Engine monitor data: explain my fuel pressure drama during climb (O-360-A1D)

[DXB]

M20C owner here - O-360-A1D, Marvel Schleber MA-4-5 carb, JPI EDM900.  I've rpeviously had fuel pressure drops during climb close to redline (0.5psi) but without discernable effect on fuel flow.  The behavior was similar before and after I had the JPI installed (the factory configuration was a fuel line all the way to the gauge in the cockpit :/ ).  The behavior occurred when I first got the plane in 2014, but I haven't seen the issue in the last few years of flying regularly. The consensus had been that these events may reflect vapor lock in the fuel line to the pressure transducer, which comes off a separate port on the carb.

Last week it happened again in more dramatic fashion, with fuel pressure dropping well below redline all the way to 0.1psi early in my climb shortly after turning off my boost pump, leading me to turn it back on.  There was a slight drop in fuel flow (~1gph) as the pressure fell below redline, making me think this is a real sentinel event.  But it ran fine and EGTs didn't budge, suggesting no fuel starvation to the engine. The behavior repeated on further brief attempts to turn off the boost pump during climb up to 6500, so I left the pump on until I leveled off.  I then had no problem turning the pump off in cruise with the fuel pressure holding 2.5psi when level.   Later in the same  flight, I intentionally stressed the system by climbing full rich at max power with the boost pump off for 1000 feet, raising the pitch up to 10 degrees.  I could not reproduce that issue - fuel pressure held at >4 psi.  

Yesterday, externally I inspected the fuel system on the ground from boost pump forward, with the system pressurized by turning the pump on.  Everything looked normal, and I saw no evidence of a fuel leak.   I then took it flying, again stressed the system with steep climbs at full power, boost pump off, at various altitudes and could not reproduce the issue.  The only thing different about the day the problem happened was very cold ambient temps (high teens F) and cold soaked engine with CHTs in the 30s at startup despite the engine heater being plugged in at the remote fbo overnight.  I did take on full fuel there the night before.  

What the heck?? Since there was a modest drop in fuel flow, it seems like this isn't just an issue in the separate line to the fuel pressure transducer. Should I investigate further?  Could this been some ice passing through the fuel system and/or a transient partial obstruction in one of the fuel screens?  Or is there an intermittent fuel leak between the boost pump and the engine driven pump? Or is this my engine-driven pump telling me it's going to die suddenly  at an inopportune moment?

 

[47U]

20 minutes ago, DXB said:

Or is this my engine-driven pump telling me it's going to die suddenly  at an inopportune moment?

How many hours/years since the engine driven pump was overhauled?  Any evidence of leakage from the case drain (I know you said you inspected, but would the case drain not leak if the system is pressurized with the boost pump)?

How full was the tank you were running on?  Did you try switching tanks?  The boost pump is towards the low point in the system, the engine driven pump at the high point.  I’ve experienced wide ranges of fuel pressures in my C, both high and low, but never outside the limits.  

Have the screens been cleaned religiously?  Any recent tank work?

What year/serial number is your C?  Do you have the parallel fuel system?  With your cold temps, is there any chance of ice crystals clogging the tank pickup or screens?  If the boost pump has better suction than the engine driven pump, it might be able to overcome a partial blockage.  Were temps warmer, above freezing, on your flight yesterday?

Just thinking out loud…

[Ragsf15e]

56 minutes ago, DXB said:

M20C owner here - O-360-A1D, Marvel Schleber MA-4-5 carb, JPI EDM900.  I've rpeviously had fuel pressure drops during climb close to redline (0.5psi) but without discernable effect on fuel flow.  The behavior was similar before and after I had the JPI installed (the factory configuration was a fuel line all the way to the gauge in the cockpit :/ ).  The behavior occurred when I first got the plane in 2014, but I haven't seen the issue in the last few years of flying regularly. The consensus had been that these events may reflect vapor lock in the fuel line to the pressure transducer, which comes off a separate port on the carb.

