High CHT on Cessna 172

FWIW this O-320 I'm flying has gone from the worst engine I've flown behind to one of the best, and here's what was wrong:

#1. Had the WRONG carburetor in it. I'm not kidding. Sure it was an MA4-SPA but setting #10-3678-32 and not setting #10-5217 as recommended by Lycoming Service Instruction. A quick call to a carburetor shop reviled the 10-3678-32 runs MUCH MUCH richer (by 2 – 3 gallons GPH) than the 10-5217, the plug fouling problems confirmed that. I could find no long entry to suggest that carburetor was ever replaced with that 10-3678-32, so I assume it was factory installed.

With all the possible cooling issues in mind we ordered a rebuilt 10-5217 and requested it be set it to the highest fuel flow for that setting # tolerance.

#2. The Induction Leaks - The pipes that come out of the sump are swaged in meaning that they can vibrate loose and then start an induction leak. On a 1400 TTSN engine we found the forward two pipes loose in the sump casting. All the induction gaskets and rubber hoses were replaced at this time.

#3.One baffle spring broken

#4. Time the mags to the engine with a piston stop and the electronic protractor. Don’t use the timing marks on the ring gear and don’t use the wheel with weighted pointer degree wheel type timing disks.
#5. Look in the cowling and see where the silicone baffle seals lay. They need to lie toward the high pressure above the engine not away.
 
I have to admit that I don't know what carb is installed.
The problem has existed (or I first noticed it) a few months after I bought it in April 2009. I bought the plane locally and the same A&P that I use maintained it previously. He said the problem was new, but nothing that I am aware of had changed. Nothing was found on the pre-buy or log book review. I suspect the problem has always been there though. Or at least, ever since the O360 upgrade was performed.

All the mechanics that I have had look at it have pretty much given up and said that all 172s are probably running the same but so few have engine monitors that nobody notices it. I suppose that used to be true but more and more people are installing monitors on these little engines.

Uh, it just dawned on me that I am hijacking someone else's thread, so I am going to stop here but I don't want to delete what I have already written. In closing, earlier this week, I had my A&P add some new seals to cover places where I saw small gaps at the flange where the top cowling meets the lower cowling. There were gaps you could stick your finger through. He also said he found and repaired a couple of very small (and old) intake gasket leaks in #3 and #4. I don't know why nobody found them before or if they will make any difference. I will see what happens if the weather lets me take it up this weekend and report back.
 
Taking to account all of the above suggestions, and flying behind an O-360 conversion in my '79/172: I've had many discussions with my A/P regarding this issue. I too see low to mid 400's on the #3 in a climb out on a warm day, then everything calms down into mid 300's in cruise. His point was that as long as the CHT's return to cruise range, no harm no foul. If you fly a 180 hp in a conventional 172 installation, you're producing a lot more heat burning those 20 or 30 more horses than what was originally designed for, even with baffles modified for the 180.

Note where your oil cooler or the oil cooler scat tube inlet is located in your particular installation. If it is mounted in a baffle cutout behind and above the #3 cylinder, then a good portion of the airflow toward that cylinder tends to move in a path above the cylinder and through the oil cooler (which is a good thing), rather than up against a solid baffle wall then down and out of the lower cowling. This tends to make the #3 run warmer since not all of the airflow moves through that cylinders cooling fins and then out the bottom of the cowl. The rear cylinders tend to run warmer anyway, since the ambient cooling air is warmed by the front two jugs.
 
Large gaps between the oil cooler duct and the oil cooler? That's easily missed and causes cooling air losses.
 
I have a 172k 1969 with O360 conversion and never see over 365 CHTs - even climbing out of redding, CA at 98 F. If I was running 400s I would be looking a lot harder.
 
JOhnH said:
And that is exactly why I have spent thousands of dollars over the past few years trying to find out. I have had two A&Ps and two IAs look at it and it still runs hotter than other similarly configured 172Ns
Click to expand...

