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High Oil Temp...

Piper J3

Well Known Member
Let’s talk Oil Temperature in RV-12 with 912ULS….

I recently discovered a crack in the muffler and was able to find an exact replacement exhaust system with only 70TT. Exhaust system is identical to original that I just replaced and it’s in pristine condition. My 12 is an early SN with muffler located close behind the oil cooler.

Rotax originally allowed using Mobil 4T 10W40 Full Synthetic Oil. I continue to use this oil and now have 750TT with no problem.

With original exhaust I would sometimes see 240F oil temp in climb on a hot day. With new exhaust I see 250F oil temp at 85F OAT. Both CHT and EGT are relatively unchanged at ~ 200F and ~1290F.

I am in contact with Mobil Oil tech support and also Rotax with no definitive answers yet. They say they’ll get back to me in a couple of days, but could be like telling a child “later for sure”.

I’ve been searching the internet (I know… not good idea) and see automotive recommends 230-260F oil temps and I also see where Full Synthetic Oils can tolerate 300F.

250F is approx. mid-range yellow arc on Dynon D180 with Van’s preset limits for Oil Temp.

I observe that oil pressure does not fall off at the high oil temp.

My question… what are max oil temps observed in RV-12 with Rotax 912ULS?

Thank you in advance…..
 
Jim,
I see 230 on oil temp typically at WOT, 120kts level flight on 80 degree days in SoCal. Easily hit 240 on climb out. At 240 I back off on RPM to 4800 and it will drop down after a few minutes but so does my climb / speed.
I noticed my lower oil line on the engine is 1/16” from the muffler and order the banjo oil line from Aircraft Specialties. I am doing a 100hr engine inspection tomorrow and changing that hose. I use sport 4 oil and have 928 hours on the engine. It runs fine and doesn’t use oil or coolant.
My coolant temps hit 230 on climb out but it comes down fast after leveling off. I have been using Preston’s (lime green) coolant and was told to change to Dexcool (orange). I will do that also on this 100 hr.
 
Jim,

I flew yesterday in 95F Phoenix. On climb out I hit about 245F oil temp which dropped to 230F in cruise with an OAT of 78F at altitude. This is about a 10F improvement after I trimmed 2 1/2” from the lower cowling aft lip and installed two deflectors to create a low pressure area at the cowling outlet. I previously sent you the pics.

Rich
 
Jim,

I flew yesterday in 95F Phoenix. On climb out I hit about 245F oil temp which dropped to 230F in cruise with an OAT of 78F at altitude. This is about a 10F improvement after I trimmed 2 1/2” from the lower cowling aft lip and installed two deflectors to create a low pressure area at the cowling outlet. I previously sent you the pics.

Rich

How much altitude did you gain before you hit 245F?
I can easily gain 2000’ on hot days before hitting 240F. My cowl is not trimmed but has been modified to move the oil cooler forward.
 
Oil temperature

On my aircraft, with that exhaust system installed, the oil temperature on a 90 degree ground temperature day would reach about 240-245 by the time I hit 2500' agl. Then I would throttle back and lower the nose to keep temps from getting higher. I opted for the new exhaust system instead of cowl modification. The new(2016) system has about 3" gap behind the oil cooler and I haven't seen temps any higher than 240 on a full throttle climb to 4000'+ agl. Most times the temperature goes down the higher I climb at wot. When the Bride is with me, I have to find smooth air and sometimes that's around 8500-9500 msl. I operate a little higher altitudes than most RV 12 folks.
 
Jim, it might help to modify the air intake of the lower cowl to make it more streamlined. There are some square corners in the fiberglass around the oil cooler. If those areas were filled in, then air coming into the cowl would be concentrated and could flow directly into the oil cooler.
 
close call

I think I have found the cause of my high oil temp. Today while doing the 100 hr on the engine I found this (see pic below).
This oil line was re-routed when I installed an oil thermostat from Aircraft Specialty Flightlines about 50 flight hours ago when I put the lower cowl / oil cooler back on I didn't see that it pushed the oil line against the exhaust manifold. This could have ended very bad had it gone on much longer.

I contacted Steve at Aircraft Specialty Flightlines and asked if the hose was salvageable and he suggested replacing it, this was around noon today. Steve didn't have the hose in stock. He was able to make it and get it shipped next day UPS, Wisconsin to California and I will have it tomorrow. The best part is he covered it all under warranty. This was clearly my faulty installation, not a bad product. Unbelievable customer service.

burnt hose.jpg
 
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I added a "Cowl Flap" sort of like a Cessna 180. This is just an experiment to see if it reduces oil temp. Sheet aluminum is held in place with 3M Body Side Molding Tape for testing...

Data point for today's flight: 49F OAT 205F Oil Temp 49F Oil Press

It's supposed to near 90F this weekend, so shouldn't have to wait long to wrap up test....
-
 

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Indicated temps read higher than actual on the stock D180 version of the 12. The error will be negligible at the bottom of the scale but can be substantial at the top end. Here’s a simple test, when you are indicating temps high in the yellow arc, momentarily turn the Master Switch OFF and see if the indicated temps drop. The lower value will be closer to the actual temperature.

