Oil pressure issues Lycoming O235.

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Fluffysgrandad

Filing Flight Plan
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Fluffysgrandad
Hi everyone.

I own a Tomahawk with the Lycoming O235 engine that was rebuilt about 50 hours back with 4 new cylinders and a new camshaft and conrod bolts plus various other parts. It runs beautifully with all normal indications other than the oil pressure.
With a warm engine full static power gives approx 2350 rpm and oil pressure in the mid green area but as speed and rpm increase on takeoff oil pressure goes high into the yellow band and sometimes touches the red band. About five minutes in cruise it slowly creeps back to mid green but if power is reduced for descent to around 2200 rpm and then increased to 2400 levelling out oil pressure returns to the high area before gradually reducing after several minutes. After a touch and go the oil pressure also rises back to the high range until power is reduced again for landing.
I haven't seen this before in a couple of thousand hours operating these engines and my mechanic is a little baffled by it
Does anyone have any suggestions? The oil temperature stabilises nicely around 180 degrees and doesn't vary.
 
Have you noticed this issue since the very first time the rebuilt engine was run, or is it a recent development? If it is a rebuilt engine, is it fully "broken" in? Has oil consumption stabilized?

Fluffysgrandad said:
Hi everyone.

I own a Tomahawk with the Lycoming O235 engine that was rebuilt about 50 hours back with 4 new cylinders and a new camshaft and conrod bolts plus various other parts. It runs beautifully with all normal indications other than the oil pressure.
With a warm engine full static power gives approx 2350 rpm and oil pressure in the mid green area but as speed and rpm increase on takeoff oil pressure goes high into the yellow band and sometimes touches the red band. About five minutes in cruise it slowly creeps back to mid green but if power is reduced for descent to around 2200 rpm and then increased to 2400 levelling out oil pressure returns to the high area before gradually reducing after several minutes. After a touch and go the oil pressure also rises back to the high range until power is reduced again for landing.
I haven't seen this before in a couple of thousand hours operating these engines and my mechanic is a little baffled by it
Does anyone have any suggestions? The oil temperature stabilises nicely around 180 degrees and doesn't vary.
Click to expand...
 
Thanks for the reply.
The first few hours after the rebuild the oil pressure was too high and that was masked by a faulty oil pressure sender which was replaced. A couple of washers were removed from the oil pressure regulator and that brought the oil pressure down to normal levels except in the circumstances mentioned.
In the fifty hours of operation the oil has been changed several times at 4 monthly intervals and we switched from red band at the last oil change recently. In between oil changes there has been no discernible oil consumption at all.
 
Before you do anything make sure it is high. Dont trus another sender, install a mechanical gauge temporarily to check it.
 
If this was a shop overhaul then they should have put it in a test cell with calibrated gauges and verified the proper operation of the relief valve. You should have also gotten a new spring and ball but perhaps not. With a flaky oil pressure gauge, now replaced, washers removed, etc. you could be chasing your tail. I would start by tying in a second mechanical O/P gauge that you've verified to be good using shop air and a differential compression tester. I hesitate to tell you to just calibrate yours because it sounds like it's electric, and there could be instrument grounding issues, etc. causing an inaccurate reading with a vibrating airframe. I would also go to the IPC and order a new spring and ball. That's a cheap way to start. Have a good look at the old ball and seat, and compare the height of the springs (yours vs new one).

Oh and by the way, your current circumstance could be perfectly normal if at power reduction during descent the oil temp cools significantly.
 
Some great suggestions. We have connected a direct reading and correctly calibrated oil pressure gauge to check the accuracy of the panel's gauge and it is accurate.
Changing the ball and spring sounds like a good place to start. The engine wasn't fully overhauled as it only had about 300 hours on it but had camshaft and cylinder corrosion from long periods of inactivity.
 
Do you have an oil cooler? How cold is the air temp when flying? Thick oil when cold may be an issue, especially during full power take off.

Try blocking off the oil cooler?

The parts list of the items replaced during overhaul should list new OP spring?
 
The tomahawk doesn't have an oil cooler which can be an issue in hot weather. The oil pressure rises above normal occur whether the engine is at normal operating temperature after an hours flying or shortly after start up. It always returns to mid green on the gauge after a while when power is reduced to cruise from full power but I have never seen oil pressure behaviour like it in a lot of time on these aircraft.
 
After start the oil pressure rises to about 65-70 psi at 1000rpm.
After landing with a warm engine right back at idle it is in the yellow band at about 40 psi. A touch and go on a warm engine will bring the oil pressure up to 95-100 psi and on downwind at cruise power it will slowly start dropping to about 85-90psi or top of the green.
Static runs at full power only ever give about 70-80 psi. As airspeed increases and with it rpm so does oil pressure. That seems to be the unusual part
 
And when the engine was torn down for the cam replacement, was anything done with the crank (ground,etc) and main bearings? Oil pump and gears, etc? Were the same number of washers replaced under the spring? If none of that stuff was touched, and you now have abnormally high oil pressure I would be suspecting a blocked oil galley somewhere, or a misaligned bearing, though it would probably be making metal by now. If the oil is warm and the pressure rises with rpm... as it should until the ball comes off the seat.. then either that's not happening, or it IS, and there is something else further downstream restricting the oil.
 
The bearings were replaced but the crankshaft wasn't touched as it looked like new. We will try a new ball and spring and then go from there.

Thanks for the advice.
 
As another guy said, look at that ball seat. If it's rough or nicked or corroded, it leaks. Its pressure will be low at low RPM since it loses much of the pump's output. The rebuilders stick more washers under the spring to get the pressure up, but that results in too-high pressure at high pump speeds, since the leaks can only handle a bit of the flow.

Dan
 
Thanks Dan. It sounds like the problem is in this area. Is the ball seat replaceable or repairable if corrosion or pitting is the likely culprit?
 
Check to see that the oil filter adapter plate PN LW-12999 which sandwiches between 2 gaskets between the filter adapter and accessory housing was not forgotten. This part is common only to the 235' and has been missed by mechanics not familiar with it. Charlie Melot Zephyr Engines
 
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