What’s fueling higher engine temps?

Q: I own a factory overhauled O-360 A4A, which I’ve owned since it left the factory. I’ve flown behind it for 600 hours. I recently had a prop strike and the engine was stripped and reassembled with no damage found. All they replaced were the exhaust valve guides and piston rings. (And all the rest as per shock load spec.)

The plane has been put together exactly as before and performs almost better than before. I have 10 years of multi probe engine monitor data to compare.

However, one or two parameters are different that worry me.

All things being equal (same old baffling, spark plugs, power settings and fuel flow, etc.) the oil temperature is running at 220° instead of the 180° that I am used to seeing, and the CHTs are spread 35° from coldest (before they were all within 10° of each other) and overall CHTs are also 20°-30° hotter than before.

EGTs and fuel flow are just as before. It’s a fixed pitch (mid) prop and the oil cooler is just behind the #3 cylinder.

Compared to the temps on my bedding-in flights on the same engine 10 years ago, this is too much and also they did not reduce at all over the last 10 hours.

I already changed to a thicker single grade mineral oil from the multi grade before, replaced the vernatherm and ultrasonically cleaned the oil cooler … No change. Not sure where to go next. Any ideas?

Christoph Andrykowsky, via email

A: Christoph, it seems like the problem you are experiencing is one that usually comes when the weather gets warmer during the summer months.

First, I don’t believe the problem is something that was caused by the sudden stoppage you unfortunately experienced, or the disassembly, inspection, or reassembly of your engine.

My first suspicion is the oil vernatherm. Since I don’t know whether you have a full flow oil filter adapter on your engine, or just an oil screen housing where the vernatherm is housed, I’d like you to remove and inspect the vernatherm. My concern is that the mating surface between the vernatherm and the seat of its housing is no longer fully compatible which, in turn, allows hot engine oil to recirculate through the engine without being forced to pass through the oil cooler.

I know you replaced the vernatherm, but unfortunately this doesn’t always solve the problem because even though it may be a new part, it may not be seating a complete 360° of its seating surface, which it must in order to force all of the hot engine oil through the oil cooler.

My suggestion is to once again remove the vernatherm and reface the seat on the corresponding seating surface. This entire procedure is covered in its entirety in Lycoming Service Instruction 1316A. My guess is when this surface is refaced according to SI 1316A and using your new vernatherm, the problem should be solved.

That being said, it also may be a situation where the engine baffling is a contributing factor. You mentioned that the “same old baffling” was reinstalled, which may or may not be playing a part in this. Please make absolutely certain that the baffles are not drooping or folding back, allowing the cowl inlet cooling air to escape rather than being forced over the entire oil cooler.  Also, since you mentioned that the cylinder head temperatures are now slightly hotter than previously, we want to make certain that the baffling is doing its job properly.

I am not concerned with the 35° spread in the CHTs or the fact that they are now 20° to 30° hotter than before. I assume new piston rings were installed during the reassembly of your engine, and this slight increase in CHTs may be a result of closer tolerances from the new rings, etc.

Since you didn’t mention any specific CHT temperatures, I’ll remind you that the maximum CHT for your engine is 500°F during continuous operation. I’m certain if your CHTs were getting close to that you would have mentioned it. There have been cases in the past where the CHT was running excessively hot and caused the oil temperature to increase, but I don’t think that’s the case here.

One more thing that I’m curious about, and that you didn’t mention, is what the engine oil pressure was. When we have higher oil temperatures, we typically expect to see lower oil pressure readings, which are usually more noticeable at low or idle RPM.

If I sum this all up, I really believe that the faces of the vernatherm and its corresponding seat are the most suspicious and causing the situation you have pointed out.

I believe this question is well timed for this time of year when ambient temperatures begin to climb.

Before any corrective action is taken, be certain to confirm the engine oil is as hot as your oil temperature gauge is leading you to believe. If you notice that the engine oil pressure is indicating somewhat lower and the temperature is higher, then I’d be looking at Service Instruction 1316A for corrective action.