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Can a bad fuel pump cause a misfire on one cylinder

Can a bad fuel pump cause a misfire on one cylinder

Technical note: Faulty fuel pumps can cause engine damage.
For an injected engine to function properly, the fuel pump must be able to generate the necessary pressure for the system to operate. Close enough is not good enough. The pressure supplied by the pump must be within specifications, as the engine is calibrated for this operation. If the fuel pressure is below specification, even by a few pounds, it can...

The fuel pump is the heart of the fuel system. On most newer vehicles, an electric pump is installed in the fuel tank to supply fuel to the injectors. When the key is turned and the engine is started, the pump will continue to run unless there is a problem with the pump and it shuts off. A faulty fuel pump causes the engine to stall and cannot be restarted.

When the engine is equipped with a carburetor, replacing the fuel pump is not a major problem or expense. The pump mounts on the side of the engine and usually sells for $20 or less. It is also relatively easy to diagnose. If it is leaking or not pumping fuel to the carburetor, it is defective.


Electric fuel pumps are a different story. Pumps typically cost between $150 and $300. They are also difficult and expensive to replace because they are in the fuel tank. Labor alone can add several hundred dollars to the cost of repairs. Diagnosis can also be a challenge, even for experienced technicians.

For an injected engine to function properly, the fuel pump must be able to generate the pressure needed to operate the system. Close enough is not good enough. The pressure supplied by the pump must be within specifications, as the engine is calibrated for this operation. If the fuel pressure is even a few pounds below specification, this can cause problems. A poor pump that does not supply the correct pressure will cause the engine to run lean, misfire and hesitate on acceleration.

The amount of fuel is as important as the pressure. A good fuel pump is usually capable of pumping at least 750 ml (3/4 liter) of fuel in 30 seconds. If it doesn't, something is wrong. The pump may be worn, a clogged fuel filter may be blocking the flow of fuel to the engine, or the pump may not be getting enough voltage in its circuit to run at normal speed. A loose or corroded wire connection in the fuel pump circuit, a faulty relay, or low voltage in the system can affect fuel pump operation.

Low fuel pressure can be caused by any of these factors or by a faulty fuel pressure regulator. The regulator is a small valve with a spring-loaded diaphragm inside. The purpose of the regulator is to control the fuel pressure to the injectors.

In most cases, the regulator is mounted on the engine's injection rail. However, in engines with "returnless" fuel injection systems, the regulator is located in the fuel tank or near the fuel pump. In engine-mounted applications, the regulator is connected to the intake manifold via a vacuum line. At idle, the high vacuum in the intake manifold causes the regulator to release fuel pressure and return excess fuel to the tank through the return line. As the engine load increases and the vacuum level decreases, the regulator releases less pressure to maintain the same relative pressure differential between the injectors and the intake manifold.

If the regulator has an internal leak, it can release too much pressure, causing symptoms that mimic a faulty fuel pump. Therefore, if the regulator is not checked, the fuel pump may be replaced unnecessarily. The regulator can be checked by disconnecting or removing the vacuum hose. This should result in an increase in fuel pressure. If the fuel pressure is low and rises to normal levels when the return line is disconnected, then the regulator is leaking and should be replaced.


Too often, these "other" causes are overlooked and the fuel pump is replaced unnecessarily. Misdiagnosis is a big problem with electric fuel pumps, costing everyone time and money. Unfortunately, until now there has been no easy way to test an electric fuel pump. Pump pressure can be checked on the vehicle by connecting a pressure gauge to the fuel manifold service port or fuel manifold line. Fuel flow can be measured by disconnecting the fuel supply line to determine how much fuel the pump can deliver in 30 seconds, or by using a special flow meter to measure fuel flow.

Why a good pump can fail

Fuel pumps are designed to last the life of the vehicle, but they often fail due to other factors. Dirt or rust in the fuel tank can quickly destroy the pump (so always check and clean or replace the fuel tank if there is dirt inside when replacing the fuel pump).

Many drivers also have the bad habit of driving with a low fuel level in the tank (less than 1/4 of the tank capacity). This can shorten the life of the pump and cause premature failure, since the pump depends on the fuel for cooling and lubrication. The fuel tank has an internal baffle to prevent fuel leakage. Even so, the pump can run out of fuel during sharp turns or hard braking or rapid acceleration. And if the pump sucks in air, it suffers the consequences.

As the fuel pump ages and its brushes wear, the pump can draw more current than normal through its circuits, which can cause the pump's wires to heat up and melt or short out. If the damaged wiring harness is not replaced when the pump is replaced, the pump being replaced may not run at all, or may not rotate fast enough to produce normal pressure.

The fuel pump is part of the fuel tank distribution assembly. The pump can be replaced individually (less expensive, but requires more work and increases the risk of incorrect installation) or as a complete assembly (easier, but more expensive).

Note: Some replacement pumps may not be exactly the same as the originals. This is because some aftermarket pump suppliers have replaced the "old technology" with newer, more efficient and reliable "turbo" pumps.

When replacing the pump, the fuel filter that runs from the pump inlet to the tank should always be replaced with a new one. If the old fuel tank is rusted, it should be replaced to prevent the pump from failing again. The fuel filter should also be replaced.

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