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Sky High CHT in Cruise


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#1 Apollo

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Posted 27 February 2013 - 08:30 PM

Hey guys,

 

Hoping you can help me diagnose this issue.

 

A friend of mine just bought a 1962 M20C model. They've been doing touch n Goes to get used to the airplane.

 

last night, I decided it would be fun for us to do a little VFR cross country. So we set out, all temps were looking great.

 

As soon as we stopped climbing and entered 'cruise' the CHT's went from 380ish to over 600. 

 

 

We immediately turned around and headed back to the airport, but as we were descending the CHT's went right back down to around 300 or so.

 

 

I must mention that the cowl flaps will not close. I THOUGHT that cowl flaps closed in flight was to help engine cooling, but I could be wrong. 

 

 

 

Any idea? Bad probe or gauge? Surely it never actually got to 600F.....


CFI/I - F82 in Lubbock, Texas


#2 1964-M20E

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Posted 27 February 2013 - 08:41 PM

Do you have a single CHT? What type of system do you have is it an RTD or a thermal couple? Unless I am wrong a thermal couple(since it works on generating a mV signal based on temperature) will give a low reading with a bad coonection and an RTD (since it is based on resistance increasing with temperature)would give a high reading with a bad connection.
John
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former 1964 M20E

#3 Hank

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Posted 27 February 2013 - 09:17 PM

Depends on the manufacturer. Almost all of the thermocouples we use at work default to a high reading when broken, whatever the display will show [with digital controllers now, that's often 999º, whether F or C].

 

However, if the wire is loose and changing attitude from nose high to level caused it to temporarily short, a high spike may be observed. The safety wire on my oil fililter shifted once, touching the Oil Temp probe and making the needle deflect to full right. You know that can't be correct when you haven't flown in several days and turn on the Master.


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#4 Earl

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Posted 27 February 2013 - 09:24 PM

Cowl flaps closed will make the engine run hotter as it reduces airflow.  Hence, cowl flaps open on departure and climb, partially closed in cruise and closed on descent.  I am betting a faulty probe or connection to the gauge.  



#5 Cruiser

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Posted 27 February 2013 - 10:21 PM

Inspect the wiring and thermocouple. If the temperature was really 600°F you would have melted the pistons. 


aka TomK
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#6 Hank

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Posted 27 February 2013 - 10:40 PM

Inspect the wiring and thermocouple. If the temperature was really 600°F you would have melted the pistons. 

 

Negative, they are steel. The aluminum block and head, however, would be significantly weakened if the temps were 600ºF for very long, and the bolts will probably all need to be tightened up from where the bolts heads sunk in.

 

Look for loose wiring connections, chafed insulation, corroded leads, etc.


--Hank
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#7 astelmaszek

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Posted 27 February 2013 - 10:57 PM

Negative, they are steel. The aluminum block and head, however, would be significantly weakened if the temps were 600ºF for very long, and the bolts will probably all need to be tightened up from where the bolts heads sunk in.

 

Look for loose wiring connections, chafed insulation, corroded leads, etc.

 

Hank,

 

Actually, the pistons are aluminum. Looking at one sitting on my desk right now. Reason they don't melt is called boundary layer.

 

Andy



#8 danb35

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Posted 27 February 2013 - 11:35 PM

Negative, they are steel. The aluminum block and head, however, would be significantly weakened if the temps were 600ºF for very long,

No, the pistons are aluminum.  The crankcase would be unlikely to have seen much additional heat from this event, but the head would almost certainly be damaged if that temperature reading were actually accurate.  Like others here, I'm suspecting an instrumentation problem.


Dan Brown, KE6MKS
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#9 aaronk25

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Posted 27 February 2013 - 11:50 PM

It's got to be a instrumentation issue. No way 600 and on a carb model too!

#10 n74795

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Posted 28 February 2013 - 01:13 AM

Hank,

 

Actually, the pistons are aluminum. Looking at one sitting on my desk right now. Reason they don't melt is called boundary layer.

 

Andy

I have seen many a melted piston......Detonation can melt a piston...



#11 carusoam

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Posted 28 February 2013 - 01:39 AM

Ahhh, the old hydraulic lubrication approximation?

