Home | What is a good oil temperature? What can cause it to be too high or too low? What controls oil temperature? How can I get some idea of whether my factory oil temp gauge is accurate?

What is a good oil temperature? What can cause it to be too high or too low? What controls oil temperature? How can I get some idea of whether my factory oil temp gauge is accurate?

What is a good oil temperature? What can cause it to be too high or too low? What controls oil temperature? How can I get some idea of whether my factory oil temp gauge is accurate?

Technical Editor:
I hope this doesn’t duplicate too much of the info that has circulated on this. Please let me know if any of you have had personal experience that differs from what I have written here. I have extrapolated this from a posting I placed on the MM List in November of 2003.

The engine’s operating oil temperature needs to be at least 165 degrees, and a normal minimum of 185 degrees is preferred. At 185 degrees, about a half-hour’s operation is supposed to be enough to drive any accumulated moisture out of the oil. If the moisture stays in the oil, the oil’s acid neutralizer and corrosion inhibitor additives are rapidly exhausted. This is why ground operation only (with no associated flight activity) is strongly discouraged.

Lycoming’s Vernatherm (thermostatic bypass valve) is designed to start opening at 180 to 185 degrees, depending on where you read the number (it is sometimes visible on the valve). It is typically mounted in the pressure screen housing or the oil filter adapter housing. Working similar to the water thermostat in a typical car engine, this valve enables colder, thicker oil to bypass the oil cooler, by leaving open a passage/bypass hole into the engine’s main oil gallery. However, because of the way it works, by plugging the bypass hole as it expands, it is always possible for some amount of oil to circulate through the cooler. The Vernatherm does not block off the cooler passage when the oil is cold; it just makes it all go through the cooler, if the oil temp makes it up to 200 degrees or so. The resistance to flow of the thick, cold oil, as it tries to go through the bypass port, can cause some small amount of oil to go through the cooler even when the oil is cold. In contrast, an auto thermostat lets almost no water make it to the radiator until the minimum temperature is reached. Until that point is reached, a bypass system just recirculates the water within the engine (to prevent unequal heating). Because of this operational difference, it can be helpful to block airflow through some or all of the cooler’s fin area, OAT and oil temperature experience permitting, during the winter months. If you are seeing CHT’s of 360 degrees or more, I would advise you to not block any of the cylinder head cooling airflow. The cylinders are very sensitive to hot spots from reduced airflow.

The Vernatherm valves are finicky. Half of the ones in service are probably out of tolerance. They are on the Lycoming mandatory parts replacement list at overhaul. Nonetheless, I have read dozens of overhaul work orders that did not show this valve on the list of replaced parts. In fact, it is almost uncommon to see a new one. There is a Lycoming procedure for testing the valve. I have yet to test a used valve that passed the test for extension, alignment, and retraction at the specified temperatures. They fail to fully extend, they cock sideways during extension or retraction, and they fail to fully retract. New ones usually pass the same test. In most cases a faulty valve will fail to extend at the required temperature, or to the required length. Most often this just results in an abnormally high oil temperature, since too much of the oil goes through the bypass hole, rather than being routed through the cooler. However, if the valve is cocking during extension or retraction, or if it is failing to retract due to binding, it could be forcing oil through the cooler when it should be bypassing the cooler.

JPI (EDM700 manufacturer) recommends that their oil temperature probe be mounted at the front right side of the case, in the oil gallery (the plug must be removed). This is intended to give a valid reading of the oil temp as it exists inside the operating area of the
engine. I have found that this location results in a virtually perfect match of the readings shown by a properly-calibrated factory gauge and the JPI instrument. If you are seeing correct temperatures from this sensor location on a JPI, the moisture should be getting driven out of the oil. A sensor mounted at the outlet of the cooler (or in that section of the oil system path) would read too low.

A sensor mounted in the oil sump might or might not read correctly. While most of the sump oil has drained out of the engine’s operating parts, some of that oil will have picked up more or less heat than the oil coming from other areas. There are also stagnant areas of oil movement in the sump, where part of the oil may sit for a while and transfer heat out through the aluminum sump cover. Mounting the sensor in an area of the case that tends to remain at core operating temperature, and where there is fairly rapid and constant oil circulation, will give a more reliable reading.

Many of the original sensor-driven panel instruments (fuel level, oil temp) have long since lost their calibration. I have found the original Mil-Spec oil temp sender itself to be highly reliable. If there is a difference between the factory gauge temp reading and the JPI temp reading (with a properly located JPI probe), I would believe the JPI. One crude way to evaluate the factory oil temp gauge is to know what the OAT is, while it is 70 degrees or higher (so the gauge needle should be well off the low-end peg). Before starting the
engine, mentally extrapolate where the ambient temperature would fall on your gauge face (the minimum shown is probably 60 degrees). Turn on the Master, and see where the oil temp needle goes. If it fails to move up to the point where it matches the ambient temp, your panel gauge is probably not showing a correct oil temp reading.

You most likely will not be able to get a valid corroborating temperature test (using some other instrument) during a ground run-up. The plane has to be in flight at 65%-75% power in order to get proper, adequate, and even heating throughout the engine. I have never been able to get a certified light aircraft sump oil temp any higher than 165 degrees with the plane on the ground, despite ambient conditions that caused CHT to start going too high.

Lycoming Service Instruction 1316A describes the repair of the oil cooler by-pass valve seat. The special tool used for this is Lycoming P/N ST-388. If your maintenance facility does not have one of these, you may be able to rent it from Lycoming (possibly through a Lycoming distributor). This should be a last resort. The seats are seldom bad, and if the tool is not used correctly, the seat can be damaged beyond repair.

NOTE: At least one club member has reported that a maintenance shop made what could have been a serious error, following an engine overhaul. When they reconnected the oil cooler lines to the engine, they connected both sides of the oil cooler to oil pressure galleries on the engine. Here is the text of that note:
“If the group can remember, a few month ago I had the same problem, and had done all the above including cooler and hoses. Then one day when we were doing some hanger flying, the mechanic said that, according to his Piper/Lyc books, that the return hose from the cooler was hooked to a different port. Sure enough, the installers in Chicago had hooked it to a pressure port on both sides of the cooler, rather than connecting one side to the banjo fitting on the top rear of the gear case. The oil temp had been staying around 200 F to 220 F. As soon as we connected the hoses to the right ports, the problem was solved. Oil temp is now stable at 185F. So even the pros can have a bad once in a while. Hope this helps. Good luck.
Dave Sheets

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