High oil pressure on startup

A few seconds of elevated pressure is not beyond reason.

Even though the Pressure SENSOR appears to be part of the Oil PUMP, it is functionally at the far end of the Oil Galleries and nearly 5 feet from the Pressure Regulator.
The Pressure Regulator is located immediately after the Oil Pump and Filter, and before the Engine.
An Oil Filter filled with cold Gelled OIl would, if anything, LOWER the Oil Pressure, NOT Raise it.

Assuming the Oil is Very cold and Viscious (Thick), The Oil pump, being a constant displacement Gerotor Style Pump, Is going to pump Oil at whatever extreme pressure it needs to move the Oil.
The Oil Regulator will open as far as Possible trying to relieve the pressure but may not be able to bypass enough of the overly thick Oil to prevent the Pressure from climbing to near limit values.
This won't last long.
The Oil that the Regulator Bypasses is returned directly back to the Oil Pump inlet to be recirculated around the loop again.
Each pass sees the Oil getting heavily massaged by the Oil Pump.
This closed-loop of oil is going to be heated up quickly by the mechanical action of the Pump.
As the Viscocity drops, so will the Oil Pressure as the Regulator can properly regulate the now thinner Oil.

The Oil may spike for a few seconds on a cold start, but the physics of the pumping action will soon warm the Oil Pump long before the engine itself reaches operating temperature.

Walter Rawdank wrote:

One more question …assuming this is metric, would this sender be appropriate replacement ?

https://www.summitracing.com/parts/vdo-360023

Assuming your Display Gauge is made by VDO or any other gauge that uses a 10-180 ohm sender and the threads are M10, then this sender would work.

So many variables. ?
- - -

But, yours is more likely NOT a sender problem.

The problem here is that I never had that issue before,.

Yes, would see on a cold day PSI go up 70 or so and then slowly work it’s way back as engine warms up but here it instantly goes to 70 and then after reaching 70 seems to accelerate all the way to 90 and last time even over 100 and then 25 second later goes back to normal - just rolls back to normal almost instantly ( I just rewatched a recording I made so I took  some notes)

Adjusting throttle correlates with pressure changes - first time this happened  , adjusting throttle would raise the pressure higher , last time adjusting throttle would lower it - regardless of that, as I mentioned in about 20-30 seconds everything goes back to normal and everything looks good until another cold start.

This worries as I know my plane/engine pretty well ( 500+ hours and counting ) and something has changed.

One more variable , the plane just came back from annual where I had them do the 600 gearbox inspection - don’t see a correlation here but who knows …

Btw.. I am sure I have VDO , old school sender ( not the new $250 one ).

Well, I am gonna give the mechanical gauge a shot and see if it acts the same way - if it does, I will let the mechanic handle it further - if this is not just a sender , that’s beyond my comfort zone ?

Hello Walter

Before you spend a lot of time and money perhaps put on a new oil filter.  Sometimes the filters (I hope it is an OEM new one from Rotax) can have an issue with cold oil and the newer filters have increased bypass pressure valves.  A bump up to even 100 psi for a short bit will not harm the engine and as long as it runs as normal after that then I would not worry.  

As far as the 600 hour work on the gearbox that should have no effect on oil pressure.  The oil pressure is governed by the oil pump and your oil lines.  If the oil lines were played with perhaps check the routing and see if anything has changed when they did the inspection.  

Cheers

I had exactly the same - i.e. brief period indicating around 100psi on start once or twice - as the first sign of failure of a VDO sender. After that it became more erratic.

A check with a gauge diagnosed it and a new sender cured it.

NOTE the VDO sender is 1/8 NPT not metric.

I think the catalogue number is 360-081-029-012C