CHT Question

bluesky74656

bluesky74656

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Todd Kooser
I'm flying a Bonanza S35 with an IO520. I was cruising at 8000, WOT, 2600 RPM and about 13 GPH LOP, which should have been between 65% and 70% power. All the EGTs were 30 or 40 degrees LOP, and all but one of that cylinders had CHTs of abou 300. The #5 cylinder, however, was reading 400 or more.

I'm wondering if that reading could be accurate. Would a truly high CHT necessarily result in a higher EGT? I'm thinking it's more likely that the sensor is faulty.
 
You want to swap probes to make sure it's a sender and not a bad nozzle delivering too much fuel.
 
My plane the #3 reads off because the factor CHT (round gauge) is plugged in there, my engine monitor has a spark plug probe on that one which reads colder rather than the screw in CHT probe.

The first question is "Is this a change?" If it's a change you need to look into it. My guess, some part of the baffle system has broke or become misaligned.

Most of the combustion issues would show in EGT changes as well.
 
It's a club plane and this is the first time I've taken this one one a cross country. They moved my preferred bonanza to another base, so I'm getting familiar with this one. So, I'm not sure if this is a change.

Henning, wouldn't a nozzle problem also show in EGT?


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The EGT is in line with the rest of the cylinders. I guess it could be an erroneously low EGT reading along with a nozzle problem, but Occam's razor suggests that assuming one problem is more likely than two unrelated problems on the same cylinder.


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bluesky74656 said:
It's a club plane and this is the first time I've taken this one one a cross country. They moved my preferred bonanza to another base, so I'm getting familiar with this one. So, I'm not sure if this is a change.

Henning, wouldn't a nozzle problem also show in EGT?


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Click to expand...

I assume nothing about the system at this point, my only concern is that you do not actually have a condition that is harmful, so I want to eliminate the most harmful possibility. Yes it would show on the EGT, however you can have that one cylinder running 40° ROP which is a hot place to be. Never eliminate the obvious worst problem by assumption.
 
bluesky74656 said:
The EGT is in line with the rest of the cylinders. I guess it could be an erroneously low EGT reading along with a nozzle problem, but Occam's razor suggests that assuming one problem is more likely than two unrelated problems on the same cylinder.


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Peak -40° happens twice, rich and lean.
 
The EGTs being in line with eachother is pretty much irrelevant. The idea that all EGTs should be equal is false.

The hot CHT could be several things:

-different mixture distribution
-probe (although probes usually don't go bad by reading high)
-baffling not providing proper cooling to that cylinder (happens - most baffles are really not in good shape, even if you think they are)
-ignition on that cylinder not firing well
 
Might want to grab an inspection/ dental cam and take a quick look at the exhaust valve on #5.
 
I wouldn't be happy with 400f at all.

For me it's 380f max for climb, 350f max cruise,
anything over and I start changing my profile, fuel, cowl flaps, power, pitch, etc.
 
Ted DuPuis said:
The EGTs being in line with eachother is pretty much irrelevant. The idea that all EGTs should be equal is false.



The hot CHT could be several things:



-different mixture distribution

-probe (although probes usually don't go bad by reading high)

-baffling not providing proper cooling to that cylinder (happens - most baffles are really not in good shape, even if you think they are)

-ignition on that cylinder not firing well
Click to expand...


Would the "You can't really hurt the engine under 65% power" advice still apply in these cases? The problem manifested both rich and lean of peak by a fair amount, so to keep that cylinder under 380 I would have to have to pull back to 55% or so for the trip home.

Either way I'm going to write it up for our maintenance folks to take a look at.


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bluesky74656 said:
Would the "You can't really hurt the engine under 65% power" advice still apply in these cases? The problem manifested both rich and lean of peak by a fair amount, so to keep that cylinder under 380 I would have to have to pull back to 55% or so for the trip home.

Either way I'm going to write it up for our maintenance folks to take a look at.


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If it's not your plane, that's all there is for you to do, good on you for noticing and squawking it.

65% power is mostly about detonation and not having enough ICP to create it. There are other ways to damage the engine.
 
High CHTs can (and will) still wear the cylinders.

Other possibility is if it has one of the under spark plug CHT probes instead of the normal location. Those read a lot hotter.
 
Henning said:
It's the CHT that's high, what are you looking for on the valve?
Click to expand...

80% of the valves cooling comes from contact with the seat. If the cyl is running hot, so is the valve. Not saying it's necessarily the cause of the high CHT, but the effect could be the start of a burned valve or worn seat. I'd want to check the color and see if the burn pattern was still concentric. Could tell you if this is new or has been a longstanding problem.

I'd also test the plug lead, do a lean mag check, and look for vacuum leaks.
 
bluesky74656 said:
I'm flying a Bonanza S35 with an IO520. I was cruising at 8000, WOT, 2600 RPM and about 13 GPH LOP, which should have been between 65% and 70% power. All the EGTs were 30 or 40 degrees LOP, and all but one of that cylinders had CHTs of abou 300. The #5 cylinder, however, was reading 400 or more.

I'm wondering if that reading could be accurate. Would a truly high CHT necessarily result in a higher EGT? I'm thinking it's more likely that the sensor is faulty.
Click to expand...

