Oil Temperature Control Systems

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There are several key items to check when the oil temps seem to be out of the ordinary. But as the staff of Light Plane Maintenance recently reported, don't wait until it's an urgent problem.

This article originally appeared in the June 2002 issue of Light Plane Maintenance, and is reprinted here by permission.


Oil temp running too high? Troubleshooting the problem isn't very difficult to do. For example, there you are, cruising along on a nice sunny day, when suddenly you notice your oil temperature gauge has developed a case of the leans.

The needle is pointing more towards the red line than you've ever seen it before, and your adrenal glands are getting ready to redline, too. You might even be pulling the seat cushion up a bit between your cheeks.

Maybe you've got a real problem brewing here. Maybe your engine is getting ready to self-destruct. Then again, maybe not. Unfortunately, you've got to figure out what's going on here and take action accordingly. But how to get a handle on this?

First things first: Don't panic. Remember to fly the plane as you start a methodical round of initial troubleshooting of your high oil temperature indication.

Is That Right?

One of the first questions to ask, and answer, is whether the oil temperature gauge is indicating the truth. These things have been known to fail and even lose their minds (such as they are).

If the oil temperature gauge is working, it will respond to changes in oil temperature -- that much is obvious. You can change the engine's oil temperature in flight.

Try adding more cooling airflow to the engine. If you've got cowl flaps, open them, reduce power, drop the nose and see what happens. If you don't have cowl flaps, just lower the nose a bit, and pull back the power.

If the gauge is working properly, you should see oil temperature start to come down. This is a reasonable indication that the gauge isn't lying to you, but it's not definitive.

Use the oil pressure gauge to confirm that the temperature gauge is working. High oil temperature usually is accompanied by lowered oil pressure. Thus, with the engine powered back and the cowl flaps open, you should see oil pressure start to rise as the oil temperature starts to fall.

If you've gone through all this and the temperature gauge seems to be indicating properly (rising and falling in response to oil temp), you'd better start thinking about landing soon. That hot and/or rising oil temperature may be trying to tell you something that needs immediate attention.

Look Here

Once you've landed, the very first thing you should do, no matter what, is get the cowling off. Make a thorough visual inspection of the engine compartment. Your problem may be something as simple as a bird's nest on the oil cooler (Mooneys are famous for this one) or between the cylinders.

Maybe you left the cowling plugs in (it's happened), or maybe you've got a sandwich bag or some similar bit of trash draped over one bank of cylinders. Look for these problems before you leap for a mechanic.

Having looked for those obvious (and common) overtemp causes, your next task is to find out if the oil temp gauge is accurate. You can confirm that the temperature gauge is reflecting reality by getting your hands on a big thermometer.

One of those three-foot scientific jobbies works great. However, so does a temperature probe wire attached to a digital multimeter, so equipped. (The temperature probe on these meters with a tiny bead at the tip of Teflon-coated wire will work just dandy, and are several feet long.) Just slip it down the dipstick tube or into the filler neck until the bulb/tip is in the oil. The thermometer/multimeter should show about the same temperature that the cockpit temperature gauge was indicating.

If the engine has cooled off too much, take it out and run it until oil temperature is giving a solid indication, preferably until oil temperature stabilizes. Then give it the thermometer check.

In general, the thermometer should show a temperature between 150 and 190 degrees F. If the cockpit oil temperature gauge is leaning towards red line but the thermometer is showing only 160 degrees, rest easy and go get a new gauge.

On the other hand, if the gauge is showing a comfortable margin from red line and the actual oil temperature is 200 degrees, not only do you need a new gauge, but you've probably got some other problem, too.

If you're proactive, you might want to do the thermometer check even if you don't have an apparent oil temperature problem. If your cockpit oil temp gauge indicates in degrees, you can make sure it's accurate, and if it's not you can determine how far off it is.

If your gauge is one of those bone-head types that simply indicate in a range, you can add your own tape markings on the glass to give you an accurate handle on oil temperature in flight. It's a helpful trick that doesn't take much money or time -- and can be done by anyone.

