Apollo

Sky High CHT in Cruise

29 posts in this topic

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.....

Share this post


Link to post
Share on other sites

*Members that donate $10 or more do not see advertisements*

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.  

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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...

Share this post


Link to post
Share on other sites

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-

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

Early C's have adjustable cowl flaps. On my 70 model they are fixed.

Share this post


Link to post
Share on other sites

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

-a-

Share this post


Link to post
Share on other sites

 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.

Share this post


Link to post
Share on other sites

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-

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

 

 

 

 

Share this post


Link to post
Share on other sites

I hate to say it, but you can melt your pistons, well soften them to where the piston pin holes get elongated, and the ring lands get triangular shaped and your oil consumption goes to 4 qts per hr.

This happens when you lean for taxi, and rush the checklist, just saying...

Share this post


Link to post
Share on other sites

Early C's have adjustable cowl flaps. On my 70 model they are fixed.

But you can adjust them. Mine were closed and I had very high CHT (never over 500). I opened them to an inch and a half and CHT went down to below 450 in climb and <400 in cruise. 

Share this post


Link to post
Share on other sites

This sounds to me like an instrument issue although I'd want to determine that ASAP. My 63 C model with a JPI installed when I'm real careful will see CHT's around 400 in climb and around 350-370 in cruise. If I'm not careful temps will climb 430-440 in climb. I went through all the usual suspects such as baffle leaks, mixture issues, etc. and concluded these models run hot. In fact after extensive research I couldn't find anyone with a similar model and JPI equipped Mooney who saw significantly lower temps. This is unfortunate since 400+ isn't good for cylinders and can contribute to shortened life although this Mooney has always had high temps and has made it to TBO twice now.

Anyway, I'd check probe connections to rule that out and maybe substitute another probe to see if that helps. I know how frustrating this can be. Hope you get it figured out.

Share this post


Link to post
Share on other sites

This happens when you don't lean enough for taxi, and rush the checklist, just saying...

Fixed that for you.

1 person likes this

Share this post


Link to post
Share on other sites

One of the other partners flew it to Colorado this past weekend. He said that the CHT's got steadily high up to around 600. He said he turned off the master, then turned it back on and the gauges reset right in the green. 

 

 

Annual is this month - gonna see what we can do to fix this.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now