Minor fluctuations in fuel flow - Normal?

[blaine beaven]

Hi all, 

I just got my 1978J with IO-360-A3B6 back from a full panel upgrade - G3X, GTN650xi, GNC355, GFC500, etc. 

I'm really enjoying all of the data from the EIS system. One thing I have noticed is that my fuel pressure seems to fluctuate up and down in cruise, with no other changes being made. Is this normal? Is this a sign of something I should investigate? I did not have an engine analyzer prior to this upgrade so my benchmark was a needle on an analog gauge. 

Attached is a screen cap from my Fly Garmin logbook. When I do a delta analysis on Savvy over a 30 minute period it shows about a 1.9psi difference average. 

Any groupthink is always appreciated!

Blaine

[MikeOH]

Just speculation, but I notice that the fluctuations appear to vary in intensity vs. time.  For example, they seem to be the worst between 11:30 and 12:00.  About half as much between 12:30 and 1:00, and much less frequent from 1:00 to 1:30.  Do you recall any changes made that would coincide with those times, especially of an electrical nature? Strobes turned off, avionics, prop de-ice, passenger entertainment (iPads, iPhones, etc.).  Sure looks like electrical noise.

[Andy95W]

8 hours ago, blaine beaven said:

I'm really enjoying all of the data from the EIS system. 

And it’s incredibly accurate, as well.

In this case, it’s measuring fuel pressure with digital accuracy from a mechanical pump that was designed about 80 years ago.  A fuel pump that is basically a plunger pushing on rubber diaphragms, which then moves the liquid into a mechanical fuel injection system that was designed 70 years ago.

Variations in engine rpm, tiny air bubbles moving through the system, slight turbulence, changes in altitude, bank angles, change in AOA could all have a slight effect on fuel pressure.  The indications show that the variations decreased as the flight progressed.  I would surmise that was from the engine (and fuel injection body and servo) becoming more uniformly heat soaked.

I'm not saying that you don’t have an issue, but for now I certainly wouldn’t lose any sleep over it unless something changes significantly.  The most important thing is your fuel flow, which is nicely consistent. 

[Ragsf15e]

My jpi930 in my F showed those as well.  I actually used a snubber on my manifold pressure and oil pressure to stop those fluctuations, but my snubber on the fp just created a leak so I removed it.  I think @PT20J had the same fluctuations and I don’t know if he ever solved it.  Sometimes the digital displays are too accurate.

[PT20J]

A lot of Lycoming IO-360s do this. There are posts about it on vansairforce as well as MS. It seems that the factory mechanical gauge doesn't react fast enough to show it but the digital ones do. The G3X seems to be more sensitive to it than the JPI. I have pretty well tracked it down to vapor bubbles forming in the fuel pump. It doesn't seem to affect operation. If your servo has a very tight mixture plate, you may also notice that the residual heat causes the fuel pressure to rise off scale after shutdown as vapor forms and raises the pressure.

[blaine beaven]

Thanks everyone! I think it is, as suggested, just too accurate of data. I’m glad to hear it’s not uncommon, so I’ll simply continue to monitor for any abnormal indications. 

[Ragsf15e]

8 hours ago, blaine beaven said:

Thanks everyone! I think it is, as suggested, just too accurate of data. I’m glad to hear it’s not uncommon, so I’ll simply continue to monitor for any abnormal indications. 

You can keep an eye on the drain line for each pump and also note how much (if any) increase you see with the electrical pump on to help monitor health of the system.  The mechanical pump drain on my F was tied off to the sniffle valve drain and is pretty small.  The electrical pump drain is below the pilots feet.

[Yetti]

Do a leakdown test on the fuel pump.  If it passes then go fly.    

[PT20J]

The RSA spec for leakage at idle cutoff is 5 cc/min max. The mixture plate is not a perfect valve. Some do better. AvStar servos have a tighter seal.