The oil boundary later would smell of being burnt as well. If your oil is dark and discolored and smells fowl, this would be an indication of overheating.

I would expect that if the CHT went unusually high like that, you would also get an equally strange result in EGT as well. If the EGT was indicating normally, then the CHT is most likely a false indication.

The value of a JPI becomes evident on days like this. When you say CHTs did he have more than one?

Cowl flaps for a C: more open = more cooling. It's winter out, in cruise, temps should be lowish. If not, open the cowl flaps. If that doesn't work, land promptly. There is a red line on the C's cht gage, probably lower than 600F, right?

Sounds like something isn't right. It's old machinery that has recently changed hands. Could be instrumentation. Could be engine.

Trying to be helpful. Best regards,

-a-

#12 astelmaszek

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Posted 28 February 2013 - 03:14 AM

The boundary layer is not oil. It's unburnt fuel. It coats the entire combustion chamber, otherwise, the whole head, cylinder and piston would melt. The temperatures inside the cylinder reach 3000F during normal operations inside the plasma that forms when the mixture ignites. This is precisely the reason why detonation is bad. It destroy's the boundary layer. If it wasn't for that micrometer thin layer that simply refuses to exchange heat, all combustion engines would last about 30 seconds before burning holes thru the piston. Without it, the piston rapidly loses its thermal inertia and continues heating up.

#13 AmigOne

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Posted 28 February 2013 - 03:27 AM

Your C model has flaps?  What did you mean that they would not close?



#14 jetdriven

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Posted 28 February 2013 - 03:29 AM

The boundary layer is made of air. When you get detonation or preignition, the hot combustion gases pierce this layer and melt the pistons.
Byron
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#15 Hank

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Posted 28 February 2013 - 03:32 AM

Early C's have adjustable cowl flaps. On my 70 model they are fixed.
--Hank
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#16 carusoam

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Posted 28 February 2013 - 03:37 AM

'65 and older had cable driven cowl flap adjustments....

-a-

#17 Apollo

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Posted 28 February 2013 - 03:43 PM

 If the EGT was indicating normally, then the CHT is most likely a false indication.

The value of a JPI becomes evident on days like this. When you say CHTs did he have more than one?



-a-

 

 

 

The EGT was very normal, and got really cool once we started reducing power and doing touch n goes.

 

We have it in for annual starting in march, so heres hoping we can find the cause of the issue. Scary stuff.


CFI/I - F82 in Lubbock, Texas


#18 carusoam

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Posted 01 March 2013 - 01:17 AM

Check the dog house for missing parts and leaks.

M20Cs have the ability of generating near 500F CHTs according to the single CHT probe. It's not good to operate there, but was acceptable back in the day... According to the '74 POH. There is a green range up to 450F and a red line at 500F.

Operating at high CHTs will wear out the cylinders at a higher than normal rate. Acceptable to the FAA, But expensive to new owners...

Best regards,

-a-

#19 Chimpanzee

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Posted 01 March 2013 - 08:57 AM

I wonder if the CHTwas creeping up slowly up and down or making the transition quickly, like a flag. I had the latter case with the oil temp on my 20C (round combi gauge) which "flagged" in and out of the red. Turned out to be an old and cracked wire from gauge to probe. Had it replaced and no problems since the. However, the old girl just had an Insight G1 installed, so I should have comparatives next time.

cheers
Norbert



#20 Apollo

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Posted 01 March 2013 - 07:43 PM

Flew cross-country last night, about 45 minutes both ways.

 

CHT's did the same thing. Once we got into cruise it went straight over into the red and beyond in the CHTs.

We kept going and sure enough after about another 2 minutes the CHT's went back down to under 400.

 

 

BUT, on the way back from our first airport, the oil temp was straight past the red(small margin anyway) but it was bouncing around +/-50F.

 

 

 

 

Really weirded out by all this. Gotta be gauges. I saw no loss of pressure or anything else that bothered me. It was about 45F yesterday at our elevation (3200ft). I have 220 hours in a C model just like this one, and I've never seen it this hot, even in the Texas summer.

 

 

 

 


CFI/I - F82 in Lubbock, Texas





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