This has actually happened twice. Both times in hot weather. In OATs below ~70 deg F oil temps are usually ok. The first time was a year ago, the last time was on Tuesday.

Oil temps pegged right at the redline (245) and held there for a while. 2200 RPM and 120 mph at ~6500 and 60 deg F oat. This was after the climb-out (which I did at full throttle, full rich, and 110 mph). After 10-15 minutes, temps dropped rapidly to ~200. This last time I saw the temp swing back and forth between 180 and 220 a few times (rapidly, transitioning in 1-2 seconds). On that same flight I descended from 6500 (oat ~60 deg F) to 1500 (oat ~90) at 500 fpm (probably something around 1800-2000 rpm and 130-140 mph). Oil temp dropped gradually to 165 and stayed there until after touchdown when it spiked and held back to 200 deg. CHTs were between 330-380 at cruise power and 280-330 in the descent.

This week I replaced the vernatherm as part of the annual condition inspection (in accordance with the new Lycoming SI). The flight described above was the first flight after. I also replaced all the rubber baffle seals as part of the inspection. Manometer test is yet to happen, but they look WAY better than before. I also sealed around the oil cooler better.



Normally, it's not unusual to see temp swings of 40 deg or more (though usually they max out below 220) so I keep oil temps in my scan because of this. When it gets to 225 (unusual) I start decreasing power, descending, and looking for a place to land.

I've been assuming it was an indicating problem because of how fast the indication swings (I'm still going to test the indicating system with a hot-plate, a pan of oil, and a good thermometer), but the last time it happened I noticed that the oil pressure was down to 65psi when the temps indicated high. Normally its near 80-85 psi. Plus digging around the oil diagrams over the past two days it looks like the oil temp sender measures oil temps before it gets routed through the engine, which would mean after the oil cooler. If I understand it right, it would mean that if the cooler were isolated from the engine, then the flow was resumed, it could cause a rapid change in oil temps as read by the oil temp sender.

My current theory (pending a successful test of the indicating system) is that there's something hanging up in the path between the engine and the oil cooler that's preventing the oil from going to and from the cooler. The brand new vernatherm which is showing the same symptoms as the old one so I don't think its that (I can test it when I test the indicating system to be sure). This is on a Lycoming spin-on adaptor (p/n 77852, I think).

When I go to troubleshoot this weekend I want to maximize what I look at, since I'll likely have to drain a brand new oil change and am trying to get it all checked out at once.

What else could cause symptoms like these? What else might cause what seems like an intermittent blockage of the oil cooler from the engine? What would happen if there was also a viscosity valve (type 1 bypass) installed with my vernatherm and that viscosity valve was malfunctioning? Could it cause symptoms like these? Any other ideas I should check out?

What kind of oil temp gauge do you have? Is the oil cooler a straight through design or is there some kind of temp control valve there also? I think you run vernatherm or viscosity valve not both at the same time, I have read proper way to do it in the past but getting to old to remember. The sudden changes sound like a gauge issue to me.
DENNNY

The first example sounds like a sticky vernatherm.

On the second example, a 40 degree fluctuation (both directions) in oil temp over the course of one or two seconds HAS to be a gage, wire or probe malfunction. I can't fathom any other explanation for that behavior, especially in a engine that's very much up to temperature.

More moderate temperature fluctuations can have a variety of causes, vernatherm principle among them, but not the only suspect.

One possibility is that there is a defect in the air flow to the oil cooler...a piece of cowl flap that folds over the vent opening, or a split in the scat tube that opens up at certain air flows and reduces air pressure to the oil cooler.

A spike in oil temperature at shut down is normal, since the engine mass has a lot of heat and the cooling has stopped. Even without a oil cooler, the oil sump acts as a cooler when the airplane is moving. Take the cooling away and the hot metal parts raise the temperature of the oil.

I have two oil temp probes...one at the factory location before the oil cooler, and one in the oil sump, after the oil cooler. It's common to get into the 225~240 range for hours at a time prior to the oil cooler, but it's very uncommon for it to get above 180 after the cooler. That said, both gages read pretty consistently...no big swings like you describe. If you want more information on what your oil is doing, installing additional temperature probes is a good way to go.

A couple things that might make you feel better: If your CHT's aren't climbing it's a pretty good indication your oil is doing it's job.

Both of my mechanics say running oil up to the red line is just fine. Don't go over, but that's the operational limit... going right up to the edge of it is completely OK. There's not much choice if you're flying at OAT's in the mid to high 90's, which is common in much of the world.

Synthetic multi weight oils have much better over-temp characteristics than single weight oils, so run them if you're worried about the oil temps getting a little warm.

Camguard increases high temperature oil performance, according to their literature.

Flying at a lower altitude generally reduces engine and oil temperature even though the OAT is higher. That's because as you climb the density of the air decreases faster than the temperature, and less-dense air has a lower cooling value. Strange but true, though how much effect there is outside of the O2 altitudes I'm not sure.

Good luck, and there's no reason in the world to discard the oil you drain out of the engine. Drain it into a clean bucket and poor it back in!

