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Head Scratcher - High CHT, drops when leaning

J

jtrusso

Well Known Member
Head Scratcher - High CHT, drops during leaning before peak EGT

I apologize in advance for the long post....

I've been fighting high CHT on cylinder #3 for years now. I have a Mattituck 0-360 A1A running 2 slick mags with a Whirlwind 151 prop. I've read, re-read and read again many of the threads here on VAF about high CHT's, all of the Deakin articles and a lot of papers by Mike Busch and I still can?t figure out why #3 is running so hot.

I've checked the baffles, and while not perfect, they're pretty good. I need to spend more time making them perfect because I'm sure there is some room for improvement here. Both mags have been checked and are working fine, timing checked to 25 BTDC. All 4 cylinders consistently compression check in the high 70's and I can?t' find any indication of an induction leak. I?ve checked all 4 CHT probes in a bath of hot oil and they all read within 1 degree of each other. I have the screw in type CHT probes and an Aerospace Logics digital CHT and EGT gauge. I need to double check the takeoff fuel flow but I believe it is around 15gph.

Climbing out from sea level at 125kts, WOT and 2,500 RPM the #3 CHT always goes to 450. When leveling off for cruise, leaning to about 11gph it comes down to around 430. The other 3 cylinders CHT peak around 380 in climb and then settle down to 325 or so in cruise. All 4 EGTs seem normal and are consistently in the 1300 range.

This past Friday my experience with the #3 CHT defied everything I have read about air cooled engines and the relationship between mixture, CHT and EGT. I leveled off at 4,500?, 24? and 2,500 RPMS and started leaning the engine. From 11gph to 9gph all EGTs rose, with #3 peaking around 1360. During this time, while the EGT rose about 200 degrees, the CHT fell from 449 to 380. The other 3 CHTs all stayed the same, around 350. The engine seemed very happy operating at 9gph, a little vibration but not at all bad, and all CHTs were 380 or less. I used to think the #3 cylinder may be running lean, but if that were the case I would have expected the EGT to peak long before the other cylinders and then start to drop in temp, with the CHT dropping after EGT peaked. This was not the case, #3 was the first to peak and the CHT began to drop almost immidiately and did so all the while the EGT was rising. I'm completely confused.

I don?t get it, why would the CHT drop on #3 as I leaned the mixture? I haven?t had time to go back up and try this again, to see if aggressive leaning lowers the CHT but I plan to do so this Friday. In the mean time, what does the VAF brain trust think? Does this make sense?
 
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Several things here I suspect.

I've checked the baffles, and while not perfect, they're pretty good.
Click to expand...
I would definitely make sure your baffling is as perfect as it can be before embarking on more exotic fixes.

It may be possible that you are inadvertently leaning on climb out.
This can happen when a mixture cable is too short and pulling the mixture arm back on climb out.
Check fuel flow throughout the climb and verify the flow meter.
15GPH is on the low side for an O-360 and 16.5 GPH is more like an average for your engine.
I had a 360 for several years and I never operated at 11 GPH in cruise,
more like 8 to 9 GPH.
The fact that Cylinder head temps drop after you lean tells me that you have arrived at a lean of peak setting at 9 GPH, this is normal for lean of peak operations. What is not normal is lean of peak at your indicated 9 GPH
on a O-360, that condition is reached more likely around 7GPH.

Something tells me that your fuel flow is indicating about 2.5 Gallons more than actual flow which would explain your severe lean condition on take off.

Uneven fuel distribution can contribute to a large cylinder temperature difference but having your # 3 climb to 450 is unacceptable in my view and
is most likely the result of a severe lean condition in that cylinder.

So, besides your baffling check and verify your fuel flow meter to make sure you deliver enough fuel for optimal take off engine performance.
 
Leaning should always result in lower CHT, so I am not surprised to see this.

A couple of other things to check which is not that difficult
Swap your number 3 injector nuzzle with one other cylinder to see if it follows it. Although this should affect your EGT more, but since there is no set number for the EGT and only relative, it may reveal some. If you have checked the air, the spark and the compression, the only other thing left is the fuel flow.
Also regarding your baffle, check under your #3 cylinder to see if you have unusual pack of stuff/wire etc. that would restrict the flow of air. I have a pretty decent CHT numbers, climb out at 95K will hit the 400-405 and then comes down to the 355-365 range in CA. If/when I lean of peak, I am in 310-320 range. I re-routed some of the cables under my #1 cylinder and it lowered my CHT on the cylinder.

