A number of issues can lead to irregular fuel pressure. If your measured value does not match the values specified in the factory manual you should first run the test again, paying special attention to any possible leaks in the fittings. If you are still getting abnormal readings here are a few potential culprits that may warrant further investigation:


Low Pressure
Leaks in the fuel line
Fuel pump issues
Fuel pump wiring, fuse, or relay issues
Upstream clog in fuel line or filter
Fuel pressure regulator failure
Leaking fuel injectors
Low fuel level in the gas tank


High Pressure
Obstruction in the fuel line or rail downstream of the tester
Clogged or obstructed return line
Kinked or pinched return line
Fuel pressure regulator failure