I have had two engine driven fuel pumps on my engine in the last 700hrs. Both wobble pumps. The first one failed in the mechanism (not the normal gasket).

Normally the fuel pressure is steady, but I noticed that both old and new pump occasionally fluctuate between about 23 (or 25) and 27 PSI. If I apply the electric boost pump, then I see the pressure rise to about 28 or 29 PSI and the fluctuation reduces, but there is still a small fluctuation. It can keep doing this for hours. Other times, the pressure will be steady for hours on end, I can't say why.

Can anyone comment on possible causes? Should I be concerned?
I thought I saw a thread about this issue somewhere recently, but I can't find it.

Fuel pressure is measured at the outlet of the engine driven pump.
Fuel pump is a certificated part / installation.

This one?

https://backcountrypilot.org/forum/lyco ... tion-23397

It was in this very forum (maintenance) just a few threads down. Kinda related, but not quite the same.

I contacted Tempest, who makes the engine driven fuel pumps. I spoke with Kim Claar about what was going on with mine and he was super helpful explaining all the inner workings of the pump. Maybe he can shed some light on what your experiencing.

Tempest
Kim Claar
336-449-5054

Zzz wrote:This one?

https://backcountrypilot.org/forum/lyco ... tion-23397

It was in this very forum (maintenance) just a few threads down. Kinda related, but not quite the same.

I didn't see anyone talking about the same issue in that thread. The search turned that one up.

I've had a failure of the low-pressure version of that pump at about 500 hours. The failure symptom was low and fluctuating pressure, but also fuel running out of the overflow fitting. My pump failed for inadequate tightness of the cover plate screws, which allowed air to be sucked into the fuel path, and for fuel to bypass into the overflow chamber. You might want to check the screws on yours.

My pump is spec-ed for 4-6 psi, and it usually runs 4.5 to 6. But the variation is slow. I don't see the pressure needle wiggling.

Normally those AC pumps are very reliable, though one of my vivid childhood memories was being stranded on a family vacation in 1967 when the fuel pump on our '64 Pontiac died. There was no overnight freight back then, so we spent a few days in Sudbury, Ontario waiting for a new pump.

DaveF wrote:I've had a failure of the low-pressure version of that pump at about 500 hours. The failure symptom was low and fluctuating pressure, but also fuel running out of the overflow fitting. My pump failed for inadequate tightness of the cover plate screws, which allowed air to be sucked into the fuel path, and for fuel to bypass into the overflow chamber. You might want to check the screws on yours.

My pump is spec-ed for 4-6 psi, and it usually runs 4.5 to 6. But the variation is slow. I don't see the pressure needle wiggling.

Normally those AC pumps are very reliable, though one of my vivid childhood memories was being stranded on a family vacation in 1967 when the fuel pump on our '64 Pontiac died. There was no overnight freight back then, so we spent a few days in Sudbury, Ontario waiting for a new pump.

Thanks for your input Dave, it all helps. I will check out those screws and make sure they are torqued up right.