Good luck
 
jtrusso said:
I don?t get it, why would the CHT drop on #3 as I leaned the mixture?
Click to expand...

I am not an engine expert but when you lean the engine, you're providing less fuel to the cylinders for them to burn. Heat is generated from combustion. At some point less combustion = lower CHTs. But that is just the answer to your explicit question. There's more to the story.

If you had allowed things to equilibrate after leveling out into cruise mode before starting to lean, initially, if you leaned slowly, I'd expect to see CHTs rise. The unburned fuel from the extra rich mixture provided some cooling and you're removing the excess fuel for cooling. As you continue to lean you'll get to the optimum fuel air mixture for combustion where you'll hit peak EGT. If you keep leaning, you're running LOP and you'll see lower EGTs and better fuel economy...and marginally slower speeds. It's that middle area where the temps go up that's the "red box" and the goal is to lean quickly to avoid time in that area.

Again, I'm no expert but that's how I understand it and there are certainly others here who will correct me if I'm wrong.
 
Bavafa said:
Leaning should always result in lower CHT, so I am not surprised to see this.
Click to expand...

Don said:
I am not an engine expert but when you lean the engine, you're providing less fuel to the cylinders for them to burn. Heat is generated from combustion. At some point less combustion = lower CHTs.
Click to expand...

Not so sure the above statements are true. See chart:

Lycoming%252520manual%252520graph%252520temps%252520vs%252520mixture.jpg


I'm curious what your issue may be as my #3 demonstrates similar to the original poster's experience.
 
John, I see that you have read and re-read all of the posts concerning this issue but I just wanted to make sure that you have a washer between the number 3 cylinder and the baffle at the screw for that area. I have found that, unbelievably, that single washer can help cure a high #3 CHT problem. Thanks to VAF, I found that helped a lot. My expertise is not in engines but just wanted to make sure that the washer in question is in place.

I have the same engine (O-360 A1A - carb) and my fuel flows are more along the lines of what Ernst stated. 16-17 GPH on climb out and more like 8.5 GPH cruise running 8500' 2500rpm LOP, 160kts. Cruise at 11 GPH sounds high. Cruising at 2700rpm I burn about 9.7 GPH and get 175kts.

Best of luck
 
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GLPalinkas said:
John, I see that you have read and re-read all of the posts concerning this issue but I just wanted to make sure that you have a washer between the number 3 cylinder and the baffle at the screw for that area.
Best of luck
Click to expand...

I do have a washer in place behind #3 on the baffle. I saw one thread where someone added 2 or 3 washers there so I was thinking about trying that to see if it helps.

Thanks for the suggestion, I appreciate you, and everyone taking the time to read this and brainstorm with me.
 
Glad to try and help. More experienced people than me will chime in.

How about cooling tubes going to the mags or other areas, maybe cabin heat supply drawing down the air being forced down thru the cylinder? Have you tried covering these temporarily to check the numbers.

ps 450 cht is really high. I know that Lycoming max cht is 500 but anything above 400 is weakening the cylinder (at least that's what I have heard)
 
Call Airflow Performance

It could be you need to tune your injectors. You didn't mention that you have done this.
If you call Airflow Performance, they will tell you how to graph the performance of your injection system and suggest changing/tuning the nozzle orifices so that they all peak at about the same fuel flow, which allows running LOP efficiently.
Could be your #3 cylinder injector is off. Maybe from the beginning or maybe due to some other reason.
Besides, they may have some other reason that you are seeing this anamoly on Cyl #3. They are the experts, after all.
Good luck.
 
jtrusso said:
I?ve checked all 4 CHT probes in a bath of hot oil and they all read within 1 degree of each other. I have the screw in type CHT probes and an Aerospace Logics digital CHT and EGT gauge.


I used to think the #3 cylinder may be running lean, but if that were the case I would have expected the EGT to peak long before the other cylinders

This was not the case, #3 was the first to peak and the CHT began to drop almost immidiately and did so all the while the EGT was rising.

I don?t get it, why would the CHT drop on #3 as I leaned the mixture?
Click to expand...

If I were fighting a problem like this, I would try to swap the CHT probe and lead on the #1 and #3 cylinders. Don't change them at the gauge. If the hot CHT changes to #1 on the gauge, you have most likely you have an engine issue and not a gauge/lead/probe issue. Just swapping probes doesn't test the remainder of the system.

