914 Low Power on Takeoff Attempt

If your differential fuel pressure is going to 0, there are a very limited number of things it could be. I think this is the exhaustive list:

1. Faulty gauge
2. Faulty fuel pressure regulator
3. Faulty fuel pressure regulator sensing (i.e. broken vacuum line)
4. Fuel line issue (clogged filter, clogged line, underperforming fuel pump)

What would be interesting to see is what happens as you pass the crossover point where airbox pressure is greater than atmospheric. Does the differential fuel starts pressure dropping a little when you first cross 30"Hg, and it increasingly drops in a somewhat smooth and linear manner? And does the drop happen earlier on days when a low pressure front is moving through, and later when a high pressure one is?

Hi Craig,

I certainly don't pretend to know the BUDS program at all well but in the "DataLogs_9576763_2023-12-03_09-30-10csv" file, the timeline is not in sequence, a feature others have pointed out in different threads.

To me, in the files I can access from your post, there seems to be only one flight and perhaps only part of that too. Was it a flight or a ground test run? Lines 6, 7, 8 & 9, for example, appear to be sequential however the change in parameters don't make sense to me re the RPM / throttle position / servo position / ambient pressure relationships for inflight performance.

Maximum airbox temp was 82.7 degrees on line 7 at 2640 seconds but look at the difference in servo position between lines 6 & 7 given the same (Max) RPM, (Max) throttle position, ambient & manifold pressures. I don't know for sure what "boost time" is telling us. (Time "on boost" in the 60 seconds between data captures perhaps?) If that is the case, that could explain the change in servo position at line 7 but given the other parameters at that time, WHY is it there and HOW can it still produce that manifold pressure? TCU intervention? Was the airbox temperature higher than 82.7 degrees at some time during the preceding 60 seconds? By the time you get to line 8, 60 seconds later, it's at (ground) idle RPM with the throttle almost closed. I don't understand that but hopefully someone will be able to explain. 

In any case the maximum airbox temp of 88 degrees is not exceeded in this data set but is it close enough that small differences in under cowl conditions could have pushed it up those extra few degrees at times not recorded? If the normal rise in temperature due to compression is 40 degrees as per the manual, it would suggest your inlet temp is somewhere around 40C+, again suggesting a leak somewhere in your induction system.

All this is me just thinking out loud and perhaps proving my ignorance of the system, please, no-one take anything here as fact or advise!

Cheers, Des.

Folks, 
If you look closely at the first graphs that the OP (Craig) posted showing fuel pressure over manifold pressure, it indicates that the differential fuel pressure is NOT the problem, it’s staying correct at about 3.6-3.9 PSI above manifold pressure as it should.  At first look it appears that there are some places where the manifold pressure is equal to fuel pressure but that’s deceiving.  It only appears that way because of the difference in units of measure, and the fact that he is using a sender that measures fuel pressure as guage pressure instead of differential pressure.  

Let’s take a closer look at the the first graph Craig posted.  There is a location where it appears fuel pressure equals manifold pressure because the fuel pressure and MAP lines overlap.  At that point the fuel pressure is 9.5 PSI (guage pressure) and manifold pressure is 41” hG.  So if we convert the fuel pressure of 9.5 PSI to hG we get about 19 hG above ambient.  Now add ambient pressure to that (assuming a 30 hG day) and our adjusted fuel pressure is 59 hG.  Now subtract the manifold pressure of 41 PSI and that leaves a differential fuel pressure of 8 hG.  Converting 8 hG to PSI shows us a differential fuel pressure of 3.9 PSI, which is close to ideal.  Other areas in the graph calculate to a similar differential fuel pressure.  At least based on these graphs, fuel pressure is not the problem. 

Sorry if this comment appears multiple times (It hasn't seemed to post properly)

I've attached some pictures.

I think I have a couple of things going on:

There is some fuel leakage from both carbs (they're overdue for the 200hr inspection... My bad), so I will be doing that within a week. I'm assuming that leakage from the carbs means that the carb can become depressurized and lose the boost?

If you look at the turbo picture, you can see there's some leakage from the hose that goes up to the airbox. I think it's oil, but I can't imagine how oil could come down that hose. Possible?

The fluid seeping out of the tube between the turbo and airbox is oil.  When the motor sits, oil can drain back into the turbo by getting past the turbo oil line check valve.  Then when you start the engine the turbo blows the oil up the air tube and some drains back and leaks out as per your posted photo. You can replace the check ball and seat or sometimes clean and reseat it. This is common maintenance item, but it’s not causing your problem.  

The carbs are also due for some maintenance, but I don’t think this is the problem either.  The engine runs smooth even when down on power, and the fuel pressure appears correct.  The definitive item on your graphs is the fall off in manifold pressure at certain points. For some reason, your turbo is intermittently not making boost pressure. 

If I were troubleshooting this I would start with a visual inspection of the TCU, the Turbo Control Servo, the Turbo Waste gate and Bowden cable, the throttle position sensor, the enrichment solenoid, and all the wiring connectors between those components.  You could also disconnect, inspect, clean and reconnect all electrical connectors to these components. Then connect the TCU to BUDS and verify all functions and calibrations. These procedures are well described in the 912/914 heavy maintenance manual.