since 1940

High oil temperature in Continental or Lycoming aircraft engines 

What are the causes of high oil temperature?

• The engine may be producing too much heat.
• The cooling system may not be working.
• The oil temperature gauge may be inaccurate.

 The first thing to check is the gauge. If the gauge is working it responds to oil temperature changes. Therefore, reduce the oil temperature and see if the gauge responds. To do this add more cooling to the engine. Lower the nose, open cowl flaps and reduce power. Does the gauge respond? If the gauge responds then it is probably working.

 The oil pressure gauge can confirm the operation of the oil temperature gauge. High oil temperature affects both the pressure and temperature gauge. As oil temperature goes up pressure goes down. As oil temperature goes down pressure goes up.

Make sure the gauge is accurate. The best method of determining how hot the oil and how accurate the gauge is to take the oil's temperature. Immediately after landing use a long thermometer and dip the oil. We use a three foot mercury scientific thermometer and lower it into the oil filler tube. The desirable cruise oil temperature operating range is 150 to 190 degrees F. Temperature changes within these limits are not cause for concern. If you confirm a high oil temperature the next step is to check the basic health of the engine.

Check engine timing, compression and other checks you would do at a 100 hour inspection. Check for insufficient oil supply or low oil pressure. Low oil pressure decreases the flow of oil through the oil cooler. If the oil temperature is still high then either the engine's dumping too much heat into the oil, or the oil's not getting cooled.

Hot combustion gas leaking past the piston rings into the oil sump makes the oil hot. This gas carries oil with it as it blows out the engine breather, dirtying the underside of the aircraft. Combustion byproducts also turns the oil black.

If the oil temperature is high, the oil's on the dipstick is coal black and the belly is dirty with oil then the rings aren't holding compression. Do a compression check to find the cylinder with the leaking rings.

If the engine's not dumping excess heat into the oil then its time to check the oil cooling system. The Textron Lycoming aircraft engines oil cooling system consists of the vernatherm valve, viscosity valve, oil cooler, engine baffling, and cowl flaps.

Lycoming Vernatherm Valve PN 53E22144

 

There are many older (obsolete) versions of this valve.   Obsolete Lycoming part numbers: 53E19600,75900,LW-10269

Airplanes where one engine runs hotter than the other. Adjust cowl flaps. You might want to have a winter and summer setting; set at the closed limits in winter and slightly open in summer. Push up on the cowl flaps to eliminate any slop in the linkage when making the adjustment.

Engine baffling that doesn't seal causes high oil temperature. Baffling should seal the top of the engine from the bottom. This increases air pressure in the cowling forcing air through the cylinder cooling fins. If the baffling isn't sealing cooling air bypasses the cylinder cooling fins. If the cowl flaps, and baffling are ok then its time to check the oil cooler. According to Continental in their Service Bulletin SID97-2 "The amount of heat energy that must be removed by the cooling air is approximately equal to the horsepower that is driving the propeller".

Is the oil going through the oil cooler? The oil cooler can be partially plugged, bypass valves can leak, the oil cooler flange can warp. If the oil is hot but the oil cooler isn't then oil isn't circulating through the oil cooler. When the engine's hot feel the oil cooler to make sure it's hot. The hottest you can hold your hand against something is 120 degrees F. If the oil cooler isn't hot then oil isn't flowing through it.

Trapped air in the oil cooler occurs on engines with low mounted oil coolers such as the Cessna 182RG. This decreases the ability of the cooler to cool the oil. If this type of cooler has recently been installed then remove the cooler and purge the trapped air.

Many oil coolers have never been properly cleaned. Flushing the cooler with solvent doesn't remove sludge and varnish deposits. These deposits can reduce the efficiency of the oil cooler. Have the oil cooler cleaned by someone specializing in oil cooler repairs.

On Continental aircraft engines with engine mounted oil coolers make sure the gasket separating the inlet side from the outlet side of the oil cooler isn't broken. The gasket can break if the oil cooler pad isn't flat. Oil bypasses from one side of the cooler to the other side without flowing through the cooler. Lap the contact surface of the oil cooler on a flat plate using 400 grit sandpaper and a light oil to restore flatness to the surface.

Oil can bypass the oil cooler because of a defective vernatherm valve. The vernatherm is a temperature sensitive valve that stabilizes oil temperature by regulating oil flow to the oil cooler in Textron Lycoming aircraft engines. The vernatherm valve modulates to maintain oil temperature at approximately 170 degrees F. The Continental vernatherm valve can be checked for proper operation by dropping it in water heated to 180 to 190 degrees F. If valve opens, it's operating properly; if not, it should be replaced. Valves marked 77C should travel at least 0.090 inch as the water temperature rises from 120 to 170 degrees F. Valves marked 173 degrees should travel at least .16 inch as the water temperature rises from 135 to 173 degrees F.

Textron Lycoming Aircraft engines with a vernatherm valve:

High oil temperature can be caused by a leaking vernatherm valve. The oil passing through the oil temperature control cavity goes either through the oil cooler or to the crankcase passages, depending on the oil temperature. If this valve leaks, oil bypasses the oil cooler.