You "used to think" that #3 was running lean? It seems to me your own data shows it is the leanest cylinder as #3 EGT is peaking first.

As for why #3 CHT is dropping..... how fast are you leaning? It doesn't make sense to me either, but maybe if you are leaning too quick it's because of the lag in the probes/gauge.

I'm not an expert, but maybe this will give you a few new ideas. I suspect the #3 isn't getting enough fuel and is a bit too lean. Maybe this in combination with baffle issues.

What happens to the #3 CHT if you turn on the boost pump at a safe altitude?
 
Folks, it's O-360 Carbureted, no injectors to tune.

I have the same engine (O-360 A1A - carb) and my fuel flows are more along the lines of what Ernst stated. 16-17 GPH on climb out and more like 8.5 GPH cruise running 8500' 2500rpm LOP, 160kts. Cruise at 11 GPH sounds high. Cruising at 2700rpm I burn about 9.7 GPH and get 175kts.
Click to expand...

Not ruling anything out but I would put my money on a fuel flow problem,
specifically running excessively lean especially on take off.
 
N427EF said:
I had a 360 for several years and I never operated at 11 GPH in cruise,
more like 8 to 9 GPH.
The fact that Cylinder head temps drop after you lean tells me that you have arrived at a lean of peak setting at 9 GPH, this is normal for lean of peak operations. What is not normal is lean of peak at your indicated 9 GPH
on a O-360, that condition is reached more likely around 7GPH.
Click to expand...

Hello Ernst,
Did you notice Johns cruise power setting of 24" and 2500 RPM?
For my IO-360-M1B, that is 75% power. At that RPM, 9 gph is LOP by about .5 gph according to the Lyc charts that have been amazingly accurate in regards to peak fuel flows on my engine.

Admitedly, I do not have charts for carbed engines so they could be different.
 
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Did you notice Johns cruise power setting of 24" and 2500 RPM?
For my IO-360-M1B, that is 75% power. At that RPM, 9 gph is LOP by about .5 gph according to the Lyc charts that have been amazingly accurate in regards to peak fuel flows on my engine.
Click to expand...
I agree!
Lean of peak is quite a large range and you can reduce FF by another gallon from that point before the engine quits.
Cylinder head temps drop drastically once you get 20 to 30 degrees lean of peak. EGTs don't change that much after LOP.
If the OP noticed engine roughness around 9GPH and as he stated his #3cylinder dropping from 449 down to 380, tells me that cylinder is way past lean of peak and consequently most likely leaned to peak for take off.
The OP went from 11 GPH down to 9 GPH to notice roughness in the engine where everybody else goes from 9.5 GPH down to 7.5 GPH to see the same change.
Poor fuel distribution is not the same at all throttle settings as it has a lot to do with the position of the butterfly valve.
You cannot easily fix poor fuel distribution in a carbureted angine but you can easily fix a fuel flow problem.
 
John,
Do you have a heat muff on the number 3 exhaust? This along with the scat tubing may be reducing exit airflow beneath the number 3 cylinder. A long shot I know but I would be interested if this is the case.

Also, is your #1 CHT running appreciably cooler than #2 & #4? If it is you may need to fit a ramp in front of #1 to encourage the air towards the rear cylinder.
 
alpinelakespilot2000 said:
Lycoming%252520manual%252520graph%252520temps%252520vs%252520mixture.jpg
Click to expand...

I am no expert and following observation is purely based on the chart above.
When lean the mixture both EGT and CHT rise. However, CHT will drop before EGT peak. If due to the uneven distribution, at 9,000' (distribution may be different at lower level) #3 was at CHT peak (before EGT peak and still ROP) and all other cylinders are much richer then #3, it will behave what described in the original post. However, this does not explain why other three CHTs did not move. They are supposed to move up then drop. I do agree 450 is too high as suggested by others, it needs to be improved.
 
Another Possible Cause?????

.....Not to add to the overload of information, but this is something that we have seen several times in the past and routinely verify on our engines. If you have access to an oscilloscope this is something that is easily verified. You made no mention of the ignitions you are running and the problem I am referencing here is vary common. It has a tendency to manifest itself when you are approaching maximum timing numbers if search of performance. You need to check each ignition completely independent of the other as either one can cause this problem. Mags are far more prone to having this problem than electronic ignitions as mechanical variations in the manufacturing process or wear issues will contribute to end result. Verify that all cylinders are firing with exactly the same timing numbers and 180 degrees apart. If one plug is being fired too early (advanced) it will cause all of the symptoms being described in this thread. High cht on effected cyl, high fuel flow as engine is working against itself, and rapid temperature drop as fuel is removed. Just checking the timing by conventional methods will not reveal this issue. It requires viewing the engine as four separate engines with identical timing numbers and 90 degree spacing working together. Two or three excess degrees of ignition timing can be huge if the other three are at or close to the maximum. I hope I have explained this adequately and not added more confusion to the problem. We have seen this many times, as we are always attempting to squeak out every last horsepower from the high performance engines we build and race. Thanks, Allan...:D
 
Whoa hold on a sec!

