Kevin Horton
Well Known Member
I've got a Lycoming IO-360-A1B6. The fuel pressure sender is between the engine driven fuel pump and the Bendix fuel injection servo. The fuel flow sender is between the electric boost pump and the engine driven fuel pump.
Historically, the fuel pressure has been in the high 20s. Two flights ago I got an alarm from my Grand Rapids EIS 4000 engine monitor as the fuel pressure indication exceeded 45 psi. It slowly drifted up to 50 psi. There was no change in the fuel flow or any other engine indications. I tried turning the boost pump on - the fuel pressure indication would decrease about five psi, then slowly increase back to 50 psi. Then after several boost pump OFF-ON cycles, the fuel pressure slowly decreased to the high 20s and stayed there for 30 minutes or so. Then, it slowly increased to 55 psi, and stayed there for about 10 minutes despite multiple boost pump OFF-ON cycles. I tried changing fuel flow with the mixture and rpm changes, but they had no effect on the fuel pressure. Then, for no obvious reason, the fuel pressure decreased back to the high 20s. On the next flight, the fuel pressure slowly varied between 23 and 36 psi.
Can anyone explain the mechanism that regulates the pressure output from the Lycoming fuel pump on a fuel injected engine? Are there failure modes that could cause the fuel pressure to vary above and below the normal range?
If the fuel pressure really is high, is there any risk of flying with it that way? Could the fuel injection servo be unable to function properly if the input fuel pressure is too high?
I suspect that this is more likely an indication issue than a real fuel pressure issue. I can't imagine that it is a bad ground, as that shouldn't lead to a fluctuating reading. But, perhaps the sender is failing. I'm tempted to replace the VDO 360 003 fuel pressure on spec.
Comments and advice are appreciated.
Historically, the fuel pressure has been in the high 20s. Two flights ago I got an alarm from my Grand Rapids EIS 4000 engine monitor as the fuel pressure indication exceeded 45 psi. It slowly drifted up to 50 psi. There was no change in the fuel flow or any other engine indications. I tried turning the boost pump on - the fuel pressure indication would decrease about five psi, then slowly increase back to 50 psi. Then after several boost pump OFF-ON cycles, the fuel pressure slowly decreased to the high 20s and stayed there for 30 minutes or so. Then, it slowly increased to 55 psi, and stayed there for about 10 minutes despite multiple boost pump OFF-ON cycles. I tried changing fuel flow with the mixture and rpm changes, but they had no effect on the fuel pressure. Then, for no obvious reason, the fuel pressure decreased back to the high 20s. On the next flight, the fuel pressure slowly varied between 23 and 36 psi.
Can anyone explain the mechanism that regulates the pressure output from the Lycoming fuel pump on a fuel injected engine? Are there failure modes that could cause the fuel pressure to vary above and below the normal range?
If the fuel pressure really is high, is there any risk of flying with it that way? Could the fuel injection servo be unable to function properly if the input fuel pressure is too high?
I suspect that this is more likely an indication issue than a real fuel pressure issue. I can't imagine that it is a bad ground, as that shouldn't lead to a fluctuating reading. But, perhaps the sender is failing. I'm tempted to replace the VDO 360 003 fuel pressure on spec.
Comments and advice are appreciated.