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How to Test a Fuel Pressure Regulator

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Fuel pressure regulator, injectors and fuel rail.

Fuel pressure regulator, injectors and fuel rail.

If you learn how to test a fuel pressure regulator (FPR), you may save some money on repairs and get your vehicle going sooner. A fuel pressure regulator commonly fails in one of two ways:

  • When it fails to hold pressure, it provides too little fuel to the engine, causing a lean mixture (low pressure) condition.
  • When the FPR gets stuck and builds up more pressure then it should, it'll cause the injectors to deliver too much fuel, causing a rich mixture (high pressure) condition.

A worn out spring or valve, a leaking diaphragm or lack of vacuum to the pressure regulator may cause these conditions.

Common bad FPR symptoms include:

  • hard starting
  • running rough
  • misfiring
  • stalling
  • no-start condition
  • hesitation

However, other worn out or failed components—like the fuel filter, fuel pump, and automatic transmission issues—can also cause similar symptoms to those of a failed pressure regulator. So you need a way to troubleshoot the regulator in your vehicle whenever you suspect a malfunction.

Here, you'll find a couple of tests you can do at home with the use of a fuel pressure gauge, a fairly inexpensive tool. If you don't have this gauge, you may buy one from your local store or online.

Also, it's a good idea to have the repair manual for your particular vehicle make and model. The manual comes with specifications for your particular fuel system, and many other systems, including particular tests for it, if applicable.

Haynes manuals are an inexpensive, aftermarket good choice. They come with step-by-step procedures, illustrations and systems descriptions for many troubleshooting, maintenance and parts replacement projects you can do at home.

Index

I. Checking for FPR Fuel Leaks

II. How to Check a Fuel Pressure Regulator

REMINDER - Fuel Filter Replacement

III. Testing for Maximum Pressure

WARNING - Fuel Pressure

Quiz - Test Your Knowledge of FPRs

Look around the end of the fuel rail to find the fuel pressure regulator.

Look around the end of the fuel rail to find the fuel pressure regulator.

I. Checking for FPR Fuel Leaks

The easiest way to test an FPR is with the use of a fuel pressure gauge. But first, you'll start your tests with a quick preliminary check:

1. Open the hood and locate the fuel pressure regulator on one end of the fuel rail. This rail holds the fuel injectors in place. A standard regulator is a small metallic cylinder with a thin vacuum hose connected on top. Also, you may see an incoming fuel line and a return fuel line connected to it, depending on your particular configuration.

Note: the tendency on newer vehicle models is to place the fuel pressure regulator inside the fuel tank as part of the fuel pump assembly. So you won't find the regulator or a return fuel line on these models. The computer helps maintain pressure along with the pressure regulator.

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2. Check the vacuum hose for a tight connection. A loose hose will prevent the regulator from working properly. Also, check the hose for damage and wear. Then disconnect the vacuum line from the regulator.

3. If you see any signs of fuel in the vacuum line, the diaphragm inside the pressure regulator has a leak and you need to replace the regulator. Otherwise, continue with the rest of this test.

Note: try to do this test right after you've taken you car for a ride on the highway which helps reveal small fuel leaks.

II. How to Check a Fuel Pressure Regulator

The order in which you perform the following tests for your particular system (key on, engine off; engine running; or just engine running, may vary. Consult your repair manual.

1. Locate the Schrader valve. Most modern fuel injection systems come equipped with a Schrader or test valve located on the fuel rail. The valve is similar to the air valve on your tires.

2. Connect your fuel pressure gauge to the Schrader valve or test port. If your system doesn't have this valve or has an older fuel system, check the repair manual for your particular vehicle model for the best way to connect the gauge to the system.

Note: if there's no test port or you don't have the correct adapter, remove the test port valve and connect the gauge directly; or disconnect the fuel inlet line and connect a T-adapter. Then reconnect the fuel line and connect the gauge to the T-adapter.

3. Then find the fuel system pressure specification for your particular model with the engine running and off. Look up this number in your car repair manual.

4. Start the engine and let it idle (or have an assistant crank the engine for a few seconds, if the engine doesn't start). Then turn off the engine.

5. Observe the fuel pressure gauge while the engine runs and after shutting off the engine. Fuel pressure should hold to specs, while the engine is running, and after turning off the engine. With the engine off, pressure should hold for about 5 minutes. If necessary, consult your repair manual.

