K fuel flow gauge erratic

[One Whiskey Hotel]

After resolving a fuel flow transducer issue last year, I've developed a new issue I need some suggestions about. My fuel flow gauge (original MP/FF instrument) has become intermittent and erratic in readings. On takeoff last flight after a short period of not flying the airplane, it would read erratically and seemed to hover around 15-18 gph. Noticing this, I considered returning to the field, but TIT kept true at 1400 and CHT's didn't spike either. I have a EDM-700 and GTN650, and both read about 21 gph consistently on T/O and 13 gph ROP cruise, so I continued the flight keeping a close eye on those and TIT/CHT's, everything was uneventful. Incidentally, in cruise the gauge continued to read erratically between 15-18 gph, contradictory to the JPI and GTN-650 indications of 13 gph. As such, with reliable FF indications on the JPI and GTN-650, I do not believe the issue is in the transducer nor do I believe there is an issue with fuel delivery to the engine.

I pulled the CB for the FF gauge to reset it, hoping it might resolve the issue (it obviously didn't), so where would a good hangar elf start to look next? Perhaps the connection behind the instrument itself? Or do you think my FF gauge has simply laid an egg? Need to resolve this as it's a primary and very important instrument.

Your thoughts? Anyone else experience this sort of issue, and how did you fix it?

Edited September 5, 2017 by One Whiskey Hotel

[peevee]

so, to clarify you saw 15-18gph on the ships gauge and the JPI read a normal 21?
I think your reasoning is sound that it's probably not a fuel delivery issue, but I would disagree that it's not a transducer issue since the two are different transducers (JPI and factory). Were it my plane I'd remove the factory transducer (orange/red plastic cap) and soak it in carb cleaner a few hours and see if that didn't resolve matters.

 

My thinking is it can only be 3 things, the transducer, the wiring, the FF gauge. God help you if it's the gauge, I think they're hard to come by. Had a flaky FF indication on our old 231 and cleaning the transducer cleared it up.

[One Whiskey Hotel]

Peevee: Correct, ships gauge wavered about 15-18, while JPI indicated 21.

When I had transducer issues a year ago, it turned out to be bad wiring. The repair resolved the issue immediately, but perhaps it's time to re-examine the repair and verify. When I was in there last, I only found one transducer and I believe the JPI may be piggybacked on the ship's gauge transducer? Is this common?

 

 

Edited September 5, 2017 by One Whiskey Hotel
Damn autocorrect

[peevee]

37 minutes ago, One Whiskey Hotel said:

Peevee: Correct, ships gauge wavered about 15-18, while JPI indicated 21.

When I had transducer issues a year ago, it turned out to be bad wiring. The repair resolved the issue immediately, but perhaps it's time to re-examine the repair and verify. When I was in there last, I only found one transducer and I believe the JPI may be piggybacked on the ship's gauge transducer? Is this common?

 

 

I do not know, the ships gauge uses kind of an odd transducer in my experience. Our 231 had an EI analyzer and it used a separate transducer. It might be more common to use one than two, I wouldn't know.

I would definitely clean yours then, they do get gummed up after awhile. It's a low cost affair anyway.

[Guest]

It would seem odd to me to have two flow transducers in series in the fuel system.  More common would be one transducer providing data to two indicators.

Clarence

[DonMuncy]

The original ship's transducer in the K (at least mine), is different than one for the JPI. My parts book shows two different ones. The real killer is that mine had the same part number as the one for the JPI,  but it is my understanding the original was modified at the factory for use in the Mooney. The JPI transducers are fairly reasonably priced, but the original ones are terribly expensive.

I have never seen it, but I don't know why you couldn't put two transducers in the line. They turn so easily, I wouldn't think they would distort the flow.

[peevee]

5 minutes ago, DonMuncy said:

 The JPI transducers are fairly reasonably priced, but the original ones are terribly expensive.

 

this was my experience when I thought I needed to replace ours, that's why I soaked it.

[DonMuncy]

Just now, peevee said:

this was my experience when I thought I needed to replace ours, that's why I soaked it.

I also cleaned mine and it worked for about an hour before failing again. I used the one for the JPI and had my JPI instrument modified to read fuel flow. Both the original instrument and the JPI read off the new transducer, but the original reads about 1/3 the true flow.

[peevee]

9 minutes ago, DonMuncy said:

I also cleaned mine and it worked for about an hour before failing again. I used the one for the JPI and had my JPI instrument modified to read fuel flow. Both the original instrument and the JPI read off the new transducer, but the original reads about 1/3 the true flow.

I think I actually dug up your experience when I was looking into it for ours. I figured for a few dollars in carb cleaner and an hour of my time it was worth a shot.

[carusoam]

Somebody reported a FF ship's gauge the other day that was an analog gauge with a FP sensor.  Essentially an FP gauge calibrated in gph.

Typical FT101 and JPI FF sensors are paddle wheels in the flowing fuel with a magnetic pick-up.

If two gauges have different output... you probably want to check the details of the two systems that you have...

If they share the same sensor, it isn't so much a sensor problem...

PP thoughts only, not an instrument guy...

Best regards,

-a-