Fuel Pressure Fluctuating PIREP

[elimansour]

Hi all.  I had a strange fuel fluctuation on the fuel pressure in my J.  Engine is 50SMOH and fuel pump was replaced with new at engine OH.  I noticed the fuel start to fluctuate as soon as I turned off the boost pump on climb.  Here is a short video - you will see the pressure go back at the end as I turned the boost pump back on to keep from having pressure go in the red.

Fuel Pressure Fluctuation

Some additional facts.  This was the first time I saw this behaviour from the fuel pressure.  Previously pressure was always near the top of the green. Temp was 92 and aircraft was fueled to top off earlier in the day (when it was cooler).  Plane was outside all day.    I noticed fuel tanks were really full when I did the pre-flight, in fact, there was fuel seeping from the overflow below the wing.  The fluctuations would occur randomly and in a couple of instances required boost pump use to keep from getting too low.   The fluctuations were jerky - not smooth and could vary from 28 down to 16 and jerk back up in a second. The really bad fluctuations seemed to stop after about 1 hour in the flight and only returned a couple of times after - but never getting so low as necessitating boost pump use.

Luckily, I had an oil change scheduled today so will also be speaking to the mechanic about this but I wanted to share with the Mooney collective and see if anyone has experienced this previously.  As this was happening thoughts varied from vapor lock or engine driven fuel pump issues or possibly air in the lines.  

Will report back what the mechanic says.

 

[johnw413]

  I just replaced the fuel pump in my '81 M20J because it stayed near the low red-line (14psi) and I thought it fluctuated a little but never really dived into real red zone. When I start the engine I get about 25psi, both with the engine pump and the boost pump. On takeoff at 2300' it goes down to about 20, which I attributed to the throttle being wide open.  As I climb to 9-10,000' it goes down to maybe 15psi.

  I tried doing different things: change throttle,mixture,prop speed, etc but the only thing that changed the reading was altitude. At what altitude do you normally cruise? Does turning on the boost pump help? It changes very little for me, maybe 2psi. People keep saying the normal pressure is about 25 but I only see that at low altitudes.

  The A&P thought I was wasting my money changing the fuel pump. I'm not sure what else could account for the symptoms I saw other than the mech fuel pump. I would've thought the boost pump would help boost, but no.

   I'd never really looked that hard at the fuel pressure gauge before, just usually while priming or take-off. I thought the pressure was high but now I'm not sure. I've been told many times that if the reading is in the green it's OK.

  I'd be curious at what altitude you were reading it. I'd also wonder if your problem was the gauge, sending unit or connections. I'm guessing this is the kind of gauge that uses a bimetallic spring that's heated by current from the sending unit. In my case, it seems pretty stable. I really wish I understood this problem better. I believe some M20Js have a purely mechanical  gauge but I'm pretty sure mine's electric.

 

 

 

[PT20J]

Also check this thread

 

[elimansour]

2 hours ago, johnw413 said:

  I just replaced the fuel pump in my '81 M20J because it stayed near the low red-line (14psi) and I thought it fluctuated a little but never really dived into real red zone. When I start the engine I get about 25psi, both with the engine pump and the boost pump. On takeoff at 2300' it goes down to about 20, which I attributed to the throttle being wide open.  As I climb to 9-10,000' it goes down to maybe 15psi.

  I tried doing different things: change throttle,mixture,prop speed, etc but the only thing that changed the reading was altitude. At what altitude do you normally cruise? Does turning on the boost pump help? It changes very little for me, maybe 2psi. People keep saying the normal pressure is about 25 but I only see that at low altitudes.

  The A&P thought I was wasting my money changing the fuel pump. I'm not sure what else could account for the symptoms I saw other than the mech fuel pump. I would've thought the boost pump would help boost, but no.

   I'd never really looked that hard at the fuel pressure gauge before, just usually while priming or take-off. I thought the pressure was high but now I'm not sure. I've been told many times that if the reading is in the green it's OK.