Note: While conducting this test, also remove the vacuum line while the engine is running. It should cause the pressure to rise. Otherwise, there's a problem with the FPR.

6. Make a note of the pressure gauge reading.

7. Ask an assistant to turn the ignition key to the On position (without starting the engine) for 5 seconds, and then to turn it off. Repeat this step at least 5 times to make sure fuel pressure is consistent; otherwise, the FPR might be sticking.

8. Compare your readings to specification in your repair manual.

  • If your gauge reading is lower than the specification while the engine is running (pressure goes down quickly or after a few minutes, or doesn't build up), possible culprits could be: a leaking fuel injector, the fuel pump (worn out pump or not receiving full voltage), fuel filter (restricted or clogged), anti-drain valve (failed—draining fuel back into the fuel tank) on the fuel pump assembly, or a bad FPR. Continue reading.
  • If fuel pressure begins to drop soon after shutting off the engine, you probably have a leaking injector, a leaking anti-drain valve in the fuel pump assembly or problems with the FPR itself.
  • When you detect low pressure, and you have not changed the fuel filter in more than a year (or at the interval suggested in your car owner's manual), it is a good idea to replace the filter, and repeat the test again. A clogged or partially clogged filter may be your problem. After installing a new fuel filter, if pressure is still too low or system loses pressures after shutting off the engine, head over to the next section Testing for Maximum Pressure.
  • If your gauge reading goes above specification and you know the vacuum line is in good condition, properly connected and not clogged, most likely your FPR is to blame.

If your fuel pressure regulator passed your tests, but you still suspect a problem with the fuel delivery system, check system volume. A restriction in the system or worn out pump will affect volume delivery and pressure under certain conditions.

Also, you may want to test vacuum supply, if your FPR operates through a vacuum line. You can use a vacuum gauge to check the line and make make sure there are no restrictions.

III. Testing for Maximum Pressure

You want to do this test if your previous test showed low fuel system pressure after shutting off the engine. The test may help locate the point of failure, provided your fuel filter is not clogged or due for replacement.

This particular test is similar to the previous. You'll use your fuel pressure gauge. But this time, you'll try to locate the likely source for low fuel system pressure.

1. Connect the fuel pressure gauge to the fuel system as you did in the previous section.

2. Now, place a rag over the fuel incoming line (hose) and another rag over the fuel return line (hose). If you need more help to locate these lines, consult the repair manual for your particular vehicle model.

3. Have an assistant start the engine, let it idle for a minute, and then turn it off.

4. As the engine turns off, use a pair of slip joint or vise grip pliers to squeeze and block the incoming and return lines. The rag should be between the pliers and fuel line to prevent damage to the hose.

5. Note the fuel pressure gauge reading for a few minutes.

  • If pressure remains steady, your fault if probably in the fuel pump anti-drain valve.
  • If pressure still drops, you have either a leaking fuel injector or bad FPR.

Check the video at the start of this section so that you have an idea how to do a maximum pressure test.

A bad fuel pump may cause low pressure.

A bad fuel pump may cause low pressure.

Whenever you have a fuel-system issue, fuel pressure should be one of your first diagnostic tasks.

As you can see, testing a fuel pressure regulator can be a simple process. With the help of a pressure gauge, a fairly inexpensive tool, you can find out if you need to replace the regulator or concentrate on other components or systems.

If you need to replace the regulator head over to How to Replace a Fuel Pressure Regulator, where you'll find the steps to install a new one. Most of the time, though, you'll learn that the root cause is a device in need of maintenance. And often, you can replace a failed or worn out component yourself and save some money over paying a repair shop.

Test Your Knowledge of FPRs

For each question, choose the best answer. The answer key is below.

  1. The FPR Builds Pressure Not the Fuel Pump
    • True
    • False

Answer Key

  1. True

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: For how long does fuel line pressure remain?

Answer: Fuel pressure decreases slightly after shutting off the engine. Then the pressure will hold for about five minutes then decrease slightly. But some pressure will remain steady usually after about 20 minutes.

Question: Is there any way you could check the vacuum on the regulator to see if the incoming and outgoing towards open, or how does that work?

Answer: You need to have the car at idle. You can unplug the vacuum hose and put your finger at the hose opening. If there's vacuum you'll feel a slight pull on the tip of your finger. If you need to know how much vacuum there is, you need to connect a vacuum gauge to it. Check the specification on your vehicle repair manual.

