Fuel Pump Overpressure

Yesterday while taxing for departure the lady in the EFIS started complaining about fuel pressure. It seemed to be intermittent and I have had this indication at times before. The EFIS would read 7.5 to 8.0 # then after a bit the pressure would go back down to much lower numbers. I thought the problem was a faulty sender as the engine didn't care. Anyway yesterday after I flipped on the boost pump for takeoff the lady started complaining about the pressure again but as the power came up the pressure fell to about 7# and she shut up. I did a couple of circuits of the field. Every time power was reduced the pressure would go up and hold at 8# or so until I powered up then it would reduce.

I know how these LW 15472 low pressure pumps work as the actuator spring is the pressure regulating device so I decided to put a mechanical gauge on the fuel system. I just knew the problem had to be a faulty sender. To my surprise the mechanical gauge and the EFIS read within 1/2 # of each other. With just the mechanical pump on the pressure was generally around 7.5# and when the boost pump was turned on it jumped up to a little more than 8#.

I had always been told that there was no way for the system pressure to build that high because of the way the pump was designed. I do know now that that is just not true. The pump has less than 400 hours on it and I have a new one on the way. I never had a problem with this before I upgraded the panel and the plane was not flying for about 6 months. The pressure before always was in the 2 to 3# range and sometimes would fall below 1# for a short period.

Perhaps some folks out there in Van's land have an explanation for this abnormal behavior as I have never heard of a spring getting stronger, just weaker with time. I sure can't think of another explanation.

Very Hot?

Was this on a hot engine? Long taxi? See this thread with references to pneumatic lock.

I have seen some high FP in these situations. No idea if this may be what happened to you. If the engine was not hot, I doubt this is the culprit.

Nope cool engine and short taxi from hanger. OAT was about 88 deg. F. and I use 100LL fuel always

Ignore me.....

Clearly I was wrong - let us know what you find!

Fixed the problem!!

Bolted the new pump on Friday evening and went up yesterday. All is well again with pressures normal. The ambient temperature was around 100 deg. and I not only did a climb to 10500 and set up for cruise but also did some low level and pattern work. The pressures were perfectly normal in all regiments.

I really wanted to disassemble the pump and try to figure out what was causing the high pressure situation but that $100.00 core charge made me box up the defective pump for return.

The moral of the story is - yes these pumps can and are able to provide excess pressure. I would have never believed it unless it happened to me. So beware if you have excess pressure. It might not be a faulty sender after all.