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Fuel Pressure Fluctuations

clutch22

Well Known Member
Attached is a link to flight data from this flight.

Tuesday morning I had some pretty strong fuel pressure fluctuations in a constant cycling pattern 24-21 psi . I didn't notice the fluctuations until after I was at altitude and my boost pump was off. Returned to the airport and inspected all connections; no leaks.

Ground running shows same fluctuation pattern. Turning on the boost pump eliminated any fluctuations, the pressure held solid at 28.0 psi. So, I think that eliminates the possibility of transducer/electrical issues.

Background:
-My mechanical fuel pump only has around 20 hours on it since new. I had replaced it at the time because I was getting some smaller fluctuations and couldn't find any leaks. The pump replacement turned out to be unnecessary, as I later found a hard to access connection, that I guess I missed torquing. It was seeping. At that point, I verified that all fittings were properly torqued. The last 20-ish hours have been trouble free until last flight.

Any ideas?
How likely is it that I have a bad mech pump with only 20 hours on it?

Flight Data:
view


Ground Run:
view


Thanks
 
Piling on the mechanical fuel pump discussion. When flying yesterday, my engine stumbled for about half a second. I checked the gauges, and noticed the fuel pressure was reading 0.0, ugh. FP in my plane tends to vary widely, but this got my attention. I flipped on the boost pump for a few seconds until the pressure rose again, then switched it off. Pressure came back up again, and didn't drop to zero again. I decided to look at my flight data for this trip and noticed that FP had dipped to zero earlier in the flight as well, but I hadn't noticed. It seems to happen right after switching the boost pump off. Is this normal / expected behavior? Is the mechanical pump on its way out? A graph of the fuel pump state (red line, 1=on) overlaid on a graph of fuel pressure is attached. The burble / stumble event happens just after the :45 mark, then the boost pump is switched on for a few seconds. Comments appreciated.

https://photos.app.goo.gl/spw7ztU2gsQn2V0m2
 
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What you are seeing is perfectly normal.

On even hotter days, after a climb, the fuel pump and associated parts become heat soaked and at altitude with warm fuel in the tanks you are getting vapour bubbles. When you see down around 12-15 PSI you might get some engine surges.

If you are using togas this gets worse. So unless you have a system designed to force feed togas do not use it.

I note you are in OK. There is a really good class taught by the best people twice a year in Ada OK. You should attend! ;) Engine management made easy.
 
I'd like to give an update on my fuel pressure issue. I've been fighting this for a while now, and I'm starting to get tired of working on the plane on days I could be flying.

To recap what I noted in my original post:
-Had fluctuations early on in phase 1. I jumped the gun and bought a new pump. I later found the loose fitting that was causing the troubles.
-At some point after replacing the pump and verifying all my connections were torqued, the fluctuations returned, and were a constant +/- 2 psi cycle.
-When the boost pump is on, the pressure is solid at 28 psi with no fluctuation.

That basically covers what I noted in my original post. This time around, I called Tempest tech support to discuss my problem. He ensured me that my pump is fine (since it's new) and told me to replace my O-rings on the inlet and outlet of the pump.
After replacing the O-rings, I put everything back together, did a ground check and flew. My pressure was rock solid. I was happy.
A few days later, I flew a second flight. I noticed a slight fluctuation, but I was in denial that the problem was about to resurface. The following flight was a 2.5 hr x-c where the fluctuations were back to +/- 2 psi for the entire flight.

After reading several old threads where the issue seemed to be from heat, I decided to go ahead and install a fuel pump shroud. I was certain this wouldn't fix it, but I wanted to say I tried. This required me to disassemble the hoses and fittings from the pump. After I got it all back together, I ground ran it, and again, all was fine (solid pressure).
I flew it for an hour yesterday and the fluctuations are starting to reappear. They're still small, but I expect them to return back to the +/- 2 psi cycle in the next flight or two.

I'm running out of options of things to replace (other than replacing the new pump). The fact that when I turn on the boost pump, the fluctuations go away sure points fingers to the mech. fuel pump.

A few last observations I should note:
-The restrictor fitting that is threaded into the 90 degree Tee fitting coming out of the pump is visible for about an 1/8~1/4 of the hole when looking through the perpendicular (outlet) side of the fitting. I don't think the restrictor fitting should be allowed to thread in that far, but with it being NTP thread, thats where it got tight.

-I have a small leak on #5 cyl intake tube at the sump. The day after flying, there will be blue stain here. So, I have something going on here. I don't think this could be causing my troubles since I'm solid with boost pump, but I felt I need to mention it.
 
I suspect you could put 100 mechanical pumps on it and the out come would be the same. I have the same system and these fluctuations drove me crazy the first year. I have a dynon and she warns me before the engine coughs. I have insulated fuel lines and the red cube and it still happens occasionally but less often. Mine seems to happen most often after a fill up but it has happened after cruising for a hour and everythings cooled off so that kinda blows my heat soak theory out. It has never happened on desent that I can recall. Most times it comes back up before I can switch the boost pump on.
 
Thanks for the input so far.
Are you guys saying that even the fact that it cycles constantly from 24-21-24 psi over a 10 second period is normal?
I would agree that it's not out of control, but I'm not sure this should be occurring given that every time I do maintenance on this area, I get about 30 mins to an hour of perfect readings before they start to show back up.

I don't mean to place any doubt, but like Mike mentioned, it's driving me crazy.
 
Plus or minus 2 PSI in flight is normal. It's most likely sensor error. You wouldn't even notice this on an analog gauge. :)

Engine coughing after turning the pump off is not normal. I always leave the electric fuel pump on until top of climb, no matter how high the climb. It helps to push the fuel rather than ask the hot mechanical fuel pump to suck the fuel uphill.
Leave the electric pump on for a minute or two after level off and you shouldn't have a problem. If you do, there is a leak in the line somewhere.

Your blue stains around the intake gasket do indicate a leaking intake gasket, and it should be replaced. But it is not the source or cause of your pump pressure problem, which you may not even have.

Vic
 
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