Start with a CHT temp probe calibration before you do anything else. They can easily be off 75f or more even when new.
Just dip them in boiling water near sea level and you should see ~212F or close.
Second thing is find out if the EGT's are all peaking at the same fuel flow or hopefully within .1~.2 GPH of each other. We don't care what temp they peak at, only that they peak at the same flow. If not balancing them becomes your next step, assuming you are fuel injected...
Good luck
Tim
 
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Check that the plug on #3 (where a fuel injection or primer line would go) is in place and not leaking air. Or if there is a primer line there, check that it is not leaking air.
 
Thank you to everyone for the replies and tips on things to look at. I'm planning to fly Friday afternoon so I'll take a look under the cowl as well as try and get more data.

N427EF - I'm pretty sure the fuel flow gauge is accurate. It gives me gph numbers as well as total fuel burned. When filling up after flight the amount burned is within 1/4 of a gallon of what I need to fill the tanks. I'll double check the full throttle take off fuel flow and repor back.

DaAV8R and BobTurner - I'll double check the primer plug to make sure I'm not getting extra air into the cylinder. But if this cylinder was running so much leaner than the other 3 that it's 70 - 80 degrees hotter, wouldn't it peak long before the others? #3 peaks first, but #1 is right behind it. There isn't much spread in fuel flow from the first to last to peak.

RZBill - thanks for the info. I'll try and track down the Lycoming power charts, it would be a good reference to double check my fuel flows against.

Allan - Can you give me a little more info on how to perform the checks you describe. What pyhsically do I attach to the scope? I have two mags and the timing checks to 25 BTDC, but I would like to confirm this with the method you descirbe.

Sig600 - I don't have any inlet ramps on the upper cowl. #1, #2 and #4 are all pretty close to each other in temp, about a 20 degree spread with #4 the coolest, followed #1 then #2.
 
jtrusso said:
Thank you to everyone for the replies and tips on things to look at. I'm planning to fly Friday afternoon so I'll take a look under the cowl as well as try and get more data.

Sig600 - I don't have any inlet ramps on the upper cowl. #1, #2 and #4 are all pretty close to each other in temp, about a 20 degree spread with #4 the coolest, followed #1 then #2.
Click to expand...

Maybe we have a BINGO. Those upper cowl inlet ramps are very important for the distribution of the incoming air. Maybe not enough air is being forced down around #3. Maybe but maybe not. But nice catch Sig600. Hope that helps....
 
I forgot to mention...

One important thing I forgot to mention, at least I think it's important, is that I have run the engine with no cowling and #3 heats up much faster than the other 3 cylinders.

I had the plane down for a long time dealing with prop issues. When testing the prop without the cowling installed I would keep tabs on the CHTs and #3 was hotter than the other 3 by 50 - 75 degrees, reaching 400 in about 3 mins, at which point I'd shut it all down.

I think that this might point to an engine problem over an airflow/baffling problem, no? My thought being that by not having the upper or lower cowling installed I've taken the airflow out of the equation and all cylinders should be receiving plenty of cooling air.
 
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Induction leak on #3?

Perhaps an induction leak on #3 has you 'peaked' early so pulling the mix goes LOP and cools.
 
jtrusso said:
One important thing I forgot to mention, at least I think it's important, is that I have run the engine with no cowling and #3 heats up much faster than the other 3 cylinders.

I had the plane down for a long time dealing with prop issues. When testing the prop without the cowling installed I would keep tabs on the CHTs and #3 was hotter than the other 3 by 50 - 75 degrees, reaching 400 in about 3 mins, at which point I'd shut it all down.

I think that this might point to an engine problem over an airflow/baffling problem, no? My thought being that by not having the upper or lower cowling installed I've taken the airflow out of the equation and all cylinders should be receiving plenty of cooling air.
Click to expand...

