Fuel Pump

There are two types of fuel pumps, mechanical and electric. Electric pumps are further broken into four types: plunger, diaphragm, bellows and impeller. A fuel pump draws up fuel through an inlet valve and then pushes it out to the carburettor. Inside the pump is a flexible diaphragm, which is forced in one direction by what is called linkage to create the necessary suction, while at the same time a spring is compressed under the diaphragm. When the linkage relaxes the spring pushes the diaphragm in the opposite direction and the fuel is pushed out of the fuel pump.

Weak and failing fuel pumps can be difficult to diagnose. While the symptoms may be easy to identify, they also may be confused with other mechanical problems. By learning the warning signs of a failing fuel pump, you can save yourself unnecessary repair costs and avoid a vehicle breakdown on the road.

Engine Misfire

The engine will misfire at highway speeds, causing the car to stutter for a mile or two.

Delayed Acceleration

As you press down the accelerator, the car begins to take off and then shortly loses power before regaining power and taking off.

Vehicle Won't Start

If the fuel pump has completely failed, the car will crank but will not start.

Car Runs Intermittently

The vehicle unexpectedly turns off, but will restart later.

Trouble Climbing Hills

The vehicle loses power when climbing hills or when needing additional power, such as pulling a trailer.

Checking

When checking to see if a fuel pump is bad, make sure there's fuel in the tank, use a gauge to check fuel pressure, and check the relay and fuses.

Problem

Vapor Lock

The biggest problem that occurs with mechanical fuel pumps is vapor lock. When a car engine is shut off, the pressure spring in the fuel pump maintains pressure in the line between the pump and carburetor. The heat that would have been dissipated by the radiator comes into the engine compartment because the coolant has stopped circulating. This heat expands the gas in the line between the fuel pump and carburetor. When the expanding gasoline tries to get back into the fuel pump, it is blocked by the outlet valve. The pressure keeps building, pushing the needle valve open, filling the float bowl to overflowing and flooding the engine. A bleed hole in the fuel pump will dissipate the pressure. This hole may be installed in each pump valve or the pump casing.

Beyond Vapor Lock

If the engine fails to start and vapor lock is not the cause, then the fuel pump needs to be tested to find the cause of the failure. Install a pressure gauge in the carburetor inlet fitting. With the engine operating at idle speed, the fuel pump pressure indicated on the gauge should equal the manufacturer's specifications. Inadequate fuel-pump flow could be caused by a restricted fuel line or filter. Check these, and if they are in working condition, then replace the fuel pump

Electric Fuel Pumps

If the engine fails to start on a car with an electric fuel pump, there are numerous points on the electric fuel-pump circuit to check before determining if the fuel pump needs to be replaced. First, place a fuel-pump pressure-and-flow tester at the carburetor inlet fitting and loosen the clamp on the fuel hose connected to the calibrated bottle. While the engine runs, check the fuel flow into the bottle. If there is fuel flow into the bottle, says Don Knowles in his book "Automotive Fuel, Lubrication and Cooling Systems," then the fuel pump is not the problem. If there is no flow, check for power at the wire connected from the inertia switch with a 12V test light. If the test indicates no power, then either the starter relay, fuse link, inertia switch or connecting wires are defective.

If power is available at the inertia switch, then disconnect the wiring connector at the fuel pump and connect an ohmmeter from the ground wire to a ground on the vehicle chassis. A low reading indicates that the ground wire is acceptable. Next, connect a jumper wire from the battery positive terminal to the fuel pump terminal that is connected to the inertia switch. If the fuel pump fails to run, the pump is defective and needs to be replaced.