C-85 low & fluctuating oil pressure

[Robert Lees]

I have a C-85 with the Don's Dream Machine mods, and full flow oil filter.

The oil pressure should be in the region of 50-60 psi (and it normally provides this pressure, with the needle steady as a rock).

Back in the summer, about 15 hours ago, which was 30 hours after the engine was majored, I had low oil pressure (about 30-35 psi). So I checked the relief valve (which appeared clean)...ran engine again, still same problem. I replaced the oil & filter, and hey presto! back to normal pressure. This was just about the end of the break-in period, so I went from the break-in straight oil to 15W50 Shell multigrade at this point. All is sweetness & light.


This week, (15 hours later) the oil pressure was suddenly again down to 30-35 psi. The flight previous was OK, so it's a rapid change. When warm & in the cruise (2350 rpm), it fluctuates by 5-10 psi up-and-down every second or so...so it's varying between say 30 and 40 psi. It's still in the green, but when at idle, it's VERY low...like 20 psi. The pressure increases with engine rpm.

Again, I check the relief valve again (still clean...run engine again...low oil pressure still).

I remove the sump to make sure that the oil pick-up tube isn't loose...and it's tight.

I check that the oil pressure line from the engine to the gauge is OK...and it appears so.

On both occasions, the coarse screen & the full-flow filter are as clean as a whistle.

Any ideas where my problem may lie? My thoughts are:

1. slack relief valve spring (how do I verify the correct stiffness of the spring?)

2. Fuel getting in the oil...but from where? And the oil doesn't smell like it's got fuel in it.

3. Air getting in (but the pick-up tube is tight).

4. Dodgy gauge.

If any of the above is the cause, why would it not happen all the time?

Any advice welcome.


Rob

[Marv]

Re: C-85 low & fluctuating oil pressure

Rob try a new oil filter again and see if that puts your oil pressure back the way it should be. I remember several years ago there was a bunch of oil filters made with defective paper and this same problem was happening with automobiles, and a lot of engines were ruined. Of course they did not have gauges, just idiot lights. Just a thought. Marv

[SideSlip]

Re: C-85 low & fluctuating oil pressure

20 psi at idle sounds just fine, I'm pretty sure they are good down to 6 psi at idle.

[Ragwing nut]

Re: C-85 low & fluctuating oil pressure

change back to straight weight oil.

[Len Petterson]

Re: C-85 low & fluctuating oil pressure

Do you have a small crack in the suction line from the screen?
If so you would have air in the oil, and the oil would be foaming.
At the very list be whiteish from the air. If you drain the oil right after a flight or engine run and observe the oil would it go darker when sitting. It would be the oil relizing the air.
Testing of the Oil pressure Relife valve spring in a A 65 according to my Manual;

for A 65 65 hp Part# 631706Spring wire diameter 0.041 To Compres the spring to 1. 56 " should take a force of 6. 06 lbs min. or 6.31 lbs max. for new parts. Used part 5.75 lbs Min
for A 75 75 Hp Part #21352 0.041 wire dia.to compres the to 1.56" Should take 5.12 lbls min and 5.37 lbs max. used part 4.87 lbs min.
To test the sping, cut a length of pipe what will hold the spring to 1.56" or a woodblock with a hole in it to the same length. Compress the spring in the hole on a scale and read the force needed.
Is there any brass or other metal in the filter? You have to cut it open to find out.
Check the seat and plunger on the relifevalve. Carbon esidues can bind the plunger clean it with alcohol
If I am not wrong the pickup tube for the oil has a pipetread. ( on the A 65 ) Check for a cracked part way into the tread.
Clean the pipe internaly and check for a crack internaly.
It is difficult to see a crack in the cut tread. Vibration vill crack a pipetread, the crack would be small and make for a very small leak , but on a suction line it will draw air into the oil. (Hint I seen this happen in machinery many times.)
Verifiy your pressure gauge aginst a new one.
Do you have an orfice in the oilpressure line? It should be one in the fitting on the engine, could be partly plugged, or haveing a shunk of dirt floating around in what area partly plugging up the orfice. Remove the fitting on the engine and blow out anything in this area. The orfice (0.030") is there to reduse oilflow in case of line brakeage.
LEN

[magman]

Re: C-85 low & fluctuating oil pressure

Some of the filter mfg have had issues with Quality as of late.
The media gets under the relief valve & pressure drops. I heard there was an article in Aviation Consumer.

Ron

[Robert Lees]

Re: C-85 low & fluctuating oil pressure

Problem is solved (and nothing suggested above).

I'll post soonest; I'm about to go to bed before driving to London to catch a plane States side.

Rob