Pressure Test and Volume Test – Ways to Diagnose a Faulty Fuel Pump

Many of the aftermarket manufacturers offer good quality fuel pumps that are put through various levels of inspections before they reach the consumers. But vehicle owners, unaware of the level of sophistication, often suspect fuel pumps to be responsible for various issues with engine.

Though, a faulty fuel pump is potential enough to cause troubles in engine, it is not always the case. Hence, there arises the need to conduct a proper and timely diagnosis that will help you identify the root cause.

Chances of misdiagnosis
Diagnosing is not a difficult task. But, the only issue is that the warning signs of a faulty fuel pump can mimic other problems (caused due to other faulty parts), making it difficult to identify the real cause of the problem. This often leads to misdiagnosis (fuel pump will be OK, problem lies elsewhere).

So, whenever you experience driveability issues like hard starting, power loss, extended crank time, misfiring, etc., check thoroughly for other possible causes like, low level of gas, ignition system, compression system, vacuum leaks, vapor locks, kinked/broken fuel lines, carburetor, bad injector relay, leaking injector, broken wiring, etc.

Proper tests for diagnosing
If driveability issues persist even after remedying all the above causes, it’s time to diagnose the fuel pump. Following two tests are considered effective.

Fuel pump pressure test: If the fuel pump is not able to deliver fuel at sufficient pressure, the engine cannot run properly. Conducting pressure test helps to determine whether or not it is delivering fuel with enough pressure.

Also called as deadhead fuel pressure test, it is conducted using a fuel pressure gauge. This test helps to determine the maximum output pressure by blocking the fuel return line. Following are the possible results you can derive from this test and see how they help in evaluating your fuel pump’s condition:

With the return line shut, if the deadhead pressure is –
50% higher than its normal operating pressure at idle – no issues with fuel pump pressure
Lower than normal pressure – causes hard starting, hesitation, stalling – may be due to electrical problems, blown out fuse, bad fuel pressure regulator, improper fittings at fuel tank, clogged fuel filter/strainer, weak pump
Higher than the deadhead pressure – engine runs at unusual high speeds – may be due to bad fuel regulator or return line restrictions
No pressure – prevents the engine from starting – may be due to faulty relay, tripped inertial switch, faulty oil pressure switch, restricted fuel pump inlet, faulty fuel pump

On finding any of the above conditions, you may rectify them accordingly. But if you find the fuel pressure to be normal and the driveability issues still continuing, then conduct fuel pump volume test.

Fuel pump volume test: Besides delivering fuel at required pressure, it should also be able to deliver adequate volume of fuel to the engine. Inadequate fuel volume starves the engine for fuel – engine loses power frequently making it hard to accelerate, especially, when driving at high speeds. Volume tests help us determine the maximum fuel output (volume).

Fuel volume test can be conducted by connecting a fuel flow gauge into the fuel supply line and by measuring the volume of fuel delivered over a specific interval of time. As a rough rule of thumb, a good one should deliver about 3/4 to one quart of fuel in 30 seconds.

Some of the causes that prevent it from delivering enough fuel volume are low voltage, worn out fuel pump, restricted fuel lines or a nearly empty fuel tank.

Evaluating the results of fuel pressure and volume tests
If all the results of fuel pressure and volume tests meet the vehicle’s manufacturer specifications (idle pressure + peak flow + peak demand pressure = normal), then it can be concluded that fuel pump is OK. But, if you find any deviations from the actual specs, then evaluate your results in the following ways:

For return type system:
Idle & deadhead pressure normal + flow below spec = check for restricted fuel lines or clogged filter
Idle pressure & idle flow out of spec + deadhead pressure & bypass flow normal = replace pressure regulator
Low values in all pressure & flow tests = suspect failing fuel pump

For return-less system:
Idle pressure, peak flow & peak demand pressure low = check for restricted fuel line or clogged filter
Idle pressure & peak demand pressure not to the spec + peak flow normal = possible faulty regulator

If you have conducted all the above tests, checked and rectified all the above causes as needed, and still find issues with the engine, then it is advised to conduct electric tests (using a digital ohm and volt meter), before you finally decide to replace your fuel pump.

Identifying faulty fuel pumps through proper diagnosis is time consuming, yet it is worth practicing as you can be relieved of huge expenses associated with unnecessary replacement.