Last week it happened again in more dramatic fashion, with fuel pressure dropping well below redline all the way to 0.1psi early in my climb shortly after turning off my boost pump, leading me to turn it back on.  There was a slight drop in fuel flow (~1gph) as the pressure fell below redline, making me think this is a real sentinel event.  But it ran fine and EGTs didn't budge, suggesting no fuel starvation to the engine. The behavior repeated on further brief attempts to turn off the boost pump during climb up to 6500, so I left the pump on until I leveled off.  I then had no problem turning the pump off in cruise with the fuel pressure holding 2.5psi when level.   Later in the same  flight, I intentionally stressed the system by climbing full rich at max power with the boost pump off for 1000 feet, raising the pitch up to 10 degrees.  I could not reproduce that issue - fuel pressure held at >4 psi.  

Yesterday, externally I inspected the fuel system on the ground from boost pump forward, with the system pressurized by turning the pump on.  Everything looked normal, and I saw no evidence of a fuel leak.   I then took it flying, again stressed the system with steep climbs at full power, boost pump off, at various altitudes and could not reproduce the issue.  The only thing different about the day the problem happened was very cold ambient temps (high teens F) and cold soaked engine with CHTs in the 30s at startup despite the engine heater being plugged in at the remote fbo overnight.  I did take on full fuel there the night before.  

What the heck?? Since there was a modest drop in fuel flow, it seems like this isn't just an issue in the separate line to the fuel pressure transducer. Should I investigate further?  Could this been some ice passing through the fuel system and/or a transient partial obstruction in one of the fuel screens?  Or is there an intermittent fuel leak between the boost pump and the engine driven pump? Or is this my engine-driven pump telling me it's going to die suddenly  at an inopportune moment?

 

You're not going to like my somewhat related story and conclusions.  
I have had a jpi-930 on my airplane for the whole time I’ve had it (10years).  It’s an F, so fuel injected.  So yes, it’s totally different. I have the downloaded engine data that shows what I’m about to say…

For the first 6 months of ownership, fuel pressure was steady.  Then it suddenly started varying by 10psi (which was concerning but still in the green).  No change in engine performance.  I had a mechanic look at it and said all my fuel system was working fine. It stopped jumping again about a year later.  Stayed steady for a year or so then started bouncing all of a sudden one day.  Does it on the ground too.  I went so far as to hook up an external manual pressure gage to the line and it was stable, but in doing that, my jpi was also stable.  Ugh.  Then I put a snubber in the line which didn’t work and I also gave myself a self inflicted fuel leak.  I took it off and have watched it fluctuating since (but happily without the leak).  I use Saavy analysis and they have noted the flucuating ff twice when I submitted data for their flight test profile.  Both times they suggested a snubber…

One other thing I did… replaced engine driven fuel pump.  No change, but it had started leaking oil so it needed changed.  I had hoped it would fix the fuel pressure issue, but alas…

So yes, different engine, slightly different presentation, same jpi sensor.  Take it for what it’s worth.

Edited February 11, 2024 by Ragsf15e

[MikeOH]

I'd be suspicious of the sensor. Try "T"ing off a pressure gauge.

[Ragsf15e]

40 minutes ago, MikeOH said:

I'd be suspicious of the sensor. Try "T"ing off a pressure gauge.

I agree, I did think it’s the sensor which is why I t’d a manual sensor.  However, putting a T and extra fuel line dampens any fluctuations, so it likely hides the issue.  I think the eis and sensors are picking up a fluctuation that isn’t perceived by the manual gauges.  Why it’s there? No idea.

Edited February 11, 2024 by Ragsf15e

[DXB]

21 hours ago, 47U said:

How many hours/years since the engine driven pump was overhauled?  Any evidence of leakage from the case drain (I know you said you inspected, but would the case drain not leak if the system is pressurized with the boost pump)?

