IO-360 fuel pressure fluctuation

[Skip Forster]

Ever since installing the G3X/EIS in my Mooney M20J with an IO-360-A3B6, I've noticed fuel pressure fluctuations, sometimes as much as +/- 5 psi (normal is about 23 psi), but usually less. Sometimes it goes away by itself with no change in engine parameters. It doesn't seem to correlate with altitude or anything else. Switching tanks does not make a difference. Turning on the boost pump increases the fuel pressure a couple of psi but doesn't affect the variation. It does not seem to affect engine operation.

I have seen a number of threads on various sites (including this one) about this, and there are a lot of helpful suggestions, but I don't recall ever reading someone reporting a root cause and fix. So, if anyone has experienced this and fixed it, I'm all ears.

Skip

 

[Avanza]

Ever since installing the G3X/EIS in my Mooney M20J with an IO-360-A3B6, I've noticed fuel pressure fluctuations, sometimes as much as +/- 5 psi (normal is about 23 psi), but usually less. Sometimes it goes away by itself with no change in engine parameters. It doesn't seem to correlate with altitude or anything else. Switching tanks does not make a difference. Turning on the boost pump increases the fuel pressure a couple of psi but doesn't affect the variation. It does not seem to affect engine operation.

I have seen a number of threads on various sites (including this one) about this, and there are a lot of helpful suggestions, but I don't recall ever reading someone reporting a root cause and fix. So, if anyone has experienced this and fixed it, I'm all ears.

Skip

The fuel system is built to supply liquid fuel to the servo and injectors. During engine operation the fuel lines and pump in the engine compartment sees different (high) temperatures. This leads to bubbles in the fuel lines and varying fuel pressure and fuel flow.
Fire sleeve on the fuel lines and cooling air around the fuel pump usually fix most of this problem.
With warm fuel in the tanks, it´s normal to see low fuel pressure during climb to higher levels (>FL100). Fuel is boiling because air pressure is reduced. This goes away when fuel temperature in the tanks drops. The fuel servo supply a pressure between 1-7 psi to the injectors depending on power setting and air flow.
The aircraft now has modern instruments that shows the imperfections in the system.
As long as the engine is running and there is no alarms, all is good.

Good luck