Piston engine troubleshooting

Piston Engine Troubleshooting By Thomas Ehresman March 1998 You work in a shop that specializes in turbine equipment. The few cabin class piston twins for which the FBO is responsible get their maintenance done elsewhere. From time to...

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A note on bad mag checks for Cessna 180/182s. In very cold weather the O-470 will show bad mag checks (200-300 rpm drop). This is because of the poor fuel atomization in very cold air. Try pulling the carb heat on, releaning (if altitude requires) and rechecking. Rough running of these engines during cruise in cold weather can sometimes be cured by flying with carb heat on to bring induction air temperatures back up to normal. A carb air temp gauge is perfect for this malady.

3. High cylinder head temperature:
You may have run into this with a partially clogged injector. If not, try these other sources. Baffling: Bad inter-cylinder or cylinder head baffling can cause localized cooling air loss. Check for stiff or loose baffle seals.

The classic birds' nest is often a common culprit here too. A healthy starling can build a very effective air block in 15 to 30 minutes.

If a cylinder has recently been changed, expect higher head temps on that cylinder for the next 30 to 50 hours of operation.

If all the head temps are high look for a more generalized problem. Check for numerous gaps and holes in the baffling. A small air leak goes a long way in dropping the cooling air pressure and causing temperatures to rise. Also, check the cowl flaps for proper rigging. Some models of aircraft have different cowl flap rigging for different years and engines in the same model.

Another, but less common cause of high cylinder head temps, can be magneto to engine timing. Advanced timing is generally characterized by smaller than normal mag drops and higher than normal cylinder head temperatures. A simple timing check will confirm or eliminate this one.

4. High oil temp:
High cylinder head temperatures and high oil temps often go together. In this case look for the common causes of both: bad or damaged baffling or its seals, cowl flap rigging, mag timing, or cooling air blockage.

If just the oil temp is high and cylinder head temps are normal, look for these common sources:

Birds are culprits here, too. A favorite spot for nests is the pockets formed above or in front of the oil cooler. Rags or other debris also lodge here easily. The oil cooler to baffling seal is also important. Air going around instead of through the cooler cuts down on the volume and velocity of the flow through the cooler as well as the cooling air pressure in front of it.

Piston Engine Troubleshooting

By Thomas Ehresman

March 1998


Dirty or excessive paint on the fins will slow heat conductivity and allow the temperature to rise more than normal also. This one will be seen mostly on hot summer days where the cooler needs all the help it can get. A quick visual inspection will cue you to this culprit.

Another common source here is what's called the "Vernatherm" or temperature bypass valve. This unit is a thermostatic valve that works much the same as the coolant thermostat in your car or truck. As the oil gets hotter, the valve closes harder, forcing more oil through the cooler. Removal of the vernatherm is required to check. The culprits here are abnormal wear of the valve face and seat, or loss of the expanding medium (some sort of wax).

An abnormal pattern for the valve face on the unit is obvious contact or wear on one side and not the other. The face will be worn on high time units but shouldn't be gouging into the seat.

Loss of expanding medium is harder to pin down. The easiest way to find out is replacement with one that is known to work properly.

With either problem, the fix is replacement of the whole unit. The valve seat may also be in need of refacing.

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