Ask Paul: Tips to troubleshoot high oil temps — General Aviation News (2023)

Q: I have a 1998 Maule with an O-360 engine. I have the original needle oil temperature gauge, as well as a JPI-730 engine analyzer, and they both read the same temperature during all phases of flight.

My annual was completed a few days ago and the first thing I noticed was that the original oil temperature needle would initially fluctuate (never happened before) and then settle down, and when airborne it was at least 45° higher than the JPI-730.

I brought the plane back to the IA and he tightened the probe connection on the engine, but the temperature split is the same. Is the original oil temperature gauge adjustable?

ROBERT BLAKE, via email

A: Warmer weather is finally here and one of the first clues I have to confirm this is the number of questions about higher oil temperatures. It seems that every time the summer weather comes and folks begin to fly more, we begin to hear concerns about high oil temperatures. Hopefully I can give you some good troubleshooting suggestions that may help you from spending lots of money to solve this problem.

The first suggestion I’m going to pass along is one that I noted in a previous column. As I mentioned then, and will do so once again, this requires extreme caution, but when done properly, will give you honest results.

I suggest you check the actual oil temperature in the engine oil sump using a laboratory type, glass bulb thermometer.

You can either fly the aircraft or complete a good ground run where engine operating temperatures are normal. Then remove the oil dipstick and slowly, using extreme caution, insert the thermometer down the oil dipstick tube. These thermometers typically have an eyelet on the end that you can attach a piece of safety wire to keep control of it when slipping it down the oil filler tube. When you feel it touch bottom, carefully pull it back out about an inch or so. Give it a minute to get a reading.

Again, with extreme caution, slowly pull the thermometer out and check the reading. You want to compare this with the cockpit gauge (or gauges in your case) to see how close they are.

Caution Note: If you don’t use extreme caution during this troubleshooting procedure, you will then learn how to remove an engine oil sump so that you can remove the glass and mercury from the broken thermometer! This is guaranteed to increase your vocabulary in a matter of seconds.

Another way to check gauge accuracy is to remove the oil temperature sending unit from the engine while it’s still connected to the gauge and put it in a stainless steel pan of engine oil that you’ve heated on a hotplate. Bring the oil temperature up to 180°F, confirming this with a good thermometer, and then using a magic marker, mark that spot on your gauge. This will give you a good reference point on your analog gauge, but I don’t think it’ll work with your engine analyzer since most of those are either digital or a bar graph type readout.

While this type of gauge calibration check has been used for years with great results, it always seemed to me to be a challenge to come up with a hotplate and go through the frustration of dragging all of this up next to the airplane.

Another key factor is where the oil temperature sending unit is placed in the engine oil system. Typically, the temperature bulb is installed in the oil filter adapter or the oil suction screen housing on the accessory housing. On a normally aspirated Lycoming engine, the hottest oil in the system is typically about 50°F hotter than the oil in the engine oil sump. This is where the location of the sending unit may cause you to see a different oil temperature reading.

Since you’ve noticed the analog gauge doing some strange things since the annual, I’d suggest carefully inspecting the wiring from the sender to the gauge because it may simply be something has chaffed the wire and it’s intermittently grounding out, causing the gauge to fluctuate and the reading to go higher.

Again, using the testing methods mentioned previously, you could confirm if, in fact, it may be a gauge problem rather than an actual high oil temperature problem.

I did take note that you didn’t actually mention that your oil temperature was going high and reaching the maximum allowed of 245°F or whether it was just giving you readings that you hadn’t seen before your annual.

One last thing that comes to mind: I’d carefully review exactly what was done at the annual inspection to see if anything may have come into play with the present oil temperature situation. I can’t imagine what it may have been, but it wouldn’t hurt to take a quick review.

Now let’s look at a different situation, but still remaining in the oil temperature area of the engine:

Q: I, too, am having this high temperature problem in my non-turbo Lance IO-540. During a 500 fpm climb from sea level to FL10-11, I’m just inside of redline. Pressure is constant 60-65. At idle it’s in the green slightly lower. At level cruise I get lower temperatures, but still on the absolute top side of the green. In powered descent it will go lower, but still above 210.The engine has a new gauge and probe. The oil cooler is clean upon external observation.

Lawrence Harrison,via email

A: Larry mentioned that the oil pressure is just inside the redline on his oil temperature gauge. This comment doesn’t excite me too much because most analog gauges are non-linear at best, so a needle width at the bottom of the gauge may not represent the same number of degrees as a needle width at the top of the gauge. Read on and I’ll tell you how confirm what the oil temperature actually is.

What Larry is experiencing is usually a problem of not forcing all of the engine oil through the engine oil cooler. This could be a result of the thermostatic by-pass valve (vernatherm) not doing its job properly.
A good place to begin is to do a close visual inspection of the engine baffling to be certain that all of the proper air is getting to the engine oil cooler and not by-passing it because a baffle is not positioned correctly or worn out.

Then I’d be sure to confirm the cockpit gauge is accurate and this can be accomplished by using the suggested procedures mentioned earlier. Once we are confident in our gauge, we can move to the next step.

You mentioned installing a new gauge and probe, which is a step in the right direction if you confirmed the old units were bad before laying out the money for the new parts. However, now that you have a new gauge and probe, let’s work with that. Even though both parts are new, it doesn’t guarantee they are calibrated and accurate, so if you use the methods mentioned earlier we’ll confirm just exactly where we stand with temperature versus gauge accuracy.

If after making these checks you find that your oil temperature is in fact running high, the next thing I’d suggest is removing the thermostatic by-pass valve from the oil filter adapter, which I believe your engine uses.

Visually inspect the chamfered seating surface on this valve where you will notice a wear pattern. This wear pattern must be a concentric 360° in order to force all of the oil through the engine oil cooler.

If you notice any area on this surface that appears to be not seating or is not concentric, then it’s this area that is allowing some of the oil to pass directly back into the engine and not being forced through the oil cooler as it should be if the valve were seating a complete 360°.I’d suggest replacing this part with a new one.

However, before you install the new thermostatic by-pass valve, you must comply with Lycoming Service 1316A, which tells you how to reface the seating surface of the oil filter housing. This must be completed so that it’s concentric and will mate up properly with the new thermostatic by-pass valve.

If you were to reinstall your old valve, it probably would make the situation worse because the two mating surfaces would be so much different.

There is a special tool mentioned in Lycoming Service Instruction 1316A required to reface the seat in the oil filter housing. Some maintenance facilities have this tool, but if you confirm that this process must be accomplished and you can’t locate the ST-388, they are available for rent from Lycoming. Simply contact the Lycoming Distributor of your choice and they should be able to handle it for you.

Since this subject is quite common when ambient temperatures increase, I’d suggest that maintenance technicians and maintenance facilities keep this article handy and show it to the next aircraft owner or pilot who mentions that he’s experiencing higher oil temperatures.

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