How full was the tank you were running on?  Did you try switching tanks?  The boost pump is towards the low point in the system, the engine driven pump at the high point.  I’ve experienced wide ranges of fuel pressures in my C, both high and low, but never outside the limits.  

Have the screens been cleaned religiously?  Any recent tank work?

What year/serial number is your C?  Do you have the parallel fuel system?  With your cold temps, is there any chance of ice crystals clogging the tank pickup or screens?  If the boost pump has better suction than the engine driven pump, it might be able to overcome a partial blockage.  Were temps warmer, above freezing, on your flight yesterday?

Just thinking out loud…

Thanks for the very thoughtful response - a few notes:

- engine driven pump is old - ~1650 SMOH in 2000, well before I owned the plane.  I observed similar fuel pressure behavior (though not dropping deep below redline) when I first got the plane >1000 hours ago in 2014.   I'm reluctant to replace it without clear evidence it's the problem though.

- I don't know if  the fuel pump case drain leaks - I see no obvious evidence of it - e.g. fuel dripping or fuel smell in the cowl after shutdown.  However I suppose a brief full power ground run followed by re-inspecting might make sense?

- My tank was near full when it happened, was filled starting from <10 gal the night before at the fbo.  Later in that flight I tried a full power climb with boost pump off to test it out, and I could not reproduce the issue. My second recent flight when I stressed the system and could not reproduce the issue was on the opposite tank.  I have bladders, and no tank work has been done since their install ~11 years ago.

- I do wonder if ice crystals lodged in one of the screens - I of course sumped tanks on that very cold morning after the plane was refueled at the fbo.  Ice or debris that cleared in a screen might explain the behavior.  I kinda hope that's it.   Screens were addressed at annual last April; I agree that should be done again (except the problem seems to have disappeared now :/ )

- It's a '68 C - the boost pump and engine driven pump are in series, not parallel.  

[DXB]

20 hours ago, Ragsf15e said:

You're not going to like my somewhat related story and conclusions.  
I have had a jpi-930 on my airplane for the whole time I’ve had it (10years).  It’s an F, so fuel injected.  So yes, it’s totally different. I have the downloaded engine data that shows what I’m about to say…

For the first 6 months of ownership, fuel pressure was steady.  Then it suddenly started varying by 10psi (which was concerning but still in the green).  No change in engine performance.  I had a mechanic look at it and said all my fuel system was working fine. It stopped jumping again about a year later.  Stayed steady for a year or so then started bouncing all of a sudden one day.  Does it on the ground too.  I went so far as to hook up an external manual pressure gage to the line and it was stable, but in doing that, my jpi was also stable.  Ugh.  Then I put a snubber in the line which didn’t work and I also gave myself a self inflicted fuel leak.  I took it off and have watched it fluctuating since (but happily without the leak).  I use Saavy analysis and they have noted the flucuating ff twice when I submitted data for their flight test profile.  Both times they suggested a snubber…

One other thing I did… replaced engine driven fuel pump.  No change, but it had started leaking oil so it needed changed.  I had hoped it would fix the fuel pressure issue, but alas…

So yes, different engine, slightly different presentation, same jpi sensor.  Take it for what it’s worth.

Yeah I'm trying to avoid replacing my engine driven pump out of general uneasiness without knowing it is the problem...      My JPI fuel pressure readings are broad trends not rapid fluctuations.  

[DXB]

19 hours ago, MikeOH said:

I'd be suspicious of the sensor. Try "T"ing off a pressure gauge.

I had seen similar behavior on the old analog gauge that brought a fuel line all the way into the cockpit before the JPI was put in 10 years ago.  The new sensor is mounted roughly at the same height as the old gauge, so pressure may be an underestimate in either situation, since the sensor is well above the carb.   I believe the JPI gauge reading though I can't exclude there's some issue like vapor lock in the line from the carb to the sensor.  Regardless, I at least partly believe the low reading because there was a small but measurable drop in fuel flow, which is measured distal to the engine driven pump before the carb and temporally correlates with the drop in fuel flow.