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Fuel Pressure Suddently drops

D

DarrylFoster

I'm New Here
I'm hoping to get some additional ideas on what my issue could be. I have an RV-7A with an IO-360-M1B engine. Standard setup with a mechnical fuel pump backed up by an electric boost pump. Close to 600 hours on the engine.

When I climb to an altitude above 4500 feet (it's never happened below this altitude), after about 20 minutes, the fuel pressure will suddently drop. A few seconds with the boost pump brings it back up but I'm really stumped on the issue.
I've installed the cooling shroud with blast tube on the mechnical fuel pump and I've redirected the hot air from the heat muff exit tube away from the fuel pump. All exhaust pipes have a heat sheild next to the fuel lines. Fuel lines are fire sleeved.

As you can see on the below output, it is a real issue and not a false reading as the EGT's starting dropping as well.


Fuel_Pressure_EGT.png
 
If it was just a drop in fuel pressure, I'd imagine air in the line leading to the sender. However, your problem seems to effect all cylinders and so I might suspect a restriction in the fuel system (debris that is overcome by the electric fuel pump?) or possibly a leak after the electric fuel pump but before the flow divider. Just guessing but definitely an anomaly to identify before further flight....
 
FredMagare said:
If it was just a drop in fuel pressure, I'd imagine air in the line leading to the sender. However, your problem seems to effect all cylinders and so I might suspect a restriction in the fuel system (debris that is overcome by the electric fuel pump?) or possibly a leak after the electric fuel pump but before the flow divider. Just guessing but definitely an anomaly to identify before further flight....
Click to expand...
Hi Fred,
I'm thinking that I'm getting vapor lock somewhere. Pretty sure it's not a leak. No blue stains anywhere and I've pressurized the fuel line and it held pressure. Only happens above 4500 feet. Once I engage the boost pump for a few seconds, it returns to normal with no further issues.
Also pretty sure it's well before the flow divider because I don't think I would see a drop in pressure if it was at the spider due to the AVSTAR fuel injector being inline past the point where I pick up the fuel pressure.
 
I have your same set up but a James Cowl.

Fuel restriction or air bubble - yes. But the root cause needs some diagnostics. Clear vent tubes, to pressurize the tanks and equalize them in a climb. Then it could be a suction leak that allows a bubble to form then clear - or restriction from something in a hose or fuel filter.

I would 1st recommend a pressure test of the fuel system from the servo inlet back to the fuse exits to the fuel tank. Purge the fuel and leak test with 30 psi air. It should hold 30 psi with less than 2 psi drop in 24 hrs. It can drop pretty fast with a tiny leak.

Possibilities could include the mechanical fuel pump.

On the temps, I have measured temps on the fuel lines from the side fuse bulkhead to the mechanical fuel pump and they get a 50F rise when hot. The fuel line in the "tunnel" should not touch the bottom skin. My source of heat is exhaust from a 4-into-1 collector that is close to the firewall corner. My issue was pressure stability at altitude and not as you describe, so don't think the temp alone is the root cause.
 
I had a similar experience with the same engine @ about 1000 hours. Fuel press would randomly drop from 25 to 18 or 15. Almost always in a very nose high pitch attitude. Boost pump would immediately bring it back up. I replaced the mechanical pump and have not had any more instances of low pressure for the last 100 hours. YMMV. Good Luck!!!
 
BillL said:
I would 1st recommend a pressure test of the fuel system from the servo inlet back to the fuse exits to the fuel tank. Purge the fuel and leak test with 30 psi air. It should hold 30 psi with less than 2 psi drop in 24 hrs. It can drop pretty fast with a tiny leak.
Click to expand...
30 psi is way too high.

As this happens on climb out and goes away with the boost pump, I would first suspect you are sucking air someplace. This could be in the fuel lines (before the mechanical fuel pump) or the mechanical fuel pump itself.

I agree with the first step to verify you tank vents are clear.

Carl
 
From the manual for the M1B (and all of the IO's for that matter...)

Fuel Pressure Limits:
Input to Mechanical pump - Minimum -2 PSI, Maximum 35 PSI
Input to Fuel Servo - Minimum 14 PSI, Maximum 45 PSI

...So 32 PSI as measured in the Van's recommended location (at the output of the mechanical pump) is just fine...

The OP made no mention of the loss of power, vibration, general panic that ensued - flipping switches, turning valves, etc...This did happen, right? :)

As others have suggested, it could have been an "un-port" or air leak in between the boost pump and mech pump -- or an issue with the mechanical pump itself (failed/failing diaphragm, valves).
 
Carl Froehlich said:
30 psi is way too high.

As this happens on climb out and goes away with the boost pump, I would first suspect you are sucking air someplace. This could be in the fuel lines (before the mechanical fuel pump) or the mechanical fuel pump itself.

I agree with the first step to verify you tank vents are clear.

Carl
Click to expand...
Hi Carl, To clarify, it doesn't happen on climb out. It normal happens about 20 minutes into the flight. By that time, I'm normally in cruise. I've done the tank vent lines and they are clear. Does it on both R and L tanks.
I do want to check the tunnel line. It leaves the boost pump and it might be in contact with the bottom skin. I haven't tried that yet.
 
BillL said:
Possibilities could include the mechanical fuel pump.

On the temps, I have measured temps on the fuel lines from the side fuse bulkhead to the mechanical fuel pump and they get a 50F rise when hot. The fuel line in the "tunnel" should not touch the bottom skin. My source of heat is exhaust from a 4-into-1 collector that is close to the firewall corner. My issue was pressure stability at altitude and not as you describe, so don't think the temp alone is the root cause.
Click to expand...
Thanks Bill, I was holding off on the mechanical simple because of the $600 price tag :)
However, I do want to check the fuel line in the tunnel. That is not fire sleeved and I'm sure it's in contact with the bottom skin. Good idea... Thank You
 
I saw the same thing (for the first time) on the way home from Oshkosh @9.5K. They overfilled my tanks at OSH and fuel was venting (collected that) a bit. Wondering if topping the tanks to the brim which I did for longer legs "blocks" the vent tube until the fuel burns down a bit and creates a vacuum? EGT, FF, and pressure all briefly dropped like OP. Think this happened shortly after switching tanks as the timing in the date is right. Had I not been looking at the display I would not have noticed anything amiss. I later data dumped it to see what happened, same as OP.
 
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