High oil pressure on startup

Food for thought:

Plus standard VDO pressure senders are inexpensive if you just want to replace it. Replacing it only takes a few minutes. Many times it is the sender going bad when readings start dropping really low and or you have a lot of erratic readings. Plus if you have an older engine with the old style oil pressure plug screw, spring and ball bearing switching the the newer screw plug, a new spring and the new style pressure control mushroom vs the old ball bearing may work out too. 

I always go after the common causes first and in this case it's usually the sender.

Walter Rawdank wrote:

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 ?

The first half of this comment is something that happened with me, the second half is what I messed with before finding the real problem: failing oil hose!

Consider that Parker, etc. will brag about 2,000 psi burst pressure or some such number, bet say nothing about the opposite, that is suction.

The oil lines leading from the oil tank to oil pump are under suction. Being a displacement-type pump, output pressure increases with rpm, and so does suction on the inlet side. Increased flow volume results in higher output pressure until it reaches the limit of the oil bypass spring (not the bypass in the filter). The bypass opening keeps pressure from rising further, *but* it also keeps flow *volume* from increasing properly with rpm. (Within reason, it's *flow*, not *pressure* that's important).

Anyway, cold oil, old or kinked suction hose? Increase rpm, the hose closes itself off, output pressure drops. Then, reduce rpm, hose opens, pressure increases. The problem starts as you describe, but will continue to worsen and under a larger range of circumstances. Replacing hoses and fiddling to reduce the bend radii from tank to pump, fixed my similar problems instantly. NOTE: "On Condition" inspection of suction side hoses DOESN'T WORK. The inside of the removed hoses looked just fine.

I did test oil pressure using a mechanical gauge and it was steady as a rock at 70 PSI ( which is normal for my engine at cold startups with oil temp below 60F)

I ordered a new sender and did a run up with it installed - everything was steady and within limits - started out with 75 PSI ( oil temp 55F ) and then slowly went down and settled at around 50PSI once oil temps went up - so all is good again.

Thanks for your help guys.

VDO senders both oil and fuel can and do go bad and it's not uncommon. I always keep a spare of each because I've had to replace quite a few. They aren't expensive and then just by a quick swap from an old one to the new one rules the sender in or out as bad. This is common enough this is one of the first things I do. 90% of the time that ends my problem.