How to check fuel pressure

Checking fuel pressure is very vital in diagnosing an engine performance issue. If you have low fuel pressure, it can cause all sorts of engine performance problems such as a hard start, low power, poor emissions and even bad gas mileage.  If it goes on too long untreated, the computer will pick this up as a lean condition and flag a lean code such as a P0171.  Depending on the application, it will take any where from a few moments to a few minutes to hook up a fuel pressure gauge.  Most fuel rails have a test port that you can screw the fuel pressure tester into in order to get a fuel pressure reading.  Others you will have to tap into the fuel line by disconnecting the fuel pressure line from either the fuel rail or the fuel filter and installing a fuel pressure gauge in between the two in order to check fuel pressure.

Some fuel systems are only at 30 to 40 psi while others range in the 50 to 60 psi range or more.  Once you know what your fuel pressure is at, you are going to want to get the correct specifications for your vehicle to see if your fuel pressure falls within the factory specs.  You can go to AllData Do It Yourself or another source to look up your specifications for your vehicle.  If the fuel pressure is low, be sure that the fuel filter and the fuel pressure line are not pinched or plugged up in any way and that the fuel pump has full battery voltage before you replace the fuel pump.

There are several different type of fuel pressure testers.  How do you know which one is right to use on your vehicle when testing fuel pressure?  The answer is as long as you have a “normal” vehicle, you should be able to get away using any type of fuel pressure gauge.  What do I mean when I say normal?  Any modern-day vehicle.  Old carburetor or hot rod vehicles are not going to apply here.  Just about any 4, 6 or 8 cylinder fuel injected vehicles (even some 5 cylinder engines) are the ones we are talking about.

If you want a good quality fuel pressure kit that will do just about everything and will last for a life time, I would recommend to go with an OTC or a Snap On fuel pressure master kit.  The OTC Master Fuel pressure Kit will run you about 187 bucks from Amazon.com.   It is a much better price than Snap On’s master kit which last time I checked it ran on upwards about $500 bucks or more.

A cheaper alternative would be one of the Actron fuel pressure gauges.  They seem to work ok as far as measuring the fuel pressure and being accurate, but have their quirks.  The seals and o rings need to be replaced regularly in order to prevent them from leaking fuel.  So just careful.  The good news is that it is cheap, simple and easy to use.  The Actron CP7838 Fuel Pressure Tester is a good choice as it comes with the basic adapters.  You can pick this up for about $44.

Another good way to check to see if your fuel pump is bad is to check to see how many amps it is pulling.   You will need either a voltmeter that has an amp setting, a voltmeter that works with an amp clamp or a lab scope.  I use a Fluke 87 with an amp clamp for this job.  It is a fantastic and extremely reliable voltmeter.  (Don’t quote me, but I believe this meter is so accurate that it is certified to work on aviation/air crafts).  You can either clamp around the power wire going to the fuel pump or you can put a jumper in place of the fuel pump fuse and clamp around the jumper wire to get an amp reading with your voltmeter.  You need to check this while the engine is cranking over or when the fuel pump is running.  On some vehicles, you can remove the fuel pump relay and jumper the hot wire going to the fuel pump relay (with the key on) to the fuel pump feed wire going to or leaving the fuel pump relay going to the fuel pump and the fuel pump should run.  (Each vehicle is different so be sure to consult with a wiring diagram for your specific vehicle.)  The amount of amps your fuel pump will pull will depend on how much fuel pressure your fuel pump should supply to your fuel rail.  A good rule of rule of thumb for this test is as follows:  14 to 15 psi 3 to 4 amps, 30 to 45 psi 4 to 7 amps, 45 to 50 psi 7 to 8 amps, 50 to 70 psi 8 to 12 amps

Happy fuel pressure testing!

Posted on July 6, 2011, in How to check fuel pressure and tagged , , , , , , . Bookmark the permalink. 1 Comment.

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