Water hammer in fuel injection system? T444E

International T444E in a 3400 transit bus. Fixed a lot of problems in the fuel system, now she runs great. However, in the course of the fixes, I added a simple aftermarket fuel pressure gauge before the mechanical pump, and I noticed when running the needle jumps up and down rapidly (it’s periodic, not random, though-- in other words, the average and the peaks look stable, it’s not just jumping around completely wildly). The gauge gives a steady reading when the engine is not running (when pressurized by the aftermarket electric lift pump), and I’ve used this model gauge before on other kinds of engines and never seen this behavior.

This looks like the “water hammer” effect to me (though obviously with fuel instead of water). Is this normal for these engines? Makes sense it might be, with a such a reciprocating-style mechanical fuel pump as the T444E has, but I would expect the regulator at the end of the system to smooth this out, and/or the fact that the mechanical pump is two-stage. Could something be wrong with either of those? I know in other kinds of systems, like plumbing, severe water hammer can lead to serious rapid wear and failure of parts, but can’t say I know much about the T444E fuel injection system, as I haven’t seen anything quite like it. It’s pretty different from even the Ford Powerstroke 7.3L it’s patterned after.

Also, I noticed there’s a proper high-pressure tap for testing the fuel injection system pressure. It’s a Schrader valve, so it looks like I could just hook a regular old tire gauge up to it, but since it’s fuel instead of air, I wasn’t sure if this would either damage the tire gauge, or give a false misleading reading. Are you supposed to use a tire gauge? If not, what should I be using and where do I get it?

Sorry for all the questions, but my auto parts store knows nothing about these and has no Haynes manual or anything like that, and information online seems to be piecemeal at best…

What kind of aftermarket lift pump did you install? Could you provide a link to its specifications? What is the psi of the pressure spikes you see on your gauge?

okay, a diesel bus engine, two fuel pumps, one low and one high pressure, you are trying to measure the low side at the inlet to the high-pressure mechanical injector pump, correct? Seem like every time the mechanical pump cycles, it creates a ripple in the low-pressure line back to the tank…Sounds normal to me…You may need to rig a return line back to the tank from the injection pump inlet to smooth things out and relieve some of the unneeded pressure. They make inline fuel filters with 3 ports…In, Out and return. They were designed for gas engines to prevent vapor-lock…Is the electric “lift pump” something you have added to overcome some real or imaginary fuel starvation? Perhaps that now pressurized fuel line is confusing a check valve in the main fuel pump allowing it to pulse back into the fuel line and you are seeing those pulsations on your gauge…