M20J low CHT in cruise

[SuperPilot]

Hey guys noticed this weekend with this cold front moving in that while in cruise in my 1978 M20J at 7,000 feet with an OAT of about -2 celsius my CHT on all cylinders was running about 275-300 fahrenheit. Any concern being at those temps? On another note Oil temp was around 150. Thanks for any help guys!

[Glenn]

Your vernatherm may be staying open.

[jetdriven]

Some folks tape up half the oil coolers with the aluminum speed tape. I hear more Bonanza guys doing that. The J oil cooler is very efficient and oversized for cool weather. My oil temp often heads for the low edge of the green band when descending for landing in cold weather. I see no problem with CHTs that low, are you running LOP at 7k feet? You might want to run closer to peak EGT

[Cruiser]

check your POH, it should have green arc CHT values for normal operations.

 

The Lycoming engine manual states "For maximum service life of the engine maintain cylinder head temperature between 150°F and 400°F during continuous operation."

[jlunseth]

I have my mechanic install an oil cooler block every winter.  It does not have to be much.  The current block is a couple of pieces of soft foam taped together.  It blocks the flow of air through the oil cooler and keeps the oil temps and CHT's warm.

 

I have had two disconcerting experiences with cold temperatures (of which we have alot here in Minnesota).  The first occurred when I had ambitions to try a landing at our Bravo airport, KMSP, and in preparation I thought I should try some approaches and landings at a higher than normal approach speed, because the might want me to "keep my speed up."  My plan was to fly about 25 miles from my base, KFCM, to KGYL, and do some fast pattern work.  It was our first cold day of winter and as I recall the temp was 14 F.  I was in a hurry so really did not spend time letting the engine warm up.  I did my first landing at KGYL and taxied to the ramp, cowl flaps open as the book says.  I taxied back out and took off.  On takeoff I noticed abnormal fluctuations in the oil temp (it was very cold) so I immediately landed.  There were large streaks on both sides of the aircraft.  When I got a mechanic out to look at it, liquid in the oil cooler had frozen causing blockage of the oil breather line, in turn allowing the crankcase to pressurize, and the oil had come out of every available orifice.  That was all repaired.

 

A year or so later I needed to fly to my mechanic in Willmar for an oil change and to have an oil cooler block installed.  It was again a very cold day.  This time I gave the engine and engine compartment plenty of time to warm up, and had no issues with the oil cooler.  However, the oil temp during the entire approx. 45 minute flight was right at the 100 dF minimum operating temperature, and I actually ran the engine agressively leaned out to try to keep the temps up.

 

Now I don't ever fly in cold weather without an oil cooler block installed.  That would be anything colder than 20 dF on the ground.  I keep the cowl flaps closed during ground operations because I need heat in the engine and engine compartment.  They are going to radically cool on takeoff as soon as the ram air flow starts.  I have flown with temps aloft in the flight levels as low as -55 dC without incident, but the air up there is not dense and does not have the same cooling efficiency as the air near sea level at or below 20 dF. 

 

So keep it warm in the winter.  The cooler block is a simple quick fix, but don't forget to take it out when operating in warmer temps next spring.

[jetdriven]

I haven't read anything other than leave cowl flaps full open even in subzero temps. You may warm up the engine faster with them closed but the airflow is very restricted, and the engine warms up unevenly which forms hot spots on the cylinders. Your CHT gauges won't show this. I do trail my J cowl flaps for takeoff, even in summer, because they really are too large.

[Marauder]

I am usually below 300 degrees for CHT temps this time of the year.

[The-sky-captain]

Lop at 22 degrees/7000' today my coldest cht was 270.

[SuperPilot]

Thanks for all the quick responses! Yes I was about 25 degrees LOP even went rich of peak to the same egt and like I suspected didn't make a difference. I'll deffinately have to get an oil cooler block installed seeing as these temperatures in central PA can be brutal at times. Noticed my heater was blowing I'd say room temp air as well during cruise, I'm assuming the oil cooler block wouldn't have any impact of that right?

[aaronk25]

Try adjusting cowl flap linkages up a bit. There is a jam nut on the cowl flap itself. The cooler block off, is also a good idea.

I adjust cowl flaps about 3/8 of a inch up in winter and down 3/8 in summer, but it's MN and 95d in summer and -20d in winter YMMV.....

