Fuel Flow issue

[Maxmill59]

Hi,

I am flying a Cessna U-206F used for skydive operations.
For 3 weeks now we are facing an issue with a fluctuating fuel flow.

Here are the aircraft info :
•Registration : FGSFZ
•Type : Cessna U-206F S/N : U20601982
•Engine manufacturer : Teledyne Continental
•Engine model : IO-550-F(20), S/N :1041847
•Engine driven fuel pump : Fuel pumpnaturallyaspirated engine (no altitude compensation)

Issue summary :

•SinceFebruary, 16th 2025 :
-Fluctuating and unstable fuel flow
-Fuel flow can strongly drops at any time
-High EGT when fuel flow isdropping

•Engine driven fuel pump and control valve replaced March,13th but fuel flow issue stillongoing
•Issue occurs on both fuel tanks
•CHT steady

Sometimes during takeoff the fuel flow can drop to low value (up to 20 GPH instead of 27GPH) resulting in high EGT and and the decision to land back.
Sometimes it can happen in flight at any time (it happened once when I switched the fuel selector, but no always)
Sometimes the fuel flow is steady on take off, but start to fluctuate and decrease slowly as I climb and without touching the mixture control.
When I switch the auxiliary fuel pump on, the FF is increasing, but decrease again when switched off.

We suspected the engine driven fuel pump and changed it but the issue is still ongoing.

We grounded the aircraft and did several checks :
- adjustment of engine driven fuel pumpperformed as per TCM OVH manual X30607 chap 73-00-00
- Check of the whole fuel system (pipe, connections, fuel bladers…)
- Cleaned fuel selector
- Checked check valves

After checked the whole fuel system I did several flight test but the issue is still ongoing.

We thought that maybe the electrical fuel pump can be involved as there is a relief valve inside to maintain pressure (even when the pump is off). Maybe this valve is starting to tire.
An other idea we have is about the fuel selector that may have a damaged seal?

If you have any ideas of what is going wrong please help us.

Thanks a lot for you help