Titan DIOX-370-A4L2T occasional fuel pressure drop

Can I ask the VAF brain trust for any thoughts on the following problem on my friend's RV-7.
He has a Titan DIOX-370-A4L2T with around 400 hours from new. Every now and again in the cruise the fuel pressure drops to around 10 psi. As he has an AFS EFIS Jennifer reminds him the fuel pressure is low. The boost pump always restores the fuel pressure. Normal cruise fuel pressure is around 23 or 24 psi. With boost pump on it rises to around 28psi. When the fuel pressure drops it is clear the engine is running lean from the temperatures. He is reasonably sure the pressure drop is real.

The question is what is causing the fuel pressure to drop? It only happens occasionally, after using the boost pump for a few minutes the pressure from the mechanical pump will be back to normal. Until the next time it takes a dive which may be 30 minutes later or after a few hours.

Has anyone seen anything similar? What cured the problem, is it new fuel pump time or could there be another reason?

Any thoughts gratefully received, Pete.

If this happens on 3.000 ft, I would suggest that you take a look at the mechanical fuel pump.
If it happens on 10.000 ft, I would suggest that air bubbles in the fuel line reduce the fuel pressure. This is especially true after leveling of after a climb + 10-20 min. You can check this. After turning ON the boost pump, you will notice a slight reduction in fuel flow. This is because now you have 100% fuel going through the red cube meter, no air bubbles.
This comes from high pressure drop and/ or high temperature in the fuel lines.
To manny 90 deg fittings, no firesleeve on fuel lines, no air cooling of the mechanical fuel pump may be the cause.

Good luck

Peter---have him check the flares and the torque on the lines on the suction side of the boost pump. May not be big enough for a leak, but may be just large enough to induce air in the system. We've heard about it quite a bit after filter maintenance.

Tom

My first suspect is the mech pump. Vapor from heat is somewhat rare in cruise, at least after the cool down, post climb. Problems like introducing air from poor fittings should show up far more often then juar every once in a while and ONLY in cruise. The fact that it only happens in cruise makes me think that the problem only occurs after a FULL heat soak and this takes quite a while. Lot of rubber in that pump and rubber is known to have issues only when cold and only when hot.

If it were my plane, I would probably replace the pump. I have had numerous occasions where FP drops to the upper teens in cruise. Apply boost pump and problems goes away. May happen once or twice more on that flight and then don't see it for MANY flights. Have never figured out what it was. However, in my case, it is dropping to a level that is still acceptable (>14 PSI). In your case, it is dropping to an unacceptable level and therefore, more serious action is required.

Larry