No starts and fuel pumps

Often when an engine will not start the first thing to pop into somebody’s mind is “it must be the fuel pump”. I often have the same train of thought. And many times replacing the fuel pump is the answer to a no start condition.

But the percentage of times that a fuel pump will cause a cranking no start condition is most likely less than 25% (just educated guessing).

I suppose the actual failure rate of the fuel pump would be determined by year make and model of the vehicle.

But because the fuel pump is so often replaced before pinpoint diagnostics are performed I thought that I would discuss the subject of how a fuel pump circuit works.

Vehicles for around the last 20 years have in tank mounted electric fuel pumps. So this will be the type of fuel pump I will discuss in this short article.

Electric fuel pumps and circuits will vary greatly depending on the vehicle year make and model. Most of these fuel pump control circuits are tied to the vehicles main computer.

Using a General Motors fuel pump and electronic controlled circuitry as an example will provide a good overview of how most fuel pumps operate and are controlled.

In a late model GM fuel pump circuit the power train control module supplies voltage to the fuel pump relay when the ignition switch is turned on. This action closes the relay contacts and voltage is directly supplied to the in tank fuel pump.

The fuel pump is energized for about 2 seconds and will remain on if the engine is cranking. If the engine is not being cranked the PCM shuts off the voltage to the fuel pump relay through an internal timer. This makes checking for voltage at the fuel pump difficult. The connector must be pulled off at the tank and a buddy needs to operate the ignition switch or crank the engine to properly check for voltage.

To further complicate the circuit and engine oil pressure switch is connected in parallel to the fuel pump relay. If the relay becomes inoperative voltage is supplied through the oil pressure switch contacts to override the defective relay and provide power to the fuel pump.

This will prevent the vehicle from having to be towed due to a defective fuel pump relay. Although keep in mind that long engine cranking before the engine actually starts up is a side effect and can help in diagnosis.

An engine cranking but not starting condition caused by a bad fuel pump can be confirmed by a fuel pressure test and checking for voltage at the pump.

If you have 12 V at the pump as described above and no fuel pressure in the line this is in strong indication of a defective unit. Often when I have run across these types of conditions I can bang on the fuel pump and it will begin to run.

What happens is the windings or brushes in the motor will have a bad spot. The vibration of the banging can get the motor past this open in the windings and it will begin to run again. Note that this does not always work and if you run into this condition replacing the pump is the best solution.

Too often both professional technicians and DIY driveway mechanics will go ahead and replace the fuel pump without taking the proper diagnostics steps. There’s nothing worse than pulling out a gas tank filled to the top with fuel replacing the pump and then finding out that it was not the problem causing the no start.

I would recommend purchasing a repair manual and following the specific steps outlined for checking the voltage at the pump motor as well as taking the time to check for fuel pressure. Fuel pressure testers can be had at your local parts stores and even department stores for around $25.

This can save you a lot of trouble and is a good tool to have in your box. Do you need more information about online car repair. This next link takes you to the homepage from this No starts and fuel pumps page.