I am impressed you had that cool of CHT's.... Must have had the cowl flaps wide open....:dunno:
 
Chip Sylverne said:
80% of the valves cooling comes from contact with the seat. If the cyl is running hot, so is the valve. Not saying it's necessarily the cause of the high CHT, but the effect could be the start of a burned valve or worn seat. I'd want to check the color and see if the burn pattern was still concentric. Could tell you if this is new or has been a longstanding problem.

I'd also test the plug lead, do a lean mag check, and look for vacuum leaks.
Click to expand...

Gotcha, you're looking for damage, not cause, indications. Fair enough, however looking at the valve won't show the most common point of damage from heat and that is cracks in the Acme threads at the top of the barrel, especially with chrome cylinders. But it's not his plane, so likely he won't be doing the looking.
 
Ted DuPuis said:
Other possibility is if it has one of the under spark plug CHT probes instead of the normal location. Those read a lot hotter.
Click to expand...

This was my first thought. Assuming the engine monitor is an add on and the original equipment CHT instrument is still functional (assuming it has one) the probe for the engine monitor on that cylinder may be the spark plug washer type while the others are in the CHT probe wells on the heads.
 
I don't know much, but 300 is a low temp for 5 cylinders to be running. What kind of cooling gets those kinds of results?
 
Is the position of the sensor something I can see quickly by just popping open the cowling, or is it more involved?

I'm most concerned about causing catastrophic damage in the next 2 flight hours. When I get back home I can report it to our maintenance folks and they'll either fix it and assess any cylinder damage or confirm that it's a sensor placement issue.


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bluesky74656 said:
Is the position of the sensor something I can see quickly by just popping open the cowling, or is it more involved?

I'm most concerned about causing catastrophic damage in the next 2 flight hours. When I get back home I can report it to our maintenance folks and they'll either fix it and assess any cylinder damage or confirm that it's a sensor placement issue.


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I would call who ever you are renting from.. Tell them the story and let them make the decision..... After all.. it is THEIR plane...:rolleyes:
 
400F CHTs for an additional 2 flight hours won't cause any catastrophic damage that wasn't already going to occur.
 
bluesky74656 said:
Is the position of the sensor something I can see quickly by just popping open the cowling, or is it more involved?

I'm most concerned about causing catastrophic damage in the next 2 flight hours. When I get back home I can report it to our maintenance folks and they'll either fix it and assess any cylinder damage or confirm that it's a sensor placement issue.


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Click to expand...

Yes, if you can pop open the cowl to see the bottom of the head, you'll notice somewhere on the top or bottom of the head will be a wire running to a thermistor probe either screwed into a threaded boss in the head, or as a washer seated under a plug. See if #5 is different from the rest.

If you are AOG away from base, call the owner/management and seek advice.
 
N801BH said:
At 2600 RPM and wide open power....:dunno:
Click to expand...


I was using the Henning method of limiting power based on fuel flow not manifold pressure or RPM. According to the chart on CSOB I was between 65 and 70% power at those settings. And OAT wasn't much above freezing.


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bluesky74656 said:
I was using the Henning method of limiting power based on fuel flow not manifold pressure or RPM. According to the chart on CSOB I was between 65 and 70% power at those settings. And OAT wasn't much above freezing.


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You lose 3% power for every 1000 up in altitude .. So 8 X 3 = 24 % less power.....

The cold air up high will help CHT temps for sure..... To a point...
 
Ted DuPuis said:
High CHTs can (and will) still wear the cylinders.

Other possibility is if it has one of the under spark plug CHT probes instead of the normal location. Those read a lot hotter.
Click to expand...
this....my #5 in my Bo reads the hottest....and has two CHTs at #5, one for the factory gauge and another for the engine monitor. The factory sensor takes the bayonet location and reads normal to the others....but, the engine monitor for #5 reads about 30-50 hotter because it is the ring-spark plug style. You can get an adapter for the bayonet to have two sensors for another +$200.....but I chose to live with the difference.

BTW.....all my CHTs read around 350F in cruise....cept for #5 up around 400. I've switched the probe with #3 and verified that the spark plug reads higher than the bayonet location by 30-50 deg F.

I have a TSIO-520D....factory turbo.
 
Last edited:
Checkout_my_Six said:
this....my #5 in my Bo reads the hottest....and has two CHTs at #5, one for the factory gauge and another for the engine monitor. The factory sensor takes the bayonet location and reads normal to the others....but, the engine monitor for #5 reads about 30-50 hotter because it is the ring-spark plug style. You can get an adapter for the bayonet to have two sensors for another +$200.....but I chose to live with the difference.

BTW.....mine reads around 350F in cruise....cept for #5 up around 400. I've switched the probe with #3 and verified that the spark plug reads higher than the bayonet location by 30-50 deg F.
Click to expand...

That is great info for the OP...:thumbsup:
 
Checkout_my_Six said:
this....my #5 in my Bo reads the hottest....and has two CHTs at #5, one for the factory gauge and another for the engine monitor. The factory sensor takes the bayonet location and reads normal to the others....but, the engine monitor for #5 reads about 30-50 hotter because it is the ring-spark plug style. You can get an adapter for the bayonet to have two sensors for another +$200.....but I chose to live with the difference.

BTW.....all my CHTs read around 350F in cruise....cept for #5 up around 400. I've switched the probe with #3 and verified that the spark plug reads higher than the bayonet location by 30-50 deg F.

I have a TSIO-520D....factory turbo.
Click to expand...


Yes, much appreciated!


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