Trouble Up Front

Okay, you've checked the accuracy of the cockpit gauge, and sure enough, the oil temperature is running high. What do you do now?

While you're in there fiddling with the dipstick, make sure you've got enough oil in the engine. Low oil, in itself, can lead to elevated oil temperature, simply because there's less oil to carry the heat burden of the engine.

(Note from your POH that most engines have an acceptable range of oil quantity levels for normal flight. You may find operating a quart or so below full significantly reduces the tendency to burn that first quart of oil -- if your engine is so inclined.)

If there is enough oil present, let the engine cool down, then start it up again. Note the oil pressure indications as the engine starts to warm up again. Low oil pressure can lead to high oil temp, since lowered pressures reduce the flow of oil through the engine, and hence the oil cooler (if so equipped).

If you've got good pressure, but the temperature is still too high, the next step is to see if the oil is picking up too much heat because of some other problem. A likely source of excess heat would be the pistons.

Bad rings in one or more cylinders will allow hot exhaust gases from the combustion process to leak past. When it does, the next stop for it is the oil, raising its temperature.

Fortunately, this condition is (usually) easy enough to check for. The oil will be turning black from the blow-by gases. Also, oil will probably be coming out the crankcase breather tube (due to case pressurization from the ring blow-by).

If the oil temperature is high, the oil on the dipstick is black and the aircraft's belly is dirty with oil, you've probably got a ring problem. To help confirm this, do a compression check.

Cool It

On the flip side, it may be that the oil is not getting enough cooling as opposed to too much heating. Depending on your particular engine, the oil cooling system will consist of the following components, give or take: the vernatherm valve/viscosity valve, oil cooler, engine baffling and cowl flaps.

When troubleshooting the oil cooling system, do it just like you would anything else by starting simple. Check the engine's baffling first. Engine baffling that doesn't seal causes high oil temps.

Baffling that's working properly seals the top of the engine from the bottom, creating a plenum chamber that increases air pressure on top of the engine. (For updraft-cooled engines it's the reverse.) Air then flows from the high-pressure area on top of the engine through the cylinder cooling fins to the bottom of the engine and out the cowl.

If the baffling isn't sealing properly, cooling air can get to the bottom of the engine without flowing through the cylinder fins, raising temperatures all around. Don't just give this a cursory check, particularly with older airplanes that have had several engine changes.

Some aircraft have particularly complicated arrangements of aluminum and flexible baffling. Neglecting even small aluminum inter-cylinder baffling components can cause cylinder hot spots. Be sure to check your maintenance manual for the exact component arrangement. It could even be as simple as the flexible baffling being bent the wrong way (common).

If the baffling looks good, move on to the cowl flaps if you've got them. Check their rigging against what value the airframe service manual recommends.

Occasionally, cowl flaps that should remain slightly open with the cockpit control in the closed position get mis-rigged so that they completely close. This cuts cooling airflow through the engine compartment, again raising temperatures.

If you find that re-rigging is needed, make sure you push up on the cowl flaps while you make any adjustments. This will help remove any slop from the linkages.

You might want to have a winter and summer setting for the cowl flaps, closing them tightly for cold-weather operations and leaving them slightly open during the summer. Some aircraft, like the Mooney 201, have a "trail" position for the cowl flaps for just such contingencies.

On To The Cooler

Oil coolers may be ineffective from internal or external damage (or bad baffling where installed). Unit on top has fin damage, and unit below had fin damage and leaks as well as internal sludge.

Having checked the baffling and cowl flaps and still come up with nothing to account for the elevated oil temp, the next place to check is the oil cooler.

Is oil flowing through the cooler? Obviously, if oil's not going through the cooler, it's not going to get very cool. Try putting your hand gingerly against the cooler when the engine is hot. The cooler should be hot, too. What's hot? Figure that you can probably stand to hold your hand against something that's 120 degrees F or less.

If the cooler isn't hot, then oil isn't flowing through it. This can be due to a number of causes. For example, the cooler may be partially or even completely plugged inside.

Varnish and sludge deposits in the cooler reduce oil flow, and cut down on heat transfer. You may think you can cure this problem by simply flushing the cooler with a solvent, but that probably won't happen. When the cooler gets bad enough for you to notice it, solvent flushing alone often won't work.

