Mechanical fuel pump pressure and bypass troubleshooting tips

This video deals only with pumps having a bypass/return valve. It mostly does not apply for pumps with a return orifice (see note below).

Some very quick troubleshooting tips for mechanical fuel pumps. How to tell if the pump internal valves are bad using minimal tools and effort. I highlight later in the video how even a gauge or a valve is not necessary to do a quick side-of-the-road diagnosis.

Note for this video: some pumps have a bypass valve, some cheap pumps use only an orifice restriction to continuously bypass a certain small amount of fuel back to the return line. On these orifice bypass pumps there will always be some bleed-back through the return line and the pump-to-carburetor line will not hold any pressure with the system off. However the same pressure principals apply during testing while running: a weak or failing pump won’t make enough pressure, and shutting off the return should raise pressure at least a little. How much fuel it is bypassing should be evident by the amount of pressure increase when the return is shut off. A pump running a fixed orifice is more prone to show issues as it weakens, because there is not an actual valve to help it out by closing the return at lower pressures.
Fixed orifice return pumps are subject to machining issues: an oversized orifice can pass too much fuel and make a brand new pump not work out of the box. Orifices can also wear larger over time and get larger and bypass too much fuel. Additionally they can get clogged easily.

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#fuelpump #troubleshooting #autodiagnostics #wrenchwranglers

Chapters:
00:00 Introduction
00:56 Setup
01:49 Demonstration of problem
04:01 Vapor lock prevention
06:00 No gauge needed
07:07 Ending