Ask Paul: Solving a mystery

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: Here is my mystery: I have a Geronimo Apache equipped with 0-360-A1As. The left engine is about 1,200 SMOH, the right engine is about 700 SMOH. They both had oil pressure of 85 psi in cruise and about 50 psi taxiing in after a flight. At about 1,150 SMOH, the LE began to show low oil pressure. It slowly, over about 20 hours, started dropping in cruise from about 85 psi to about 65 psi and down to 30-35 psi taxiing in. I thought bottom end bearings. But if that was happening, and over a short period of time, I should definitely get metal in the screen. I change oil every 25 hours. There is no metal in the screens top or bottom. I put washers under the relief valve and that brought cruise pressure up to 75-80 psi, but it still drops to 30-35 taxiing in. The other engine? Still no change. I have about 150 hours on it since the pressure started dropping in the LE. On cold startup the pressure tops out right at 115 psi then drops back to about 75-80 psi when the temperature and pressure levels off in cruise.

I talked to Lycoming customer service. They said I should change the spring under the relief valve, which I already did. They say measure the pressure drop across the engine. They say I should get about 15 psi differential. I got 4-5 psi differential. I talked to a Lycoming expert at OSH last year, he said 4-5 psi is normal.

The engine runs perfect; temperatures have not changed with all these pressure problems. I had an oil analysis done and the LE, the problem one, showed better than the RE. Both showed normal for the hours TIS. I have calibrated the gauges and they are accurate.

Could a prop governor or the prop itself cause this?

I’ve decided that it ain’t gonna break so I’m flying it. What do you think?

KENT TARVER, Fallon, Nev.

A: Kent, thanks for the mysterious challenge. I decided to take the Peter Falk, “Columbo” approach and went with the raincoat, the cigar, the slouch, the false exit followed by the catchphrase, “One more thing…” and came up with my theory of solving the “mystery.” Here are my crime-solving thoughts, so let’s see if this might point the finger of guilt in the right direction.

One of the best things Kent provided was detailed information he had collected during his time of trying to track down the mystery. Like Columbo, I reviewed all of the facts and went straight to what I thought was the guilty party. My suspicion is the oil cooler by-pass valve on the left engine. Now, I’m not certain what type you might have Kent, but would suggest you check this out next. Not knowing the age of the engines, they may have either the thermostatic by-pass valve (vernitherm) in the oil pressure screen housing or it may be using the original spring and plunger configuration, which was very common when the Geronimo conversions were done years ago.

As I mentioned in an earlier article, the spring and plunger are located in the accessory housing about the 11 o’clock position and comes off the seat as the oil temperature increases versus the vernitherm located either in the oil pressure screen housing or the oil filter adapter, which goes against the seat. Either configuration causes the engine oil to be forced through the engine oil cooler in order to control the oil temperature.

From the information Kent provided, it sounds like the classic case of the engine oil temperature increasing during flight and the oil pressure becoming lower. This is a typical sign that all of the engine oil is not being forced by the by-pass valve to pass through the oil cooler. My first recommendation would be to consider complying with Lycoming Service Instruction 1316A. I truly believe that after following compliance with this publication, your “mystery” will be solved.

Kent, you never mentioned how high the oil temperature actually went on the left engine so I’m guessing it never got close to the red line (245° F) on your gauge. Even with the oil pressure dropping to 30 to 35 psi during taxiing following a flight, you are still well within the specification for oil pressure since the minimum is 25 psi at idle. Also, 55 psi is the minimum OP during cruise, so you’re still OK there without taking any corrective action but, like most pilots, you’d prefer the OP to be close on both engines and a guy like that can’t be all bad. If I were you, however, I would remove some of those washers you installed under the oil pressure relief valve spring on the left engine in order to bring the cold start-up oil pressure down so it’s closer to the other engine.

Not to dismiss anything the Lycoming factory folks told you, but measuring the differential pressure across the engine is done usually to determine whether there may be excessive clearance between the crankshaft and the front main bearing. From my personal experience, I’d say the reading you got, 4-5 psi, was what I would have expected rather than the 15 psi they mentioned at the factory and that tells me the Lycoming guy at OSH knew what he was talking about.

In your follow-up email you mentioned your oil consumption was about 11 hours per quart on the engine with the “mystery” and about 7.5 hours per quart on the RE. Maybe you shouldn’t have mentioned this, because that’s great, and some people will be envious and question how they can get that kind of consumption from their engines.

I hope the “Columbo” approach has provided enough evidence to solve this “mystery” and may help others solve some of the mysteries they might have similar to this. Please don’t hesitate to let us know the final outcome when the “mystery” is solved.

Send your questions to: [email protected].