This month’s Ask a Tech inquiries come from a Cessna 172 owner who can’t seem to keep the engine’s oil temperature off the redline and a Mooney owner who got a letter from the FAA about altitude reporting problems. First, that hot-running Lycoming engine.

“I bought a Cessna 172M with a Lycoming 160-HP engine with under 300 hours SMOH. The problem is when the temperature goes above 70 degrees F, the engine oil temperature goes in the red after about 40 minutes in straight-and-level flight. The engine runs we’ll and there is no overheating smell that I can tell.

“In the winter the oil temperature stays normal (in the green), plus the oil pressure stays normal year-round. I have sent the temperature gauge to Air Parts of Lock Haven for overhaul and we double-checked the instrument for accuracy.

“I removed the vernatherm, tested it and found it to be working properly. I also switched it out with another unit to no avail. I replaced number six oil hoses with number eight hoses with some benefit, but not enough. I also replaced some SCAT tubing and I cleaned up the engine baffling and closed up all holes. What am I missing here?”

IA Mike Berry responds: At this point, I might suggest sending the oil cooler out for service. One shop that’s done we’ll in our research is Pacific Oil Cooler Service in California. Find them at www.oilcoolers.com, 800-866-7335. It’s important that you source a shop that’s skilled with oil cooler pressure testing and repair, while avoiding field shops that flush or reverse-flush the cooler. These aren’t vehicle radiators.

It sounds like your cooler is partially blocked. I assume that you have enssured that a straight shot of ram air through the cooler is available, and that there is no physical damage to the fins. A cooler has super-fine internal passages that require very specialized equipment if there is any hope of clearing the blockage. A specialized cooler shop can first pressure test the cooler for efficiency, and then try to clean and clear the blockage.

What causes blockages? The buildup of deposits from long periods of the engine sitting without use is one culprit, as is going long periods between oil changes.

Also, the seat that the vernatherm seals against could be worn.When the vernatherm was pulled was the seat carefully checked, and was the latest model used? Older vernatherms can be trouble.

Another cause contributing to hot oil is blowby past the piston rings, which blows hot combustion gasses into the sump and then out the breather. Usually there will be black oil and a dirty belly. A good static compression check is no guarantee of the condition of the oil control rings, according to Lycoming.

“On several occasions, I’ve had controllers tell me to reset my Mode C altitude reporting and while they never made the request more than once, I recently got a letter saying my equipment was faulty while transitioning controlled airspace. It seems to be the most problematic above 5000 feet. I had my shop replace the transponder when they installed the ADS-B Out system a few years ago and admittedly, I’m due for my FAR 91.411 and FAR 91.413 two-year certifications. Which system is broken—the transponder or ADS-B? Thanks for your excellent magazine that I read front to back every month.”

It’s likely neither system, and it’s clearly time to get the airplane to the shop for the required FAR biennial inspections. It’s the Mode C altitude encoder that’s partly (mostly) responsible for squawking the correct pressure altitude and if there’s a problem with the encoder’s wiring or the encoder itself, you can get fluctuations in altitude data, totally erroneous data or no data at all. These altitude digitizers are plumbed into the static system and can be a source of leakage, for one thing, and when the electronics fail, they might work at some altitudes but not others. A bench test will sort it out. —Larry Anglisano

Got a question for Ask a Tech? Drop us a line with a good description and we’ll help noodle it.