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Fuel pressure issue

bpattonsoa

Well Known Member
Here is an interesting fuel pressure issue I saw the other day in the -10. Looking for input.

Flight from KSBP to 7S5, cruse altitude 14,500, good tail winds up at altitude. Aircraft was fueled the day before and sat outside with an OAT approaching 95. I assume the fuel temperature was high. Spent 15 minutes on the ground after engine start so plenty of time for things to heat soak, very conservative tower, used to fly out of there. Takeoff about 1630 with an OAT of 80 (had been over 90 earlier) rising to 91 six minutes after takeoff. Takeoff with fuel pump on, fuel pump off at about 1,500'. With the high OAT and a long flight ahead, I decided to do a slow climb, around 145 knots. Going thru 5,000' (much higher PALT) I leaned it down to about 15 GPH.

Looking at the data on Savvy Analysis during the climb, the fuel pressure was wiggling about .1 - .2 PSI and slowly dropping. I did not notice until I got a low fuel pressure alarm (at 17 PSI) 11 minutes into the flight at a pressure altitude of 8,500', Turned fuel pump on and it immediately recovered.

Continued on (plenty of airports around) another two minutes and then turned fuel pump off at about 14 minutes. Dropped in about 15 seconds to alarm and fuel pump went back on. Now it had my attention! Thoughts of Lefffler's issue running thru my head, a long way from home.

Switched tanks and waited about 5 minutes and turned fuel pump back off. Pressure did a quick drop to around the alarm and the fuel pump went on again. OAT was now 55.

Case of get home today, so I continued until getting to 14,500, OAT around 49. Turned the fuel pump off 30 minutes into the flight. Pressure went to normal, about 28.6, but looking at the data reveals that it was bouncing around about .1 - .2 PSI for at least another 40 minutes. It then stabilized into a straight line with and occasional bump for the rest of the flight.
For the first 15 minutes at cruise altitude, it did not seem to want to lean normally. GAMMI spread (Skyview system) was normal, around .7 GPH, but TAS would not remain stable after leaning. I tried several times and then it finally remained steady at its regular 172-3 knots. Rest of flight was normal.

I have a tunnel thermocouple, tunnel temp was 132 during the weird behavior and 113 the rest of the flight. The fuel system is normal Vans design except the Red Cube is between the injector and the Spider. (Solved the strange behavior with the fuel pump on)
I am hoping that I was seeing the effect of hot fuel, not a mechanical pump problem.

Comments
 
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I have had exactly the same issue with my Glasair. It is definitely heat related. Current practice is to leave boost pump on to level off. This has solved the issue for the time being. My thought is the increased airflow from the much higher speed at cruise in high, cold air allows the cooling. My current plan is to put a cooling shroud on the fuel pump supplied with a tube from the upper deck cooling air. Have never had the issue when it is not very hot and at low airspeed. Never an issue in cruise. The RV4 has a carb and never sees this issue either.
 
I seen the same thing happen on a long climb to 17500. I now just leave the pump on throughout the climb and never see an issue. I have same setup as you regarding the fuel system layout.
 
I have had exactly the same issue with my Glasair. It is definitely heat related. Current practice is to leave boost pump on to level off. This has solved the issue for the time being. My thought is the increased airflow from the much higher speed at cruise in high, cold air allows the cooling. My current plan is to put a cooling shroud on the fuel pump supplied with a tube from the upper deck cooling air. Have never had the issue when it is not very hot and at low airspeed. Never an issue in cruise. The RV4 has a carb and never sees this issue either.

My Tiger POH checklist requires the fuel pump to be ON all the way to cruise altitude.

This is for an O-360 with a carb.

What do certified IO-engined POH checklists require?
 
Good question, Gill. In my 40 hour test period, I did a 500 FPM autopilot climb to 17,500'. The fuel pump was off the whole time. It was on a cool day, and cold at altitude. The attitude got interesting around 17K, and the autopilot was working real hard, flying like it was drunk!

I also did a takeoff (on a long runway) with the fuel pump off and got a low pressure alert, set at 17 PSI, but the pressure stayed above the Lyc. Published minimum of 14 PSI.

I normally shut off the pump just after I go from 2,700 RPM on takeoff to my 2.500 climb RPM. Never seen any strange behavior before. I may be changing my takeoff to keep the pump on, especially on hot days or looking at a pump cooling shroud.
 
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Same here Bruce. If it's hot my mechanical pump will sometimes lose pressure to the point the alarm activates. I've installed the shield and a blast tube but it will still lose pressure.

I'm convinced that vapor lock is taking place in the lines, and the pump just isn't designed to pump vapor.

Let us know what you find.

~Marc
 
I recently had some low pressure alarms but the engine ran fine. Talked to Airflow Performance and they said if it was vapor lock, I should feel a stutter as the bubbles pass through. We thought it was a sending unit. Was told my IO-360 would run fine all day long at moderate power settings on 15 PSI. My pressure varied from 15-25 PSI.

Ended up being the mechanical FP. I have the cooling shroud already. Have flown several flights on hot days recently and the pressure was rock solid at 29 PSI.
 
29 PSI ?

I am just curious as to why a pump designed for a maximum pressure of 25 psi produces 29PSI, especially the mechanical pump.
The AFP electric pump delivers precisely 25 psi except for a momentary spike to
28-29 psi a few seconds after it is turned on.
 
I have looked all over after installing the new pump for "normal pressure" and could not find that. Plane has 35 hours on new pump and is mostly running 29. Have a Dynon Skyview for EMS, pump is a Lycoming, new pump.

Any suggestions?

Sorry to hijack the thread.
 
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What do certified IO-engined POH checklists require?
I bet it depends a lot on the installation. The POH for my 200 HP IO-360 Beech Super has the pump on for start, and "as needed" the rest of the time. Of course the cowling is the exact opposite of an RV - tons of room in there and I don't think Beech was too worried about cooling drag. That being said, never had an issue even in the NM summer. No shroud or blast tube on the pump, either.
 
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