  I'd be curious at what altitude you were reading it. I'd also wonder if your problem was the gauge, sending unit or connections. I'm guessing this is the kind of gauge that uses a bimetallic spring that's heated by current from the sending unit. In my case, it seems pretty stable. I really wish I understood this problem better. I believe some M20Js have a purely mechanical  gauge but I'm pretty sure mine's electric.

 

 

 

I typically cruise 9-12K.  This occurred yesterday at 9K.  When I turned on the boost pump it went back to 28psi and stayed there solid.  I spoke to the mechanic this morning and he will be checking but he thinks because there were no other indications of something wrong (no engine surging, fuel flow, fluctuations, the gauge being solid when boost pump is on, etc.), it is not likely to be air trapped in the lines or a mechanical issue but perhaps an indicator issue.  He will be troubleshooting and I will report findings.  It was a bit strange to see it go down so low when it had been right at 28psi for the last 50 hours since the enginge OH.

[HIghpockets]

I had a similar problem with my 1988 J. Fuel pressure would swing towards borderline low after turning off boost pump. I had the gauge checked, no problem there. Finally, on a flight in cruise, the needle dropped into the low zone and had a brief hiccup in engine. I replace the engine driven fuel pump and now it has been rock solid in the green even when switching tanks.

 

 

[johnw413]

  That's what I hoped but replacing the pump didn't help. I'm now wondering about an air leak.

[carusoam]

Fuzzy memory...

Somewhere is an altitude sensing and adjusting fuel pressure or mixture device...?

probably related to the fuel injection system...

It appears that the device may need some cleaning...

Anything sound familiar?

Something to ask the mechanic about... he may be aware of these details.

what is the name of the fuel injector system, how does it know what altitude it is at?

Sorry,  I don’t have more detail to offer...

Best regards,

-a-

[bradp]

O ring on your gascolator / fuel selector is another place to check.  Air gets entrained.  Simple fix.  

It’s very easy to disconnect the fuel pressure line at both ends and blow out the line. 

Start there before going further 

[johnw413]

  That's what I will discuss with the A&P tomorrow. It seems to account for the change in psi with altitude.

[Firehawk335]

I’ve had similar problems a couple times over the 1000 hours or so in my J.  Pressure bouncing around has been the fuel pressure sending unit (they are relatively cheap and easy to replace).  Have also had the fuel flow meter act up and it is remedied with cleaning and lubrication by the A&P.

[johnw413]

  It doesn't bounce around so much as stay low. It was checked against a mech gauge and seemed OK.

I'm going to da plane this morning and going over stuff w/theA&P. Here's hoping!

[elimansour]

UPDATE - I just got the plane back after the oil change.  The mechanic could not replicate the issue.  He inspected all wiring and fuel plumbing/ plugs and could not find any issues.  He secured the relevant wire bundles and applied stabilant 22 to the plugs.  When taxiing back today I also saw steady fuel pressure w/o fluctuations.  Will have to see how it goes on the next flight...

[elimansour]

UPDATE - finished 2 flights and 6 hours of flight time - I have not seen the fluctuations again...so I am stumped about what caused them in the first place or if it was fixed by the mechanic tightening bundles and applying stabilant 22 to the plugs.  Hopefully it is the latter and maybe there was something loose in the indication system...

[Pilot boy]

On 7/19/2019 at 8:44 AM, elimansour said:

UPDATE - finished 2 flights and 6 hours of flight time - I have not seen the fluctuations again...so I am stumped about what caused them in the first place or if it was fixed by the mechanic tightening bundles and applying stabilant 22 to the plugs.  Hopefully it is the latter and maybe there was something loose in the indication system...

Did you ever have further issues with this?  Any further resolution?  Did it get worse or come back?

[elimansour]

6 hours ago, Pilot boy said:

Did you ever have further issues with this?  Any further resolution?  Did it get worse or come back?

Thankfully, have never had the issue again in 200+ hours.