Send the cooler off to someone who specializes in oil cooler repairs. For example, Pacific Air Cooler (800 866-7335) has a very thorough flushing protocol that's effective in our opinion.

Trapped air in the oil cooler can also lead to high oil temps. In general, this happens only on airplanes with low-mounted coolers, like the Cessna 182RG.

If your oil cooler was recently removed and reinstalled, take it back off and purge the air from it. Probably the easiest way to do this is to fill the cooler with oil on the bench, then make sure it stays filled during reinstallation. (Note, cooler removal is not preventative maintenance, so get the supervision of an A&P.)

Maybe oil isn't flowing through the cooler because it's flowing past the cooler. On Continental engines with engine-mounted coolers, if the cooler flange isn't flat, the mounting gasket can break.

This allows oil to bypass from one side of the cooler to the other without actually flowing through the cooler. If you suspect this is what's happening, remove the cooler and check the gasket for integrity and the flange for flatness.

You can lap the flange flat, provided it's not too badly warped. Use a flat plate and 400-grit sandpaper that's been coated with a light oil. Make sure you clean the flange thoroughly and flush the cooler completely after this operation, otherwise you may end up pumping sandpaper grit through the engine, and it won't like that.

Oil can also bypass the oil cooler if the vernatherm valve is defective. The vernatherm is just like the thermostat on your car's radiator. It's a temperature-sensitive valve that regulates oil temperature by controlling oil flow through the cooler. As a rule, it strives to keep oil temp at around 170 degrees F.

You can check the vernatherm's basic operation the same way you would a car's thermostat. Just drop it in boiling water and it should open wide. That's nice, but it really doesn't tell you what you need to know.

Be scientific and methodical about this: Get a good, accurate thermometer (or temperature probe on a multimeter) and put it and the vernatherm in a pan of cold water. Start heating the water. (Some mechanics insist that water is not as accurate as oil, and use oil instead, but it's a messier proposition.)

Continental vernatherms, for example, should open when the water temperature gets up to 180 or 190 degrees. If it doesn't, chuck it and get a new one. A Continental vernatherm marked 77 C should travel at least 0.090 inches as the water temperature climbs from 120 degrees up to 170 degrees. Those marked 173 degrees should travel at least 0.16 inch as water temperature rises from 135 to 173 degrees.

Lycoming Vernatherms

Lycoming vernatherm showing contact pattern. Contact should occur around entire circumference of valve face or oil may leak through, defeating the valve.

In Lycoming engines, oil passing through the oil temperature control cavity goes either through the oil cooler or the crankcase passages. Which way it goes depends on the oil's temperature. It also depends on the condition of the vernatherm. If the vernatherm leaks, oil bypasses the cooler.

Not all Lycomings have vernatherms. On those that do have them, the vernatherm's location on the engine varies. Engines with both an oil filter and a vernatherm usually have both of them together, with the vernatherm mounted on the filter adapter.

On some engines, the vernatherm is mounted on the accessory case. On engines without an oil filter, the vernatherm might be found on the oil screen housing above the oil temperature bulb. If you can't track down the location of your vernatherm, check with Lycoming Service Instruction 1008B for some clues.

Once you've found your vernatherm, remove it from the engine (A&Ps or supervised owners) and examine the contact seats. The contact seat of the vernatherm is seldom inspected during overhaul, and sometimes the seat, if left uneven, allows oil to bypass.

Examine the contact pattern, and if it's uneven, reface the seat. Lycoming Service Instruction 1316A has detailed instructions on this, and you'll need the ST-388 seat cutting tool to repair the vernatherm seat.

If you elect to reface the seat, as opposed to springing for a new vernatherm, don't remove more than 0.010-inch of material. Refacing the seat face increases the distance from the vernatherm to the seat, changing the amount the vernatherm can open. Obviously, if too much material is removed from the seat face, the vernatherm will no longer function properly.

On many Lycomings, the vernatherm screws into the oil filter adapter. If you have this style and find the seat is uneven, you're probably better off just replacing the adapter. It will cost you more to try and save it than to simply replace it.

