1 Change the fuel filter before testing the fuel pressure. Dirt and debris can clog the filter, and water often settles there, making it difficult for fuel to pass through. Testing fuel pressure with a dirty fuel filter will result in inaccurate readings.

2 Install the proper adapter as close to the engine as possible. This is where the fuel pressure is needed, so testing further back in the system might not yield an accurate test. Start, or attempt to start, your vehicle. Let it idle for a moment or two. This gives the fuel pressure a chance to stabilize.

3 Check your owner's manual to determine the desired range for the reading. A general rule is that mechanical pumps for carburetors run from 6 to 12 psi. Generally, fuel-injected engines run in the 18 to 35 psi range, although some are higher.