Ask Paul: Why is the oil pressure dropping?

Q: Dear Paul, greetings and thank for your vast information on the web for solving mysteries.

I recently overhauled a Cessna 182 IO-540-AB1A5 engine. The challenge we are having is the oil pressure drops gradually after a four to five minute run-up to 25 PSI at about 1,500 rpm. Oil temperature seems to settle at 150. All other parameters are normal.

The pressure seems to move with the increase in rpm. We looked at the CSU and the oil filter adapter gaskets and all correctly fitted.

I would appreciate your insight on this matter if possible.

Paul Wegoye

A: Paul, this is one of those situations that could be caused by several factors, any of which could result in what you are experiencing.

My first inclination is to do some testing and I’d suggest you begin by complying with Lycoming Service Instruction 1462 A. This test will give us the results of what is happening with the oil pressure across the front main bearing.

If, for some reason during the recent engine overhaul, you ended up with some excessive clearance between the crankshaft and the front main bearing, this test will provide valuable information.

This test should be conducted with the engine at normal operating temperatures.

The Service Instruction provides detailed instructions that should be closely followed.

Next, maybe I should ask how the prop action is when this problem occurs?

What we are attempting to do is determine whether the oil pressure is dropping due to excessive clearance.

There is way to do that that does not require any special tools or fixtures, but it does present a challenge for the maintenance technician conducting the test.

This approach requires installing a calibrated oil pressure master gauge at the oil galley port on the front left half of the crankcase just forward of the governor pad on the front of the crankcase. You’ll find a 1/8″ x 27″ Allen plug directly behind the starter ring gear support, which is the main oil galley.

I think it would also be better if you install a master gauge at the 1/8″ x 27″ Allen plug on the right side of the accessory housing located near the vacuum pump pad. This is the closest to the oil pump and the best port to use for this type of troubleshooting.

Now, with the engine at normal operating temperatures, we want to compare the difference in the oil pressures between the rear of the engine compared to what you are reading at the front left half of the crankcase. There should not be more than a 10 to 12 pound maximum differential. If the difference is greater than that, it tells us that we have an excess clearance between the crankshaft and the front main bearing.

Another thing I should probably ask that may offer other clues: Did this crankcase have any weld repair at the time of overhaul? The results in oil pressure you are experiencing could also be a result of a crack somewhere in the crankcase that is opening up as the engine temperatures rise, resulting in a drop in oil pressure. 

Paul, I realize what I have suggested is no simple task, but these are the best tests that I can think of at this time. I guess I can close this response by saying “don’t shoot the messenger” and I wish you luck.