Old-style Lycoming vernatherms (pre-1972, with part No. LW-10269) have metal disks on the valve diameter, instead of a spring. Lycoming Service Instruction 1255 (August, 1972 -- still current) called for replacing these units, but some are still in service.

In the words of Lycoming, "It has been determined that the ... LW-10269 [vernatherm] does not function properly in some instances." If you've got one of these vernatherms and you're having one of these instances, replace it with a P/N 75944 vernatherm.

No Vernatherm?

Viscosity valve plunger, showing its location within the oil channel under the oil screen housing pad.

Some engines don't have a vernatherm. Instead, they came with what is called a viscosity valve to regulate oil cooler bypass flow. Viscosity valves have a spring, a plunger valve and a plug. You can usually find them hiding on the accessory case just under the tachometer drive.

Viscosity valves allow oil to bypass if oil pressure is too high, regardless of oil temperature. The idea is that cold oil, having a higher viscosity, will lead to higher system pressures. The raised pressure pushes the spring back, opening the bypass valve and allowing oil to skip going through the cooler.

As oil temperature rises, viscosity drops, lowering pressure. The spring gradually pushes the plunger back down, forcing more oil through the cooler as the temperature rises. Thus, although it's an indirect control, the viscosity valve regulates oil temperature.

Vernatherms and viscosity valves don't go together. If the engine has a vernatherm, there should be no viscosity valve. If you manage to find both of them on your engine, remove the viscosity valve.

On accessory cases with the vernatherm in the oil screen housing, if the viscosity valve isn't removed the tip of the vernatherm valve hits the viscosity valve plunger.

This prevents the vernatherm from closing and oil winds up bypassing the cooler. You may find this situation if someone installed an oil filter adapter on an engine that didn't have one originally (or if you've just had one installed). So if you elect to go for a filter adapter, make sure the viscosity valve gets removed.

(Incidentally, for those few of you out there who own Lycoming IO-720 engines, make sure the adapter to line up the oil filter holes has been installed. Without it, you're going to get high oil temperatures no matter what you do.)

You can check for leakage at the viscosity valve by removing the spring and plunger from the valve and applying a small amount of "blueing," dye penetrant or even food coloring to the plunger seat.

Install the plunger and rotate it with a dowel or screwdriver. Then remove the plunger and examine the contact pattern on the seat surface of the accessory housing.

You should see full contact all the way around the seat. If you don't, you'll need to lap the plunger to the seat to get full contact and form a leak-tight surface.

No Relief

If you've gone through all this and your oil temperature is still too high, there's some further legwork you can do. To an extent, you or your mechanic should have done this stuff long before any problem became apparent, but just in case, the following items are worth checking into.

Take a quick trip through your aircraft manufacturer's service letter/ bulletin file to see if there have been any changes to reduce oil temperature. For example, on 1977 through 1979 Cessna 172s, make sure Cessna Service Information Letter SE79-13 has been complied with.

SE79-13 calls for adding a cooling lip at the aft center of the lower cowl opening. This will produce greater air flow through the engine compartment.

Also, the oil cooler gets relocated to the right rear vertical baffle to give it a more direct blast of cooling air. All in all, these changes reduce oil temperature by about 14 degrees F during climbs.

Similarly, Maule owners might want to check into Maule Service Letter No. 12 if they're having oil temperature problems.

Finally, make sure you've got the right oil in the engine to begin with. Putting the wrong grade of oil in will have a definite effect on oil temperature. Too thin a viscosity in summer and temperatures will skyrocket.

If, after exhausting all of these possibilities, the oil temperature is still high, go back to square one. You probably missed something somewhere along the line.

You'd be surprised at how often something that later seems obvious gets overlooked during an initial round of troubleshooting and investigation. This is why it pays to be methodical and deliberate in your approach.

It's the only way you'll figure out what's going on without a lot of wasted time and money. Take your time.

Consider the system as a whole at each step in the process, and you will find the answer -- it's not rocket science -- or is it? No, it's just a matter of attention to detail, and being methodical, as are so many aspects of effective mechanics.