Engine is an IO-360. In cruise, fuel pressure on the engine driven pump is generally high 20s, but sporadically fluctuates and I?ve seen it drop into the teens. JPI fuel flow will be 9.5gph and fluctuates +/- .4 gph. When I turn on the electric pump, fuel pressure becomes stable at 28 PSI, and indicated fuel flow will drop to 8.7gph and fuel flow fluctuation is reduced to +/-.1 GPH. EGTs stay the same, so I believe the mechanical pump is causing a fluctuation in pressure and an erratic and somehow inaccurate fuel flow reading.
The engine driven fuel pump was overhauled, but sat dry and in warm temps for 5 years before flying began. Does the pump have an issue? Any idea what is going on here? Should I just replace the mech pump?
Where is your fuel flow transducer? What you described might indicate that there is some vapor being pushed through the transducer (same actual fuel flow indicates higher because of bubbles). Vapor will also reduce the pressure as they are probably coming somewhere upstream of the engine pump. If it is following increased OAT's, it is likely fuel vapor creation on the suction side of the engine pump somewhere. It could be a leak also, as others have noted.
Parts replacement is not recommended as a sole diagnostic tool.
I have had this occur rarely. At 9.5 gph, I assume you are not in a leaned condition. As Alex (& Vic) mentioned, one possibility is you are sucking a tiny bit of air into the line. To be absolutely sure, I would apply 30 psi air pressure at the wing root within plugged before the inlet of the mechanical pump. A PITA to be sure, but it is the only definitive way to determine if you have a leak and then to find it with a liquid leak detector. Use a pressure gage, and a valve on the wing root end. Adjust pressure up to 30 psi then shut off the valve. If it drops steadily over a couple of hours, you have a leak.
The condition may be aggravated by having a hot fuel in the tanks on the ground, then climbing to cruise altitude quickly. I.e. hot fuel.
The last factor is heating of the fuel in the tunnel up to the mechanical pump. I have a 4-into-1 exhaust and it radiates a lot of heat up to the skin and firewall. Fuel temperature rises have been measured. Installation of a thermal barrier has eliminated the dropping of the pressure when in cruise, but still I have a slight GPH wander at a very lean condition. 7.4
+0.2GPH, 10k altitude 60F OAT.
My red cube is between the servo and the spider.
To summarize, I have had this happen two times. 1. hot day, full gross, rapid climb to 17,000, then leaned to 7 gph. 2. 9000 ft, testing leaning at 2300 rpm at 6.2 gph. Each time the pressure dropped, alarm sounded and FP turned on. These were phase I, under 20 hrs, IO360 M1B. Leaks had been checked before first flight, again at condition inspection, and no leaks. Proceeded to measure temp rise - it was 50F from wing root to engine pump. Applied thermal barrier and rise dropped to ~30F. More data to be analyzed. Condition has not resurfaced. Reason for GPH wander is still under leisurely investigation.
