Aborted 3rd flt - high fuel psi - chart included
 

My third flight yesterday had to be cut short after 20 minutes due to erratic and high fuel pressure readings. I’m hoping that VAF can help me troubleshoot. I’ve read some previous fuel pressure threads that have provided some ideas, but perhaps I can provide a bit more detail that will help someone who knows more diagnose things a bit and give me a recommended place to start…

A bit of background that may or may not be relevant: this is only the 3rd flight since the plane first flew a couple weeks ago. It was the first flight since replacing the D180 (that failed during the plane’s second flight) with a replacement unit. Otherwise, the plane has flown very well on all flights with no apparent issues with fuel pressure. The manual that came with my carb’d O-320-D2A from Aerosport indicates acceptable fuel pressure range of 0.5-8.0 psi. Thus, my high yellow range is 8.0-9.0, with redline at 9.0 psi. [Edit: see end of this post for corrected ranges] The Dynon transducer is the 2 wire variety, one of which is a ground that goes back to the D180. And finally, FWIW, I checked to make sure I had the correct model transducer programmed into the D180, particularly since it was a new unit that I had just programmed.

Observation #1: On my third flight, about 10 minutes in and while leveling out at about 75% power, I started getting fuel pressure warnings on my D180, indicating that the fuel pressure was getting above 8.0 psi. It soon went into the red above 9.0, topping out at about 10.5. I tried turning the boost pump on and tried turning the boost pump off. Pressures soon dropped, with or without messing with the boost pump but a few seconds later the pressure would start to rise, again getting above 9.0. The chart below shows my psi’s and I’ve thrown in altitude as a reference just to show at what stage of the flight I was and to show that I was having issues at both high and low power settings, including once just as I was crossing the fence. As you can see, as soon as I started getting the high pressure warnings, I started a descent and landed.


Note: the flat altitudes at ~1700 ft show my time before takeoff and after landing. The total duration in flight was probably not more than 15 minutes.

Observation #2: Back in the hangar about 4 hours after shutdown, I turned on the Dynon. It showed 0.9 psi at rest. After turning on the boost pump, the fuel pressure rose right up to 10.6 psi (well into the red) and stayed there for at least 10 minutes, even after I turned the boost pump off. (Question: with engine off, should the pressure stay up for that long even though the boost pump is off?)

Observation #3: Tried again in the hangar after about one hour later still. Turned on the Dynon and the boost pump brought the pressure up to 5.5psi where it stayed there for at least 20 minutes. Don’t know how long it stayed there total b/c I went home. (Even if the 5.5 psi is more normal in this case, the fact that I’ve got wildly different psi’s in each of my three observations, and very erratic readings in my first, suggests something’s funky.)

Anyway, do any of my observations lead to a particularly likely culprit? Even though the engine ran very well regardless of what the fuel pressure read, I’m not interested in flying again until I get it figured out.

If I can provide any more detail that would be helpful, please let me know. I do have the datalog if needed. Thanks in advance for any help.

[edit from above: when I set the ranges on my replacement unit, I unwisely did so on the fuel pressure from memory. Instead of red line being the 9.0 I set, it should have been at 8.0. This doesn't change my problem any, in fact it suggest my high psi's are even higher than they should be, but figured I should correct my original post to reflect what the values should be in case someone unwisely follows my numbers.]

Pressure gauges
 

in the lower range as used for carb engines have a reputation for being eratic. As long as my boost pump caused the reading to go up I considered all was well. If you want the reading to give you confidence I think you would need to find a gauge outside of what most of us are using. My 2nd RV6a has fuel injection and runs about 25 psi and at that pressure the common gauges are much more consistent. Please throw this in with other responses, do not use it alone.
Good luck!

Fundamental troubleshooting rule I teach all my operations engineers - rule out instrumentation issues first! In this case, find a portable hardware pressure gauge and plumb it in - see what it reads while you're running things. Your problem could be a flaky transducer, air in the lines, or simply that those transducers don't really work very well at low pressures. I pay very little attention to the fuel pressure gauges in our Carb'ed airplanes because they don't tell me that much. Injected engines with the higher pressures are much more reliable.

For me, it would be worth the time and effort to chase this down to give myself confidence in the system, even though I might be pretty sure it's not a real pressure problem. Frankly, I don't know how the mechanical or boost pumps on the carb'ed engines can fail to give you high pressure - maybe someone else knows!

Paul

I had the exact same issue last week with carbed O-360.
Flew it home, removed cowl, checked for any obvious faulty fluid and electrical connections. Found none, but did disconnect the fuel pressure line from the transducer manifold, thinking maybe a vapor lock issue. Turned on the boost pump and watched fuel stream out. Reconnected, flew another 6 hrs. No more high fuel pressure. On my flight back from Traverse City ( after watching the worlds largest air show formation act, Team RV perform):D, I did see my fuel pressure drop to .2 psi. It recovered to the normal 2 psi a little later.
On a side note, my DYNON oil temp probe has failed yet again(4th time).
This last probe lasted all of 5 hours!

I suspect your transducer is faulty. Then again, maybe they've always been faulty and we never noticed in our Cherokee's, because there wasn't a "Master Warning" light, to let us know about these normal fluctuations??????

I sure don't know how but I have good evidence it does happen. I replaced a mechanical fuel pump because I would get occasional high pressures after start which would cause very rough engine operation. I could get the engine to sort of run OK with extreme leaning.

Replacing the pump with another one greatly helped the problem but I still get a touch of it from time to time, especially when the temperature is cooler.

I have had no one suggest the mechanism for this problem yet. And oh, I don't think this is likely the OPs problem.

Thanks for the suggestions. Do I do this just in a static ground run? Or in flight? In either scenario, how do I read the gauge with the prop spinning, or do I?

For a first test, you don't need the engine running at all. Just flip on the boost pump as you did before (your Observations #2 and #3) and see if the pressure readings on the Dynon and on the second pressure gauge match each other. Next diagnostic step depends on the result.

You gin up a hose and gauge and prop it up (duct tape helps) where you can see it for a ground run - as a first step. That will give you an idea if the electronics agree with real-world. If you have to go to flight testing, then it gets more complicated....

Fuel pressure and oil pressure test gauges
 

I recently posted a few threads on low oil pressure and high fuel pressure. I ordered new transducers, believing that these new units were defective. I also dropped by NAPA and bought test gauges for each of the above and was pleased that the numbers were oil=about 75 psi and fuel= about 5 psi. A few folks commented that the transducers or wiring may have been switched. As much as I hate to admit it, my wires were switched! All is well now. Advice: begin with the most simple check- go to Napa and get test gauges (about $25 each).

Jim Diehl
N891DD Phase 1 starting soon
Headed to Fl to transition with Jan Bussell this week.

Another good thing to check. Unfortunately for me, Dynon indicates that polarity does not matter for the two wires connecting to the fuel pressure transducer. Thus, for me, it looks like for Step 1, I'm going to try bleeding the system. Step 2, if necessary, will be to check my transducer against a different gauge. Not sure if that test will address whatever it is that's causing the erratic nature of my readings, but at least it will tell me which if any of my observed fuel pressure readings/observations have been most accurate.