CamTom12 wrote:...I'm still going to test the indicating system with a hot-plate, a pan of oil, and a good thermometer......

An easier check is to dunk the probe in a pan of boiling water. 212 at sea level.
Less messy, plus you don't have to worry about the accuracy of your check thermometer.

hotrod180 wrote:

CamTom12 wrote:...I'm still going to test the indicating system with a hot-plate, a pan of oil, and a good thermometer......

An easier check is to dunk the probe in a pan of boiling water. 212 at sea level.
Less messy, plus you don't have to worry about the accuracy of your check thermometer.

That's a good idea, thanks!

Had similar experiences with 0-300 in 170. It was a loose ground on gauge. Good luck


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Ping, interesting you mention that...

Good news/bad news: I shifted around from days to nights to days at work this week so I slept in this morning and won't likely make it to the hangar today. But I did some more research this morning and found some new info.

The oil temp sender is manufactured by Datcon (http://www.datcon.com/en-US/Products/Da ... (Low-Range)--P-N-02017-00.aspx). It's rated between 100-240 deg, and since both of my weird situations involved pegging exactly at 240 for extended periods of time (plus the weird temp fluctuations) it's making me think more along the lines of an indicating problem. I noticed a little play in the top stud for the sensor during the condition inspection, so it may be that or a grounding problem. I'm going to boil the sender and wiggle the stud to see if that makes a difference. I have a 1k potentiometer so I might try that in place of the sender to see how the gauge side is working.

I'm also going to make sure my engine/gauge is grounded properly.

Tomorrow after church my wife's going to the hangar with me to sit in the cockpit and read the gauge so hopefully I'll have an answer then!

Looks like the sender is bad - new one is on order!

Glad u got it[emoji106]


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Still isn't fixed. New sender has been tested accurate, though.

I have messed with airflow cooling solutions thinking it might be indicative of reduced airflow through the cowling, but now I think it's really isolated to the oil cooling system.

Here is the current situation: I have replaced all baffle seals (marginal improvement in pressure differential between the upper and lower cowl), sealed off some gaps in the upper cowling fiberglass that did, in one remote circumstance, allow high pressure air to bleed to the low pressure side of the cowling (no change in pressure delta), and changed oil cooler from the Niagara 20002a to a Meggit-Troy 8406R (10-15 deg oil temp improvement across the board). Pressure differential is around 3.2 to 3.5" h2o (speed and altitude dependent), CHTs are usually 340-360 deg F in cruise, and oil temps are around 220 with 55 deg F OATs.

I temporarily sealed some additional areas with tape and created cowl gaps on the low pressure side that allowed ~4.3" h2o pressure differential, and saw CHTs of 297, 320, 321, 317 and oil temps at 185. This was at 5,000', 135 mph indicated, 2400 rpm and 42 deg F OAT.

Closing the temporary cowl gaps brought pressure differential back to 3.3" h2o with 320, 350, 330, 330 CHTs and 190 deg F oil temps. This was at 8,500', 133 mph indicated, 2475 rpm and 33deg F OAT.

A huge anomaly that I can't account for is that on my first flight after the oil cooler swap, oil temps were right around 170 for a 1.5 hour local area jaunt that included slow flight and some pattern work in ~50 deg OATs. Next flight I had mis-layed some of the baffle seals when I put the cowling back on and created a HUGE pressure air leak, which led to oil temps of 240 and some really high (~450) CHTs before I could get it back on the ground (this flight caused the upper cowl fiberglass work mentioned above, to make sure I couldn't make that mistake again). Since then they've been what I noted above, my normal CHTs and high oil temp.

My worry for permanently opening up more cowl area is that it didn't have an appreciable effect on oil temps and I had a hard time keeping CHTs up in 40 deg weather. Something pilot controllable will be required if I go that route in the end.

I still think oil temps are way higher in relationship to CHTs and OATs than they ought to be. I think my next step is to pull the hoses and take them to a local hydraulic shop for inspection. Looking through them didn't show any issues, but who knows. Then maybe pull the oil filter adapter (lycoming model) and inspect it closely. Not sure where else I'd need to look. Trying to get this figured before the summer rolls around again.

Your thoughts?

FWIW, it sounds like it might be a sticky gauge needle. The bearing in the middle of the indicating needle might be crusty. Takes a good push to get it to move, so it waits for a long time before moving until the force moving the needle reaches some friction threshold... and once it starts moving it goes to where it should have gone smoothly before. Hysteresis friction, "breakout force", etc.

It swings smoothly from 0 to whatever the temp of the oil is if I remove and reapply electric power to the aircraft.


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Not sure I understand what you've done on the gauge side of the system. You replaced the sender, but did you test all the wiring between the gauge and the sender, and also make sure that the gauge was properly grounded? Are there any other electrical anomalies that you've noticed, like suddenly brightening or dimming lights, avionics glitches, etc.?