[Marauder]

Thanks for all the quick responses! Yes I was about 25 degrees LOP even went rich of peak to the same egt and like I suspected didn't make a difference. I'll deffinately have to get an oil cooler block installed seeing as these temperatures in central PA can be brutal at times. Noticed my heater was blowing I'd say room temp air as well during cruise, I'm assuming the oil cooler block wouldn't have any impact of that right?

The heater is a shroud on your exhaust. You could have a collapsed SCAT or SCEET (can never remember which one) or perhaps the access door for airflow is partial blocked. My F produces so much heat that I need to use the cool vent to blend it.

[Guitarmaster]

The heater is a shroud on your exhaust. You could have a collapsed SCAT or SCEET (can never remember which one) or perhaps the access door for airflow is partial blocked. My F produces so much heat that I need to use the cool vent to blend it.

Mine too. It will drive me out of the airplane without mixing with cold air!

[SuperPilot]

Wow, yea must be an issue there. Air is blowing hard out of the heater vent just isn't warm. With an egt of around 1400 degrees it should be nice and warm

[ArtVandelay]

Wow, yea must be an issue there. Air is blowing hard out of the heater vent just isn't warm. With an egt of around 1400 degrees it should be nice and warm

Depends on how efficient the heat exchanger is

[jlunseth]

These points may have been mentioned.  Oil cooler block will make some, but not alot of difference in the output of the heater.  It works just like a car heater of course, so getting the engine warm and keeping it warm is what generates heater output.  Actually, it uses the heat from the exhaust and to a degree that is separate from the engine temps. but it still helps if the engine is normally warm.

 

I think someone mentioned that the Air Vent and Heater output come from the same outlet.  If you want maximum heat output you need to close the Air Vent knob in the center console.  The heat output of my heater with the Air Vent knob closed is hot enough that the fuel switch that sits on the floor in the heater air flow gets quite hot.  You can mix cold air in with the warm air from the heater just by opening the Air Vent knob.  If both the Heater knob and the Air Vent knob are full open, in my aircraft, there will be quite alot of air flow but it will be barely warm.  Adjust the two knobs to get the temp and flow mix that you want.

[garytex]

Physically check that your cowl flaps are actually closing.  As the control cable wears, the sheath can move in its attachment, or be worn clear through and the sheath moves rather than the cowl flaps, mine failed full open.  Happened last summer, and I was marveling at how much cooling margin I had, all proud of the great baffle seal job I had recently done concurrent with the cowl flap actuator failure  ;-) 

[jlunseth]

Byron, on the question of closing the cowl flaps in cold temps, it is not getting the engine warm that I have had any trouble with.  It is the accessories in the engine compartment, particularly the air/oil separator, and also getting the oil warm.  I know you know this, but others may not.  The air/oil separator is basically just a can (like a soup can) and it is separate from the engine.  It does not get oil flow in the true sense.  It gets pressurized air from the crankcase which contains oil, and the oil is separated out and returned to the engine.  It also gets moisture, and there may be some ice in the can from the last flight.  If there is ice in the can, or if icing starts because the can is never warmed, the ice will block the pressurized air from leaving the can which in turn leaves the crankcase with no functioning breather line. 

 

There is also an issue on cold days that the engine temp and oil temp can be less well related to each other than on warm days.  The oil is going to cool rapidly on takeoff, particularly if you don't have a cooler block installed.  If the OT was not allowed to get hot before you took off, you can certainly see CHT's in the high 300's (like 380), but OT's that are just too low because you don't have oil flow.  The OT minimum redline on my aircraft is 100, and you really don't want to operate below that.  You don't have sufficient oil flow to allow the oil to help with engine temperature dispersion.

 

So the reason for operating on the ground with cowl flaps closed in cold temps is not to get the engine warm, it is to make sure the oil is warm, and that the engine compartment is also warm so that detached accessories such as the air/oil separator have come up to temperature. 

 

The capacity of cold, dense ground air to cool these things in the engine compartment is pretty amazing.  Startling, actually, when you takeoff and have good CHT's, but the oil has gone stone cold and won't warm up. 

[aaronk25]

I keep my cowl flaps closed on the ground when temps get below the neighborhood of 10f. During run up I will inch the cowl flap cable out a inch or so, but that is it.

