Note: The information contained in this article describes only general procedures used in testing fuel pressure on a vehicle with a typical gasoline engine. For vehicle specific procedures, it is recommended to purchase and use the repair guide for your vehicle.

Q: Why is testing the fuel pressure important?

A: In order understand the answer to this question; one has to understand that an engine needs four basic elements to allow it to run properly. These are fuel, air, compression and an ignition source. If any one of these are missing or not operating fully, the engine will not run properly, or possibly not at all. As we are only interested in the fuel portion of these elements, we will discuss its importance. It is necessary that the fuel being delivered into the engine have correct pressure and volume. If the fuel lacks in either pressure or volume the vehicle may "starve", causing poor performance or possible internal engine damage. To ensure this does not happen, fuel pressure testing is performed.

Q: When should the fuel pressure be checked?

A: The fuel pressure should be checked whenever a regular tune-up is performed on the vehicle. If the fuel pressure is too low or too high it will be an indication that further diagnosis or repair is needed to correct the problem. Unfortunately, many times the fuel pressure goes unchecked until the vehicle stalls out on the highway. Indicators of a fuel related problem would be:

• Lack of power going uphill or when pulling a load
• Backfiring through the intake when accelerating
• Poor fuel economy
• Fuel mileage that is too good to be true

These little indicators could lead to larger problems if left unchecked.

Q: How is fuel pressure checked?

A: There are several ways in which to test fuel pressure, but before you get started you must first determine if the vehicle if carbureted or fuel injected. This will dictate which type of fuel pressure tester is appropriate. If the vehicle is carbureted, the Mityvac“ P/N 05511 Vacuum Gauge/Fuel Pressure Tester is perfect for the job. It will test the suction/vacuum side and the pressure/output side of any mechanical pump that is associated with this type of system. (Note: If the fuel system has been refitted with an aftermarket electric fuel pump, this tester will still work provided the pressure is below 10 psig). Most normal mechanical fuel pumps will range between 4-6 psig. The P/N 05511 comes with different style adapters that will allow you to "T" into the fuel line to monitor running pressure and straight fittings that will allow you to clamp directly onto the fuel pump to check the suction/input or the pressure/output side. Occasionally with a mechanical pump, the problem may arise that the tester might work on the vacuum side but not on the pressure side. This is usually due to the internal diaphragm of the fuel pump developing a pinhole or it has stretched from wear. If this is the case, the mechanical pump will have to be replaced. It is important to check both sides of the mechanical pump to ensure a proper diagnosis.

If the vehicle were fuel injected, the Mityvac P/N 05515 Fuel Injection Pressure Test Kit would be the ideal tester. Most domestic and some foreign automobiles are equipped with a pressure test port. This test port is usually located on a steel line of the fuel system and typically located on top of the engine near the center. (Refer to manufacturer’s literature to determine the exact location.) If the vehicle is not equipped with a test port, then an assortment of adapter fittings may be required in order for the tester to work. These adapters are usually available at a local hardware store or from a mobile tool vendor. The fuel injection pressure test kit will test pressure ranging from 0-100 psig; the exact pressure will vary depending on the vehicle manufacturer. Listed below are a few common pressure ranges, but please refer to the manufacturer’s data as listed for your vehicle specific information.

Once you have connected the fuel injection pressure tester to the vehicle, turn the ignition key to the on position (with engine not running) and monitor the tester’s gauge reading. If the gauge comes up to the proper reading (pressure range) then the system has passed. If it does not come up to the proper reading here are a few things to check:

1. Ensure the tester is not leaking and is hooked up correctly. Sometimes the o-rings that are inside the end of the pressure hose need to be tightened enough to allow the push pin to depress the Schrader valve that is located on the vehicle’s fitting.

2. Listen for the pump to activate when the key is turned to the on position. This may require a helper to listen closely at the fuel filler door (with the cap removed) for the "whirring or buzzing" noise that the pump will make when in operation. On most vehicles this will only occur for about 10 seconds. If there is no pump activation, the next step is to check all of the fuses and relays that are associated with the fuel system. This information can be found in the vehicle owner’s manual or in the vehicle repair guide.

3. If the fuel pump does activate but there is low or no pressure at the tester then check for:

a. Clogged fuel filter
b. Restricted or damaged fuel line
c. Defective fuel pump
d. Damaged fuel line at the pump connection
e. Bad fuel pressure regulator

After any repairs are performed (if needed), be sure to test the fuel pressure again to ensure that the reading is correct and no further repair is needed. Also be sure to check for any leaks or loose fittings that can cause problems later down the road.