Note: on the D180 12, turning OFF the Master Switch removes power to the oil pressure sensor and it will indicate zero pressure.

This test will NOT work if your engine is running and the regulator wiring was modified with the C pin jumped to battery voltage.
 
Indicated temps read higher than actual on the stock D180 version of the 12. The error will be negligible at the bottom of the scale but can be substantial at the top end.

First I've ever heard of this... Can you elaborate on why this happens and amount of possible error at high end of scale? Inquisitive minds want to know....
 
... Can you elaborate on why this happens and amount of possible error at high end of scale? ....

The error is related to the voltage drop in the grounding to the D180. Worst case was calculated at 180mV and best case, in practice, is about 25mV. It’s dependent on the alternate ground paths through the tray.
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If you do the test I recommended, you can calculate the the system’s voltage drop. Using the two values from the test, it’s the difference between the low temp voltage and high temp voltage from the table on the attached chart.
 

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Mike -

Thanks for explanation. I will perform this test soonest... I assume I can shut off Master Switch in the air? Just thought I'd ask before doing something risky....
 
Jim,

I’ve been offline and just saw your question about my climb temperatures. I usually get to about 2500-3000 feet before I level off, throttle back to 4800 RPM to cooldown back to the top of the green arc. Then I resume the climb.

The paddle deflectors and trimmed canopy seem to speed the cooldown process considerably.


Rich
 
I have add’l data that suggests I will have high oil temps in the heat of summer. I may be restricted to flying early morning or evenings to mitigate problem to get me through flying season and then make permanent fix. I’m thinking about moving oil coolant radiator forward to add clearance to muffler.

Does anyone have photos / info on this modification done on an early SN legacy RV-12? Thanks in advance….
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Jim, I’ll attempt to attach photos taken as I modified my lower cowl to move the oil cooler forward.
 

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The photos aren’t in sequence but you get the idea.
I uses a length of 1” painters tape to establish cut lines.
I used some of the removed fiberglass to make “clips” to hold the duct/radiator mount to glass it in place.
Note that I also cut and re-glassed an indentation for exhaust pipe clearance.
Hope this is helpful!
 
This is Very Interesting….

azonslow pointed me to Rotax Service Instruction Titled: Selection of Suitable Operating Fluids for ROTAX® Engine Type 916 i (Series), 915 i (Series), 912 i (Series), 912 and 914 (Series). The publication is dated May 28, 2021. pdf file is d06902.pdf in the Technical Documentation Section of https://www.flyrotax.com.

Section 6 states:

6) General Engine Operation Requirements and Operating Tips

1. Keep the engine oil temperature below 120 °C (250 °F) over most of the operating period.
2. Always ensure that the oil type used is adequate for climatic conditions and peak engine operating temperatures. If operational oil temperatures exceed 120 °C (250 °F), use of a mineral or petroleum based oil is not recommended.
3. For turbocharged engines ensure an adequate running cool-down period to prevent deposits by coking of oil.
4. When operating with unleaded fuels or MOGAS and when engine oil temperatures often exceed 120 °C (250 °F) use of a high quality full synthetic oil is recommended.
5. To avoid formation of condensation water in the engine oil, the oil temperature must rise at least once every operational day to at least 100 °C (212 °F).
6. Avoid extended use of carburetor air pre-heating when safe and reasonable.
7. AutomotivefuelshaveseasonalblendsandMUSTbeusedinthecorrectseason.Failuretodo so can cause hard starting or serious issues like vapor lock.
8. Depending on the type of fuel used, operating conditions, and the demands of the engine mission profile it may be necessary to increase the frequency of oil changes to avoid the excessive build- up of lead and other residues in the engine oil. Always adjust the engine oil change intervals to avoid excessive build-up of sludge in the engine oil.
 
Well, Jim, running full synthetic Mobil 1 4T motor oil and Mogas that's free of lead and up to 250F oil temps seems to be OK, if only done on occasion? I'd keep an eye on oil temperature, myself.

I ran Mobil 1 4T exclusively with Mogas, and almost always tried to keep my oil temps at 230F or less, not letting it get into the yellow range. Summer time and 1800 to 2000 feet AGL is all I would get, before backing out of throttle to 5000 rpm and level flight to cool down my oil temps. Those were personal limits I set, myself.

Biggest problem was that summer time around here almost always had temperature inversions that caused 4000- 5500 ft of altitude climbing necessary to get to the cooler atmospheric air above. Takes a long trip to make it worth the climb.
 
Has anyone found a larger oil cooler that fits the -12?

I know other planes with the 912 have larger coolers.

I would think that would solve the heating issue in hot climates.
 