Not all Lycoming engines have a vernatherm valve and those that do are not all located in the same spot on the engine. If the engine has an oil filter then it has a vernatherm valve. Usually this valve is on the oil filter adapter. On some engines the vernatherm is on the accessory case.

Engines without an oil filter may have the vernatherm valve on the oil screen housing (part number 69510) above the oil temperature bulb. See Lycoming Service Instruction 1008B for details.

Textron Lycoming aircraft engines often have worn vernatherm seats. Remove the vernatherm and inspect the seating surface on the vernatherm valve for contact around the entire circumference. Replace the vernatherm if the contact pattern isn't even all the way around the seat face. Inspect the contact seat in the housing. Reface seat if there's not an even contact pattern.

Here's one method for checking vernatherm seat contact: Coat seat face with layout glue (Dykem is available at industrial supply stores). Fly once around pattern. Pull out vernatherm and look for a rub mark all the way around the vernatherm seat face. If there isn't 100% seating, replace vernatherm and reface seat. Lycoming makes a tool, part number ST-388, for refacing the seat.

Don't remove more than .010 of an inch of material when refacing the vernatherm contact seat. The removal of material from the seat face increases the distance from the vernatherm to the seat and decreases the amount of valve opening. This decreases the amount of oil that bypasses the oil cooler at any given oil temperature.

On many Lycoming aircraft engines the vernatherm screws into the oil filter adapter (part number 77852.) If you have this style then it's less expensive and easier to replace the adapter then refacing the seat.

Older style, before 1972, Lycoming vernatherm (part number LW-10269) valves sometimes didn't function properly. The old style vernatherm can be identified by metal discs on the valve diameter instead of a spring. See Lycoming service instruction 1255 for details.

Textron Lycoming Aircraft engines without vernatherm:

Engines without a vernatherm valve have an oil cooler bypass valve (viscosity valve). The viscosity valve allows oil to bypass the oil cooler if oil pressure is too high. This valve has a spring, plunger valve, and a plug. The viscosity valve is usually located on the accessory case just under the tachometer drive.

The viscosity valve prevents oil from going through the cooler until the oil's warmed up. Low temperature oil is more viscous and pumps at a higher pressure. In high pressure conditions the viscosity valve allows oil to bypass the cooler. As the oil warms up it becomes less viscous and pump pressure falls. The viscosity valve closes and oil flows through the cooler. In this manner the viscosity valve regulates oil temperature.

The viscosity valve must be removed if the engine has a vernatherm valve. If the viscosity valve isn't removed the tip of the vernatherm valve hits the viscosity valve plunger. This prevents the vernatherm valve from closing allowing oil to bypass the oil cooler. This problem usually occurs when installing an oil filter adapter on the engine. If the oil filter adapter has a vernatherm valve then it is also necessary to remove the viscosity valve from the accessory case.

On Textron Lycoming aircraft engines without a vernatherm the only way oil bypasses the oil cooler is via the viscosity valve. If this valve leaks, then some oil doesn't get cooled by the cooler. Although a valve leak is rare, it can occur.

To check for leakage, remove spring and plunger. Apply a small amount of "bluing", Dye Penetrate dye, or food coloring to plunger seat. Install plunger and rotate with a dowel or screwdriver. Remove plunger and inspect contact pattern on the seat surface of the accessory housing. At overhaul we recommend that you lap the plunger to the seat to form a leak- tight sealing surface.

If after all this you still have an oil temperature problem then it's time to check manufacturer service letters for any changes that reduce oil temperature. Cessna 172 models year 1977 through 1979 comply with Cessna Service Information Letter SE79-13. Changes recommended in this service letter reduces oil temperature during a maximum performance climb by approximately 14 degrees F. The service letter adds a cooling lip at the aft center of the lower cowl opening. The cooling lip induces greater air flow through the engine compartment. The firewall mounted oil cooler is relocated to the right-hand rear vertical baffle of the engine compartment to provide more direct exposure to cooling air. Maule M-4 owners, refer to Maule Service Letter No. 12.

Lycoming IO-720 aircraft engines use an adapter to line up the oil holes for the oil filter. High oil temperature results when this adapter isn't installed.

On 07 November 1991, Schweizer issued MSB B-248, pertaining to compliance with the Precision Airmotive Service Bulletin No. PRS-94 issued 21 September 1989, and states that, "there are persistent reports of low power and high engine temperatures (cylinder head, oil, and EGT on new aircraft, and on overhauled engine/servo combinations". It also states that, "failure to comply with this Service Bulletin could result in engine failure, which may lead to loss of control of the helicopter and subsequent injury or death".

Cessna aircraft with O-200 and O-300 engines

One some of these aircraft, Cessna (with FAA approval), used a air blast tube directed to the oil temperature probe to produce erroneous oil temperature readings to the pilot. If you move the blast tube (by installing a spin-on oil filter for example) so that the amount of cooling air on the probe is reduced you will get higher oil temperature readings on the gauge. Your oil temperature is not higher - its more realistic.

Source for a long thermometer: Contact Cole-Parmer at 800-323-4340 or http://www.coleparmer.com/

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.