I have a head scratcher type puzzle. On climb out my 350i occasionally has an unscheduled power drop of around a hundred rpm. Not every time, just occasionally. In static tests on the ground it is a brief drop then climbs back up. Like in flight, not every time. Static rpm is around 2600 to 2650 at current prop pitch.

Ideas on how to troubleshoot? Checked gasolators, they were clean. Small leak on hose fitting, turned out to be on a return line, not likely source of air in lines. Fuel pressure is around 48+-.5 psi at full throttle. Pumps run either or, pump 1 or pump 2 runs, never both at same time. Might happen only with pump 1, or it could be coincidental. Seems to be more of an issue on first take-off of the day. Or it might be that I just quit flying when the engine is not running to expectations.

Something temperature related? If so, what?

Engine has been flying for 300 hours since I last did any significant changes/work on it. 650 hours over all. Burning a small amount of oil, leaking a very small amount onto engine mount and fire wall from mystery source.

Ideas? Suggestion’s on how to troubleshoot? Ray did not have an immediate answer. Not something he has come across.


N750RP Cruzer 

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I'm new here and only have worked on my car engine, but a valve check using a compression tester will help diagnose various issues and where it is possibly being affected such as a possible stuck, burnt valve, etc., leading to power loss. maybe good to use a bore scope to check for other valve train issues too.

Never thought of stuck valve. The temp nature of the issue has had me leaning towards fuel restrictions or air in lines, or possible electrical issues. Tomorrow I will chat with local A&P about mechanical issues. I have been using mogas so I do not expect much carbon or lead build-up which sometimes contributes to stuck valves. I did use Av gas Dec to March.



What kind of engine monitor do you have? In case it is Dynon or Garmin you can download the EMS data and analyze it on your computer and maybe able to pinpoint the issue this way.

The software to analyze the data can be found here: 

Feel free to contact me if you need any guidance in this.


Herman Eshuis

Wheels and Wings, LLC

ULPower dealer!


I have hesitated to use the data dump from my Dynon D-120 due to the hassle of finding a serial to USB cable. I know, $15 on Amazon’s. Rule number one of flying, pilots are cheap. Also, with the Dynon EFIS I can visual scan some of the data. Not as well as the software, but enough for a warm fuzzy. Yes, I will order the cable tomorrow in spite of being cheap.

UL has diagnostic software, but the cable is almost $200. Have you ever used it?

Are there any methods for testing the ECU?


The engine is in compliance with all service bulletins. Ray’s shop took care of them when I sent the engine for review following a significant prop strike. 

After I get the cable you may get a phone call from Idaho about the software at savvy.


The ability to analyze the data on your convenience at home is much better than trying to catch it while the engine is running as you have many external factors to keep an eye on.

I use the cable on every install for initial run and cross check with the instruments in the panel. I have the ULP UL-Read E080511 cable in stock, let me know if you want to order it.

What engine number do you have?

For an ECU test it has to be send back to Belgium, In all the years installing and maintaining ULPower engines I have not experienced any symptoms like you're experiencing.

Herman Eshuis

Wheels and Wings, LLC

ULPower dealer!


Actual performance/power drop or gauge indication?

Actual. Attention grabbing at about 250’ AGL. Seems to occur about same time into flight. Take-off is also about the only occasion that I fly WOT except when performing flight tests or playing with prop settings. Coincidence or not.

Out of curiosity I tried turning off the cabin fuel valve today to see what impact fuel restriction would have on engine rpm. The half inch ball valve could almost be closed before engine rpm drops. Fuel psi drops before rpm drops. This was on the ground testing.

During ground, static tests today, the rpm drop occurred only once in 10-20 minutes of testing. Occurred in first couple of minutes of WOT. Could not get it to repeat.

Hesitant to start replacing parts without being able to prove that the problem is fixed.


Total brainstorming here....

Throttle linkage "slop" at far end of movement possibly associated with WOT position?

Clean connector on TPS and reseat?  Do you have the older style mechanical TPS or Hall sensor type?

How do tell the difference by looking?

Ralph, any further headway on this issue for you?

Have only spent a little time recently working on plane. Additional static tests indicate a slight rpm drop as engine warms and rpm’s fluctuate around 50 rpm as engine runs with 100 plus drop randomly. 

Changed a couple of spark plug caps that were loose, no change. Will swap out gas filters soon. Will follow with compression check. Will use bore scope while plugs are out. Any other ideas?

My major airplane time issue is that it is currently whitewater float season here in Idaho. Two trips on the Middle Fork of the Salmon so far (100 plus mile floats) with a Selway River trip in a couple of days.  Should provide excellent fishing opportunity. Almost feel sorry for myself with this overloaded schedule of playing.



Running out of water and rivers to float so I am returning to the sky. 

During ground static tests engine appeared to run even poorer. Larger rpm drop, ran rough, and almost quit during one extended test. 

Actions: replaced a couple of loose spark plug caps. No change.

Compression tests: 79/80. Plugs clean and in good shape. Not stuck valves. Not bad for engine with over 650 hours Hobbs time.

Attempted to capture data on laptop via Dynon EMS port. Not successful, operator error. Lacked drivers to use serial cable for data transfer. Nothing another $20 wouldn’t cure. 

Purchased and installed new fuel filters: Problem resolved. Fine fuel filter was replaced and results were obvious. Changing pre filters did not appear to make a difference. With new fine filter the engine ran smoothly, with good power on climb out, slight gain in fuel pressure. RPMs held steady on ground. Close to or just under 2700 at current prop setting. Two hours flying time since, over 15 minutes at 2800 rpm or WOT. Opened fine filter and inspected for visible particles-none. Gasolators were also clean when inspected earlier in testing. No idea why power fluctuated instead of just dropping, but it did. Lesson, when traveling and switching between Av gas and Mogas from various suppliers, be prepared to swap filters at first hint of power loss, or maybe even before. Filters are cheap.



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