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Advanced fuel pressure testing – going beyond basic pressure

Fuel Injectors

Last month, we talked about fuel pressure testing, covering the basics for testing the fuel pump. If the pump is operating (covered as the first test to conduct), then testing to see that it's putting out the correct amount of pressure is next. If the pump is operating, but not producing the pressure you expect, then most likely the pump has an issue. There are other things it could be and you may need to know if there is a blockage, a problem with the regulator, an issue with volume (rather than pressure), etc.

To check those things, you'll need a more advanced knowledge of the fuel system in your fuel-injected vehicle and how to test for these problems specifically.

Fuel Pressure Regulator
A common issue is a faulty fuel pressure regulator. Outside of clogged filters and circuit issues for the pump, this is generally the most common problem in fuel delivery issues. Testing the regulator is relatively simple and, in hindsight, we should have included this in our basic testing as the procedure to test is done using the same fuel pressure gauge.

With the pressure gauge still connected, unplug the engine vacuum hose from the pressure regulator. Fuel system pressure should immediately increase by 7-10 psi and stay at that point until the vacuum line is restored. While the line is off, check it to make sure the inside is dry. If not, you have a bad regulator that is sending fuel into the vacuum system. If there is no change in fuel psi and the hose is dry, check for a leaky/plugged vacuum line (the easiest test is to just buy more vacuum hose and replace the line itself, which should be inexpensive). If that is not the problem, you likely have a faulty pressure regulator.

Fuel Volume
A fuel pump may be delivering plenty of pressure when you test it, but your vehicle may still be showing signs of fuel starvation or over-pumping (sluggishness, inconsistent misfires, or flooding). You can test this by measuring the volume of fuel being sent through the system into the injector rail(s). Sometimes, fuel pumps are doing fine with pressure, but not the amount of fuel. The reasons can be several: blockages, bad fuel, a broken portion of the wheel in the pump, etc.

To measure fuel volume, you'll need a fuel flow gauge. You can also do it with a container and bypass line connected to the fuel pressure regulator. Use caution as fuel can be volatile and a health hazard. If you've already completed the basic tests from our last article, you're likely ready to do these. If you have not, then depressurize the fuel system before continuing (and go back and do those other tests first, they're more likely to find your problem).

With the engine off, bypass the fuel relay with jumpers or a jumper relay plug and energize the fuel pump. Run it for the amount of time specified in your shop or repair manual. This is usually 15 or 30 seconds, depending on the vehicle. Most direct-injected gasoline vehicles will deliver somewhere between 2/3 and a quart of fuel in thirty seconds, depending on engine displacement.

If your volume is low, check the voltage to the pump (which likely would have also resulted in low fuel pressure in earlier testing), check for a plugged fuel filter, a kinked or obstructed fuel line, and that there is enough fuel in the tank that the pump doesn't need to struggle to pull from it (1/4 tank or more).You may also have a plugged inlet sock, but you will likely have to remove the tank to check. If everything checks out, you may have a worn pump that needs replacing.

Injector Test / Drop Test
Called either a fuel injector test or a fuel pressure drop test, this is a way of determining if one or more of your fuel injectors are faulty. It's very much like a static fuel system test from our previous fuel pressure testing article, but checks for pressure drop as each injector is energized. This test requires an injector pulse tool in order to activate the injectors individually, but can also be done via the engine computer on some vehicles (very common on advanced diesel systems).

With a fuel pressure gauge attached, but not bypassing the fuel rail, activate the fuel pump (turn on the engine or engage the relay override). Once pressure is attained, de-activate the pump and then pulse the injector to be tested for the amount of time specified in your shop or service manual. Note the pressure drop and repeat the process, going back to full pressure and testing, for each injector. In most applications, the test will call for about 100 pulses per five milliseconds and pressure should drop from 1 to 3 psi in that time. Some big engines may go as far as 7 psi.

Each injector should have only small differences in the amount of pressure it's dropping when compared to the others on the rail. Generally 1-2 psi difference is considered okay while more than that indicates a possible problem with an injector or its head. No drop at all means the injector likely isn't pulsing. Cleaning and re-testing is a good idea, replacement is also likely on older injectors. You can test injectors by unplugging them and measuring the resistance they have between terminals. Too much resistance or not enough will cause the injector to misfire or over-fire, requiring replacement.

Scope Tests
Although beyond the breadth of tests a home mechanic is likely going to be performing, a fuel system scope test can be performed on the fuel pump to determine if the motor is wearing down with use. It involves connecting the pump to an oscilloscope and doing wave form observations on its operation. Some auto parts supply stores can conduct these tests, but if you are doing them or asking for them to be done, you are likely a tuner rather than a DIY repair person as most tests of this sort are more for perfecting fuel supply than they are finding faults.