Ask Paul: What’s up with my oil temperature?

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: I’ve been having trouble with my Lycoming O-360-AIA engine for some time. The oil temperature is running about 200° on a cool day. The head temperature is in med green.

I have completed the following to determine the problem: A few months ago a new temperature control oil cooler by-pass unit was installed; and I checked the oil temperature probe by placing it in boiling water. The instrument in the aircraft read about 200°.

The oil cooler is a Positech with six tubes. The air source is an outside air inlet. I believe that the air flow is enough to cool the oil. I have no way to check. I have asked around the airport for help but this has not been fruitful.

GARY QUESTA, via e-mail

A: Gary, after reading your e-mail, I could hardly get the smile off my face. I don’t quite know how to tell you this, but most Lycoming engine operators would really like to have that kind of oil temperature on their engine.

Let’s look at some of the important things about oil temperature. First of all, the maximum continuous oil temperature on all Lycoming engines is 245°F. Ideally, the desired oil temperature during cruise is 185° to 210°, assuming you are confident in the accuracy of your oil temperature gauge. We want the oil temperature to be at least in the range you’re seeing to assure we burn off the condensation in the oil which can lead to internal corrosion.

From what you mentioned in your e-mail, you’ve checked the probe and it came out fairly close, considering the aircraft instrument is not that finely marked and realizing it is a non-linear gauge. A non-linear gauge is one that means a needle width at the bottom of the gauge may represent a range of 20° and the same needle width at the top of the gauge may represent 35°, making it appear to be running very close to or at the redline while in actuality it’s 35° lower than it appears.

One other thing you might check is the oil pressure with the engine at idle rpm (650-700). If the engine oil is actually running hot or near the redline, lower oil pressure at idle would be a sign that the oil temperature is too hot. The minimum oil pressure at normal operating temperatures is 25 PSI, so if your pressure is above this, then I’m OK with that. If it turns out to be less than 25 PSI, then we’ve got to do some additional troubleshooting.

Going by what you’ve told me Gary, I don’t think you have an oil temperature problem at this point. Since we’re in the midst of winter, you may have to wait to do any further checking until warm weather returns.

If there is anything that you may have overlooked in the operation or troubleshooting of your engine don’t hesitate to pass it along and we’ll take another shot at it.

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  1. Andy Gompert says

    I’m looking for a temp probe that fits on a IO 540. The parts book says you need a Stewart Warner
    362-EW or Piper PN 550 487. I can find some, but there really high in price and hard to find.
    Is there a newer part number I can cross reference it to.



  2. Dave vasquez says

    Please advise if you narrow down a cause for creaping oil temperatures. I have an 0320 that does the same thing.

    • LarryH says

      Changed both oil pressure and temp gauges and probes. No change. I get just about redline in the climb with top of the green in cruise. No loss of pressure, mid green. Winter months temp is good 180. I can’t explain it I have 200 hours on the engine with this anomaly. I will give any updates should they occur.

  3. Larry H says

    I too am having this high temp problem in my non turbo Lance IO 540. During a 500fpm climb from sealevel 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 temp lower but still on the absolute top side of the green. New gauge and probe. In powered descent it will go lower but still above 210. Oil cooler is clean upon external observation.

    • Jace Curtis says

      Larry H and Paul – I’ve got a piper saratoga turbo, with the -S1AD up-draft cooled Lycoming. We’ve been fighting high old temps for years now. The cylinder temps are great, but the analog oil temp gauge creeps slowly towards redline in level flight at constant power. The analog gauge accuracy is verified and I also have a JPI that shows 20 degrees cooler. I know that because the vernatherm opens at 180 Deg F, I can see the analog gauge twitch at 180 F (on the white tick mark) and the JPI shows 160 Deg F. Needless to say, the 2 oil temp gauges are at different places in the flow loop. We’ve replaced the baffling. We’ve replaced the vernatherm, replaced the oil cooler. WEVE REPLACED THE ENGINE! Replaced the turbo. Same problems. Last week, I had to divert as I watched the analog gauge push to redline and the digital one climb through 220 Deg F. It’s gotten to the point that it’s just ruined my enjoyment of flying. I’m always distracted by the creeping oil temp. My latest theory is maybe the oil cooler is plumbed backwards (I think oil is supposed to flow up through it and not down).

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