELECTRIC FUEL PUMPS

Most all cars today have electric fuel pumps. Often the fuel pump is in the gas tank.

ADVANTAGES OF ELECTRIC FUEL PUMPS

They can't leak gas into your engine. They are less likely to develop external leaks. The electric pumps are capable of developing the high pressures needed for fuel injection at all engine speeds and conditions.

DISADVANTAGES

They're usually more expensive than mechanical pumps and harder to replace. Many pumps are in the gas tank, making it a chore to get to them. Because they run off the battery/charging system, if the alternator stops charging you can't make it as far as if you had a mechanical pump. Vehicles with electric pumps often have larger alternators than non-electric pump vehicles.

CHECKING ELECTRIC FUEL PUMPS

Usually electric pumps work fine until they fail. Sometimes their normal "whirring" noise gets louder before they break. Sometimes they will stop when they heat up, and then start again when they cool off. If your car breaks down and you suspect the electric fuel pump, you can test it like this: Take the gas cap off. Have a helper turn the engine over while you listen at the gas filler. You should hear a "whirring" noise from the tank. (If the fuel pump is outside of tank, listen to it wherever it is. Most of them are in the tank, though.) Electric pumps also can fail anytime after 50,000 miles. Most cars run the fuel pump for a few seconds when the key is turned on. This pressurizes the fuel system so the car will have adequate fuel pressure immediately when the starter is engaged. Try turning your ignition on just before the "start" position: you shouild be able to hear your pump run for a few seconds.

WARNING: FUEL FILTER CHANGES

Many electric fuel pumps pump a lot more fuel than the engine could ever use. This excess fuel is returned to the tank via a fuel return line by a fuel pressure regulator. This is done to assure a constant fuel supply at a steady pressure. This is essential for electronic fuel injection to work properly.

Because of this you might pump 100 gallons of fuel through your fuel filter, but only consume 15 gallons of gas!

Fuel filters on fuel injected cars are bigger because of this, but they still should be replaced every 50,000 miles!

FUEL PUMPS RELY ON FUEL FLOW FOR COOLING AND LUBRICATION!!!

If the fuel filter becomes clogged the car will still run fine, but there won't be enough fuel flow to keep the fuel pump happy.

MECHANICAL FUEL PUMPS

ADVANTAGES

Mechanical pumps have been used for years and are less expensive than electric pumps. During a charging system failure, a car with a mechanical pump can drive for hundreds of miles, so long as the lights, A/C blower, wipers, etc. aren't used. The battery only has to fire the spark plugs.

DISADVANTAGES

When a mechanical pump fails, it can leak gas into your engine, thinning the oil and eventually destroying your engine. It can also leak externally and cause a fire. Mechnical fuel pumps would have a hard time providing the pressures and accuracy of flow and pressure needed for computerized fuel injection, thus all computerized fuel injected cars have electric pumps.

CHECKING MECHANICAL PUMPS

Look at your pump every oil change (or have a qualified mechanic check it once a year at least). Look for gas or oil coming from the pump. Check out any "gassy" smells. When checking you oil, watch for a smell of gas. I "feel" my oil when I check it: I wipe the dipstick off with my finger and rub it against my thumb. You can tell if it gets thin way before it does any damage. By all means suspect the fuel pump if your oil level starts to rise above the full mark! Cars can't make oil, they only lose it. That extra volume is probably gasoline! Mechanical pumps can fail anytime after 50,000 miles.

HOW MECHANICAL PUMPS FAIL

The best way to test a mechanical fuel pump is with a fuel pressure gage. Normal pressure is at least 4 pounds. The "backyard" way is to disconnect a fuel line, have an assistant spin the car engine over, and see if gas pumps out.

DANGER!!! Gas is flammable!!! It's also highly Inflammable!!!

That's why the gage is the preferred method.

Assuming tests determined a bad fuel pump, pumps fail in the following ways:

DIAPHRAGM LEAK

A mechanical pump has a rubber diaphragm which is moved back and forth, pumping the fuel. This diapragm cam get a pinhole leak and fill your crankcase full of gasoline!
Most pumps have a relief hole which keeps SOME of the gas from getting in the crankcase if the rubber diaphragm breaks.

CHECK VALVE FAILURE

There are inlet and outlet check valves which can fail. On a pump like this a fuel pump pressure gage will "pulse" between a pressure and a vacuum, but will pump no gas.

CAM TO PUMP MECHANICAL FAILURE

On some motors the pump can be installed incorrectly, bending the pump arm or damaging the pump. GM V-8's have a plate and pushrod which drive the pump. Getting the pump arm under the pushrod can be tricky.

On some Ford engines the pump is driven by a roller cam bolted on the front of the timing gear. This cam can break and disable the pump.

A GM TRICK

There is a bolt on the front of the engine by the fuel pump. Remove that bolt and screw in a sightly longer bolt. This bolthole intersects the bore that the fuel pump pushrod rides in. This longer bolt will lock the pushrod in place, holding it up while you insert the fuel pump arm under the pushrod. Swap out the longer bolt for the original, bolt up the pump. This is a neat trick, because often the pushrod will keep sliding down quicker than you can get the fuel pump in place.

FUEL LINE LEAKS

If a fuel line gets a leak on the suction side the pump can "suck air" and stop pumping fuel. That's one good thing about checking the pump with a fuel pump pressure gage: it will show a pressure being made even if fuel is not being pumped. If you have a fuel pump that makes pressure on one side and creates a vacuum on the other then it's a good pump: there's a fuel line, filter, or tank problem.

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Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.

George G. Scott, Jr.

Fuel Pumps

What a fuel pump does: gasoline engines

ELECTRIC FUEL PUMPS

ADVANTAGES OF ELECTRIC FUEL PUMPS

DISADVANTAGES

CHECKING ELECTRIC FUEL PUMPS

WARNING: FUEL FILTER CHANGES

MECHANICAL FUEL PUMPS

ADVANTAGES

DISADVANTAGES

CHECKING MECHANICAL PUMPS

HOW MECHANICAL PUMPS FAIL

DIAPHRAGM LEAK

CHECK VALVE FAILURE

CAM TO PUMP MECHANICAL FAILURE

FUEL LINE LEAKS