Low fuel pressure, no symptoms

[RideOrFly]

Still new to airplane ownership...plane is a 1979 M20J. 

I was out flying last night and everything was looking normal for an hour or so. After a touch and go, I climbed up as usual and turned my boost pump off. I checked the fuel pressure immediately after and it was down at 5. I turned the boost pump back on and turned back toward the runway. The fuel pressure didn't change. The engine sounded completely normal and the performance of the aircraft was also normal. After landing, I tried turning the master off and back on. The fuel pressure came back to 5 and stayed there. Then I shut the engine down. When I pulled the mixture to idle, the fuel pressure went back up into the green immediately. I restarted the engine and it stayed in the green. 

Does this sound more like a gauge malfunction or some actual problem? It seems like fuel pressure that low would have caused some symptoms at full power (during takeoff), but it also seems like a faulty gauge would more likely read nothing at all, rather than a specific number. 

[Pinecone]

Gauge or sensor (if not a direct plumped setup).

[carusoam]

First thing people around here will ask…

Is the gauge reading correctly…

Time to plumb a temporary gage in and see if the reading is the same…

Gauges and sensors aren’t digital… they can give all kinds of false readings before actually failing… 

If the 5psi is outside the green arc… it might be time to have the instrument OH’d… or swapped with known working one…

PP thoughts only, not a mechanic…

Best regards,

-a-

 

[RideOrFly]

Thanks for the input. I went out today to start it up. When I turned the boost pump on the gauge didn't move at all even though I could hear the pump running. It cranked up right away without any problem. The needle still hadn't moved. So I tapped it with my finger and instantly had normal fuel pressure. I think that pretty much clears it up.

Between this and my kinda sketchy annunciator panel, a JPI 900 is looking very attractive. 

[Marauder]

Thanks for the input. I went out today to start it up. When I turned the boost pump on the gauge didn't move at all even though I could hear the pump running. It cranked up right away without any problem. The needle still hadn't moved. So I tapped it with my finger and instantly had normal fuel pressure. I think that pretty much clears it up.
Between this and my kinda sketchy annunciator panel, a JPI 900 is looking very attractive. 

If you’re on a budget, an EI FP-5L will give you fuel pressure and fuel flow. And if you hook it up to a GPS, time to empty, etc.


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[201Steve]

I believe that the 79 is like my 77, there is a fuel line going to the gauge. I think they get gunked up

[ArtVandelay]

I believe that the 79 is like my 77, there is a fuel line going to the gauge. I think they get gunked up

My 78 didn’t have a fuel line going to the gauge.

Bad probe, bad connection (probe end, gauge end, or engine ground) would be the first place to check.

And yes, a JPI 900 can replace some of the annunciator functionality, you’ll need a couple of lights for gear indicators.

[carusoam]

Great follow-up ROF!

Thanks for sharing it…

Tapping analog instruments used to be a normal procedure…. 
 

Visit EI and JPI to see what they have to offer for primary instruments…

EI is a gateway from small gauges that don’t work very well… to full color screens…. Everywhere…

Best regards,

-a-