If I was getting up to 250F I'd verify the oil temp using a Laser thermometer or dip probe into oil tank or maybe change out temp sensor
 
As a follow-up to my original post #1… I modified the lower cowling to move the oil cooler 1” forward. Worked a charm… Now in cruise flight, at 75 OAT, I see 207F oil temp. I should have no problem staying in green arc with exception of long climb on really hot day. :D
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Too bad the mold for the original lower cowling didn't take this into account. Fiberglass is really itchy work. Plus if your lower cowling is already painted, more rework.
 
As a follow-up to my original post #1… I modified the lower cowling to move the oil cooler 1” forward. Worked a charm… Now in cruise flight, at 75 OAT, I see 207F oil temp. I should have no problem staying in green arc with exception of long climb on really hot day. :D
-[/QUOTE

That is an amazing change from your previous numbers of 231f with OAT of 68.

I also have a 1" gap between my muffler and cooler. At 75f OAT, level flight, 5400 RPM, 118 TAS, gross weight, I have 235f on the oil temp.
I can lower the oil temp to 220f if I lower the RPM to 4800 but that drops the TAS to 105 KTS.
 
I also have a 1" gap between my muffler and cooler. At 75f OAT, level flight, 5400 RPM, 118 TAS, gross weight, I have 235f on the oil temp. I can lower the oil temp to 220f if I lower the RPM to 4800 but that drops the TAS to 105 KTS.

There is a “Heat Dome” forecast for Tues/Weds with temps in the mid-90’s. I’ll fly then and report-out with data….
 
As promised... I flew today to test oil temp with ground temp at 90F. Per post #24 I modified lower cowling to move oil cooler 1" forward of the muffler.

Today's Flight: 80 F OAT, 222 F Oil Temp, 50 PSIG Oil Pressure.

Looks like I'm good for summer ops in northern OH... :D
 
Oil temperature

Jim, I'm glad you got the oil temperature back in the normal ops zone. I was starting to think my old muffler was most of the problem. The temps you saw were very much what I had with that exhaust system. I didn't want to modify the cowl , for only a little temperature relief, so I replaced with the new (2016) setup. I flew over the weekend,with ground temps in the mid 90's and never got over 220*.
 
I did a few things that lowered my oil temp, I sprayed mineral spirits through the cooling fins of my oil cooler then blew it dry, also the replacement of the oil return line with the TS Flight line version (banjo fitting). I'm in SC and have been flying 1500' agl 95f at 5400...oil temps between 219-222F...I used to see 230-235F...I'm not sure which had the greater effect, washing the oil cooler or the new oil line. I did re pitch my prop to 71.4 from 71.
 
I did a few things that lowered my oil temp, I sprayed mineral spirits through the cooling fins of my oil cooler then blew it dry, also the replacement of the oil return line with the TS Flight line version (banjo fitting). I'm in SC and have been flying 1500' agl 95f at 5400...oil temps between 219-222F...I used to see 230-235F...I'm not sure which had the greater effect, washing the oil cooler or the new oil line. I did re pitch my prop to 71.4 from 71.

An over pitched prop can definitely cause elevated oil temps.
I am doubtful that either of the other things had any influence on oil temp.
 
I did a few things that lowered my oil temp, I sprayed mineral spirits through the cooling fins of my oil cooler then blew it dry, also the replacement of the oil return line with the TS Flight line version (banjo fitting). I'm in SC and have been flying 1500' agl 95f at 5400...oil temps between 219-222F...I used to see 230-235F...I'm not sure which had the greater effect, washing the oil cooler or the new oil line. I did re pitch my prop to 71.4 from 71.

What is your airspeed @ 5400 RPM?
 
About 110 KIAS @5400...replacing the oil return line created some needed space from the muffler, old line had about 1/8" clearance
 
oil cooler misalignment on lower cowling

I kept having high oil temps to on climb out. Discovered that the lower cowling had been reinstalled incorrectly. The oil cooler lower portion was not seated on the lower pins. Therefore, it was pushed backwards nearer to the muffler. Less airflow and much higher temps. Once the lower cowling was reinstalled correctly, that ended the excessive oil temps.
 
As promised... I flew today to test oil temp with ground temp at 90F. Per post #24 I modified lower cowling to move oil cooler 1" forward of the muffler.

Today's Flight: 80 F OAT, 222 F Oil Temp, 50 PSIG Oil Pressure.

Looks like I'm good for summer ops in northern OH... :D

A few of us modified the cowling and moved the oil cooler about an inch+ forward 8-10 years ago by modifying the cowl. It made a world of difference with hot OATs. I believe Van's changed the design in the later kits to increase the distance between the muffler and the oil cooler, finally acknowledging, this was the cause of high oil temperatures on climb during hot days (think they modified the exhaust pipes to move the muffler aft). Well done, Jim!!!
 
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I think most early builders did the move the oil cooler 1-2" forward soon after getting the sign off - not sure it's needed with the later exhaust and newer oil cooler?
 
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