I am in the same boat with my Cherokee 140 with O-320. In the latest round, I had heard that Power Flow Exhaust brings the CHTs down, so I called them. They said that, yes it does bring them down an average of 15 degrees, but I have a problem that needs to be solved first. They sent me a PDF on a running lean test and had me run it. Their guy took a look at the results (Darren, owner?). Darren said that without a doubt I am running lean. There are a few things that can cause this and I have been triple and quadruple checking all of them. I ended up sending my carb in for a rebuild (even though I only had this one installed in December). It sounds like QA on these carbs is pretty poor (earlier in the fall I had three carbs come back from Kelly bad. The first two wouldn't even go over 1,500 RPM). I had the last -32 rebuilt by another shop and then switched it with a rebuilt 10-5817. Now the 5817 is at D&G Supply for rebuild (shop recommended by Darren). I will post the results when the carb comes back. PM me if you want the PDF or call Power Flow (They are great).


Sent from my iPad using Tapatalk HD
 
Avstar is the company we got our carb from. It's been flawless for 25 hours
 
D&G has had a solid rep. i have yet to hear anything good about kelly.
 
patrick wentworth said:
I have a 172k 1969 with O360 conversion and never see over 365 CHTs - even climbing out of redding, CA at 98 F. If I was running 400s I would be looking a lot harder.
Click to expand...

That makes me wonder what kind of CHT indication system you have, because that sounds too good to be true.
 
That model of 172 has the oil cooler hose outlet in the rear baffle right above the #3 cylinder. Air rushing into that can cause a pressure drop right over the cylinder, reducing the differential between top and bottom at that spot.

Just maybe.

Might cover that hole and run it up on the ground (cowling on) for a few minutes and see if #3's CHT comes closer to the rest of them.

Dan
 
Dan Thomas said:
That model of 172 has the oil cooler hose outlet in the rear baffle right above the #3 cylinder. Air rushing into that can cause a pressure drop right over the cylinder, reducing the differential between top and bottom at that spot.

Just maybe.

Might cover that hole and run it up on the ground (cowling on) for a few minutes and see if #3's CHT comes closer to the rest of them.

Dan
Click to expand...

Also check to see if there are large gaps on the sides of the cooler duct

 
Hi guys,

Sorry for not updating before. I installed the exahust fairing and put new silicone baffles. We also discovered that one of the intercilinder baffles was cracked.

New temps:
Climb out:
1: 370
2: 385
3: 425
4: 390

Cruise
1: 345
2: 370
3: 400
4: 380

Not optimal at all:mad2::mad2::mad2::mad2::mad2:
A least in cyl # 3
 
Cmercado said:
Hi guys,

Sorry for not updating before. I installed the exahust fairing and put new silicone baffles. We also discovered that one of the intercilinder baffles was cracked.

New temps:
Climb out:
1: 370
2: 385
3: 425
4: 390

Cruise
1: 345
2: 370
3: 400
4: 380

Not optimal at all:mad2::mad2::mad2::mad2::mad2:
A least in cyl # 3
Click to expand...

What was the outside air temp ?
 
Cmercado said:
90 degrees
Click to expand...


IAS during the climb?

Thats about what I see at 70-80 mph IAS climb at that temp with higher humidity, so when its above 90 degrees, I climb 90-95 and it keeps the hottest cylinder (#3) around 405 {EI bayonet type CHT probes (not spark plug gasket type)}
 
Last edited:
bnt83 said:
IAS during the climb?

Thats about what I see at 70-80 mph IAS climb at that temp with higher humidity, so when its above 90 degrees, I climb 90-95 and it keeps the hottest cylinder (#3) around 405 {EI bayonet type CHT probes (not spark plug gasket type)}
Click to expand...

After take off and obstacle clearance I climb at 100 mph to try to cool the jugs..
 
JOhnH said:
He also said he found and repaired a couple of very small (and old) intake gasket leaks in #3 and #4. I don't know why nobody found them before or if they will make any difference.
Click to expand...


Now that is a rarely noted problem, but it can happen. I once found a series of them on the flight school's 172s, and thought at the time that the gaskets were defective. Then I noticed that we were hearing the exhaust BOOM more frequently, caused by students turning the mags to Off and On again during the runup. There' a bit of overlap in the intake and exhaust openings, and the backfire flame can set off the mixture in the induction system and rupture those gaskets. We addressed the inadvertent Off problem and the busted gaskets quit showing up.

A split gasket will lean a cylinder and make it run hotter. We never did notice rough idle, though. Strange.

Dan
 
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