Question: What if the car just doesn’t start and I’ve never had it running? How can the fuel pressure system be tested?

Answer: Locate the fuel pump relay; you may be able to connect battery power to it. Have a fuel pressure gauge connected to the test port. Check the specification for initial pressure on your vehicle repair manual. The manual will help you locate the relay as well.

Question: I have replaced everything fuel related except the FPR in my 86' Ford F-150 302 cid. Now the truck won't start. It acts like it wants to though. Checked the Schrader valve and all I get is a little air. Could it be the FPR?

Answer: It's possible. The line can also be clogged. You may want to remove the fuel filter and open the ignition key and see if you get good fuel volume coming from the fuel tank. And test after the filter. It'll give you an idea if the problem is in the fuel pressure regulator before you replace it.

Question: The only way I can keep my Datsun 280z running is by clamping off the return line with small vise grips. This can't be right. Any answers?

Answer: The problem could be a worn out fuel pump or faulty pressure regulator. But check the fuel filter as well. If it's an old filter, it may be restricting fuel flow.

Question: I have a Nissan 180SX CA18DE engine. I have fuel coming out of the exhaust and the exhaust manifold. I've had the injectors cleaned and tested and still having this issue what can it be?

Answer: This usually happens with stuck-open fuel injectors. You can try removing one spark plug at a time and have someone crank the engine to see which cylinder is causing the issue. If the fuel injectors are working properly, you may want to check the fuel system, possibly a bad fuel pressure regulator or fuel return line.

Question: When turning off the engine, the fuel pressure is 43psi running spec on Tahoe 5.3L Flex Fuel. But after 10-15 minutes, pressure slowly drops to about 15 psi. Is this normal or bad? My injectors and FPR and fuel filter are new. Per article it seems like my fuel pump drain is allowing pressure backflow. Thoughts?

Answer: It seems like the fuel delivery system is draining fuel back into the tank. This could be a fault in the fuel pump or a valve, depending on your model. Your vehicle repair manual will give you the specs for your model and may help you diagnose the problem as well.

Question: My car has low fuel pressure that won't hold and the pressure increases with the removal of the regulator vacuum line. What could be the problem with my car's fuel pressure?

Answer: The pressure regulator might be working fine. The problem might come from a weak fuel pump or assembly, clogging fuel filter, leaking fuel injector perhaps.

Question: 2003 Chevy Tahoe 4.8l. It cranks but shuts off. Changed fuel filter. Now cranks but won’t start. Sprayed starter fluid and it runs but shuts off. Fuel pump or fuel pressure regulator?

Answer: Seems like a fuel system problem. You'll need a fuel pressure gauge to confirm low pressure. It'll help you to locate the issue. This could be a bad fuel pump or pressure regulator.

Question: I have a 2003 Acura CL type s, 6 speed. I recently had the injectors rebuilt, EGR valve and manifold cleaned out and a new fuel pump. My problem is, I get random hesitation usually from 1st gear to 2nd. It hesitates then kicks in hard. Also if I park the car then 0-15 minutes later I drive it, it will bog and hesitate then kick in and work fine. If I disconnect the vacuum hose from the FPR it doesn’t hesitate. I had it tested and the pressure is 55psi. What do you think is wrong?

Answer: Compare your readings to the specifications for your application. You can find the specs in your repair manual. Also, see how pressure behaves after a couple of minutes of engine running, when the hose is disconnected and after shutting off the engine. Go over the description in the post. Also, check the vacuum hose for damage and a tight connection. Also, this could be a sign of misfires at low speed or a problem in the ignition system.

Question: I have a 2003 Chrysler Town & Country. The van will start but usually takes about 4 or 5 turns of the key, about 5 seconds each turn. Once started, it will run fine with no problems. Once the engine is turned off, if it is cranked again immediately, it will start after one or two turns of the key. If it is allowed to sit for a while, it will take the 4 or 5 turns of the key again. Does this sound like a fuel pressure regulator problem?

Answer: It seems like the fuel system gradually loses pressure. This could be a leaking fuel pressure regulator, fuel injector or another component in the fuel system (pump, valve). If you have a fuel pressure gauge, measure pressure before you start the van, and after driving and compare measurements. This will tell you if system pressure has changed. You may need your vehicle repair manual to check specs.

© 2016 Dan Ferrell

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