FWIW, my oil temp gauge is actually a thermometer, not electric at all, with a long flexible tube between the engine and the gauge. My engine tends to run warm, so that the gauge is typically on the higher side of center (no temp markings on the gauge) but in the green. Only once that I can recall did it get to redline, pulling out of Lincoln, NE, on a 108 degree day. But it never fluctuates suddenly, as if it was getting a shot of hot or cold oil, and I doubt that's your problem, either.

If a vernatherm sticks, the oil temp might be too high or too low, but it's unlikely there would be sudden 40 degree swings. I have little doubt that you have an electrical problem of some sort, either in the gauge, the wiring to it, or conceivably in the electrical buss, as an under-voltage or over-voltage spike would likely do the same things that you've described.

Cary

Cary wrote:Not sure I understand what you've done on the gauge side of the system. You replaced the sender, but did you test all the wiring between the gauge and the sender, and also make sure that the gauge was properly grounded? Are there any other electrical anomalies that you've noticed, like suddenly brightening or dimming lights, avionics glitches, etc.?

FWIW, my oil temp gauge is actually a thermometer, not electric at all, with a long flexible tube between the engine and the gauge. My engine tends to run warm, so that the gauge is typically on the higher side of center (no temp markings on the gauge) but in the green. Only once that I can recall did it get to redline, pulling out of Lincoln, NE, on a 108 degree day. But it never fluctuates suddenly, as if it was getting a shot of hot or cold oil, and I doubt that's your problem, either.

If a vernatherm sticks, the oil temp might be too high or too low, but it's unlikely there would be sudden 40 degree swings. I have little doubt that you have an electrical problem of some sort, either in the gauge, the wiring to it, or conceivably in the electrical buss, as an under-voltage or over-voltage spike would likely do the same things that you've described.

Cary

Replaced the sender, checked continuity to the gauge and the ground is to the main ground cluster. The sender grounds on the oil filter adapter where it is screwed in. I have an ammeter and voltmeter and both are very close to my oil temp gauge. I haven't noticed any power swings when the oil temp has shown swings in the past (not saying it couldn't have happened, though). I will say that I fly a lot of night VFR and I've never ever seen power fluctuations during those, when it would be really noticeable.

The gauge is on the main bus, so any fluctuations would effect everything on that bus.

Oil temps haven't fluctuated since the new sender though.

Temps are still high. Disproportionately high over CHTs considering the ambient temperatures I've been flying in. This is what I'm really concerned with.

I would recommend that you step up the sender range. The one you have "stops" at 240*. It may give you some additional higher reading BUT - that is beyond it's specified accuracy range of 240* and you can not be SURE of what the temp really is above 240. Seems I remember Lyc. lists 240 as not too bad but RED LINES their literature at 250.
The 02022-00 sender has a spec. range of 100 to 260. With this simple change you would have a better idea of how close to Lyc. red line limit your engine is actually getting to. If this sensor gives you a reading of 250 or higher you would have better information to go forward with.

Nufffornow

I recently read that Lycoming says 180-220 is fine for continuous operation. 245 is red line in my operating limitations. Right now I hit 220 in cold weather. When it warms up again it'll go higher again.

I understand what you're saying about sending unit fidelity, but I don't think there'd be much difference with either sender in the range I'm concerned with (180-240) and I'd much rather focus on cooling the oil more efficiently. Everyone else I know has to block their oil cooler off in winter while I'm still worried about high oil temps even in cool weather.

Has anyone heard of any strange oil temperature situations with lycoming 4-cylinders in the past that might give me another area to look at?


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CamTom12 wrote:Has anyone heard of any strange oil temperature situations with lycoming 4-cylinders in the past that might give me another area to look at?

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Cam,

Not what you want to hear, but I had a Cub with O-320 years ago that ran oil temps similar to what you're describing. Called Lycoming tech (which I HIGHLY recommend you do), and they suggested getting a automotive compression tester and check compressions with it.

That resulted in very low compressions in a couple cylinders, caused by broken rings and one burned piston. Blow by gasses (extremely hot) were entering the sump, heating the oil. A major overhaul was required on that engine, which it turned out had a paper overhaul. Not saying your engine is there, but one cylinder could have a broken ring.

But, first thing I'd do is call Lycoming Tech Support. They are outstanding in my experience. And they answer the phone.

Good luck,

MTV

MTV, that's a good suggestion- I'll do that when I get home from vacation next week. Just curious, how were compressions when checked aviation style?


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CamTom12 wrote:MTV, that's a good suggestion- I'll do that when I get home from vacation next week. Just curious, how were compressions when checked aviation style?


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Compressions when checked with a differential tester were all 70s. The automotive tester measures the ability of the cylinder to make compression as the engine is turned over with the starter. On that engine, by fiddling with the prop, you could get it to hold pressure, but the slightest rotation it dropped off.

That was the first thing the Tech Rep suggested. I later attended the Lycoming Piston Engine Service School, and they discussed using the automotive tester as a diagnostic tool.

MTV

Yeah, mine are all 76-78. I'll get an automotive tester and give that a shot as well as calling Lycoming.


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