Not true, without the cowling in place there is nothing to make the air flow down and around the cylinders. Hence the caution for keeping ground runs short, especially with the cowling off.
You would be amazed at what tiny leaks in the baffles do to cooling. I saw a 30 degree decrease just sealing up even the smallest leaks in my baffles which I had taken great care building them as tight as possible.
 
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jtrusso said:
Allan - Can you give me a little more info on how to perform the checks you describe. What pyhsically do I attach to the scope? I have two mags and the timing checks to 25 BTDC, but I would like to confirm this with the method you descirbe.
Click to expand...
...The best is to use an engine scope with an induction pickup that can view the coil discharge spikes and their spacing in degrees. Or, a timing light with an induction pickup you can move from high tension wire to wire. Mark off the flywheel at top dead center on each cylinder and compare the running timing on each wire of each cylinder. If either plug on number 3 is firing advanced beyond the cooler cylinders then you may have found the culprit. Thanks, Allan... :D
 
jtrusso said:
One important thing I forgot to mention, at least I think it's important, is that I have run the engine with no cowling and #3 heats up much faster than the other 3 cylinders.

I had the plane down for a long time dealing with prop issues. When testing the prop without the cowling installed I would keep tabs on the CHTs and #3 was hotter than the other 3 by 50 - 75 degrees, reaching 400 in about 3 mins, at which point I'd shut it all down.

I think that this might point to an engine problem over an airflow/baffling problem, no? My thought being that by not having the upper or lower cowling installed I've taken the airflow out of the equation and all cylinders should be receiving plenty of cooling air.
Click to expand...

John, this is indeed interesting. Is there perhaps a junction somewhere in the cht wiring for #3? A junction or improper splice in the wires, near something hot, could throw the indications off. Hard to believe that it can reach 400 in 3 minutes. I'm leaning towards thinking this is an indication problem. Another poster mentioned switching 3 and 1 probes around, all else unchanged. If the cables will reach, that is a good idea.

I'll second another poster's suggestion - establish level cruise at some power settings, but well rich. Gradually lean (by .1 or .2 gph at a time) the mixture, allow to stabilize a minute or more, and record egt and cht's for each cylinder. This data will then be used to generate two curve sets for each cylinder: cht vs mixture and egt vs mixture for all four cylinders. Curves can be generated like those here.

One other baffle thing to measure if you haven't is the gap below each cylinder, where the baffles wrap under the cylinder fins.
 
Inductions leaks are very hard to find and they are never obvious.
One way to do this is run the engine at low RPMS (low manifold pressure) without cowling and have someone spray
ether around the #3 induction tube where possible leaks could occur.
An immediate rise in RPM would be noticed when ether gets sucked into the induction tube.
Not sure this is an "approved" method but a sure way to find out if you have an induction leak.
 
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How about an engine monitor data file?

This would be far easier to help diagnose the problem with.

It sound very much like an induction leak.

Download the instructions for doing a GAMI lean test at high level and then do one at low level, compare the results.
 
An interesting thread -

If I had this going on, some things I might look into would be:

-- pull the cowl, run the engine for about two minutes, and use one of those point & shoot temperature gauges to verify that the cylinder was, indeed, hotter. (You might also be able to take measurements at several points - for example, the cylinder side walls versus different points on the head, etc. If this is happening with the cowl off - probably not a baffling issue.)

-- take a look with a borescope to see whether there was any evidence of damage in the cylinder (heat damage to the piston, scoring or something unusual on the cylinder wall, etc.)

-- do a "automotive" type compression test to verify not only the leakdown, but the breathing of the cylinder.

-- does this move or change if you rotate spark plugs (sorry if I missed it and you already have checked that).

Just some other things to consider.

Dan
 
I think this chart says why leaning lowers EGT (at some point) as posted before. Note lean under 75% power. Keep in mind each jug is separate. In theory FI is going to have better fuel distribution, but each jug is physically different with different compression.

The worst place to be is 50F rich of PEAK to about 100F rich of peak. You might get 3 jugs go be -20F lean of PEAK... but one may be at +30F. The question is that bad? Well if your CHT's are over 400F (an arbitrary number but cones from old Lycoming notes about longer life). The red line is ridiculous. 420-430F transient is OK but not continuous, opinion only.

I typically see 380F in hot conditions. However really hot day, heavy, climb I might level off periodically to cool... As far as rich, yes I would be on the RICH side. However over 150F Rich of Peak EGT is a waste of fuel and you lose power, but fuel does cool...

tJAao.png
 
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