The reason is im attempting to get the engine as close to operating temp as possible before applying 100%.

I've heard the argument about creating hotspots with the cowl closed, but in cold weather opps, I don't buy it. There is still some airflow and at idle I believe there just isn't much heat created.

Also even if parts of the cylinder see no cooling airflow the cooler parts of the cylinder still absorb heat and vise versa..

Also with the cowl flaps partly opened there is zero heat. Where with them closed the defrost will keep the windows from getting frosty from my passengers that spew to much hot air.

At 10f and below its cowl flaps trail for take off and closed at 120kts.

The only evidence I have supporting this as OK operation is I ran my cylinders to TBO and there on there 2nd run. All barrels specd within factory new tolerances.

Also run at peak EGT at 24-25" when low. A lot of folks said I'm gonna need a top overhaul....prematurely. Cylinder shop said the cylinders looked beautiful.

IMO if the engines are warmed up good, oil changed, and run often it is pretty hard for a pilot to do something to shorten the life.

[slowflyin]

My F model, 201 wanna be front end, will always run that cool if I run to far lean of peak this time of year.   My oil temp  will still be rock solid at 182 just like always.  I usually push the red knob forward until I'm around 325 on all four.  Still LOP but I pick up a knot or two and mine runs smoother at that setting.

[Guest]

The heater is a shroud on your exhaust. You could have a collapsed SCAT or SCEET (can never remember which one) or perhaps the access door for airflow is partial blocked. My F produces so much heat that I need to use the cool vent to blend it.

Either a collapsed duct or a disconnected duct. Two vowels means two plies, ie. Scat, Sceet, Cat, Ceet.

Clarence

[aaronk25]

I also don't buy it that the lead will build up on valves ect if not continuously operated above 300d. I'm a mike busch fan but, I call BS on this. I cruise in my J in the winter with CHTs cooler than typically in the 250-280 range if it's -10- 25c. I've borescope my cylinders several times after a long winter and they look exactly the same as summer. No build up.

The flash point during ignition is so hot I just don't see how some of these arguments can support this. The difference between 250f and 400f is so small on the heat spectrum compared to combustion temps or even egt temps I don't buy it.

I do but the fact that a cylinder will likely last longer the cooler it operates.

Boat engine cylinders' CHTs are 145-210f. They last a lot longer. Similar power settings. I think it's heat that kills our cylinders and I wouldn't be worried about a cooler CHT.

[bd32322]

I also don't buy it that the lead will build up on valves ect if not continuously operated above 300d. I'm a mike busch fan but, I call BS on this. I cruise in my J in the winter with CHTs cooler than typically in the 250-280 range if it's -10- 25c. I've borescope my cylinders several times after a long winter and they look exactly the same as summer. No build up.

The flash point during ignition is so hot I just don't see how some of these arguments can support this. The difference between 250f and 400f is so small on the heat spectrum compared to combustion temps or even egt temps I don't buy it.

I do but the fact that a cylinder will likely last longer the cooler it operates.

Boat engine cylinders' CHTs are 145-210f. They last a lot longer. Similar power settings. I think it's heat that kills our cylinders and I wouldn't be worried about a cooler CHT.

I hope you are right. I typically see sub 300 CHTs in at least two cylinders in winter, the other two will be 310 or 320 or so. I have also blocked off the oil cooler with 1 strip of HVAC tape and I get 160 for oil temps. Probably need to add a bit more tape and get the temps up a bit more

[aaronk25]

I hope you are right. I typically see sub 300 CHTs in at least two cylinders in winter, the other two will be 310 or 320 or so. I have also blocked off the oil cooler with 1 strip of HVAC tape and I get 160 for oil temps. Probably need to add a bit more tape and get the temps up a bit more

I do the same with my cooler same tape too. I think the vernatherm is set around that point as I see the same 165 oil temp top. My last oil analysis showed zero moisture so I think we're good there too.

[ArtVandelay]

I hope you are right. I typically see sub 300 CHTs in at least two cylinders in winter, the other two will be 310 or 320 or so. I have also blocked off the oil cooler with 1 strip of HVAC tape and I get 160 for oil temps. Probably need to add a bit more tape and get the temps up a bit more

Or fly at higher altitudes where the water evaporates at lower temperatures

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