High CHT on #4 during clilmb

[JOhnH]

I bought a 1977 Cessna 172N with a carburetted PenYan 180 upgrade in April. The engine had about 1180 hours and now has 1225. At first it was fine, although it used/uses a of oil (about 1 qt every 1.5 hours). I am learning that the more I top it off the more it uses to lubricate the belly. After I put about 25 hours on it, the EDM began flashing high CHT on #4 during climbs. This started about the time I had the oil changed, but I am not certain it didn't do it once prior. The first time it did it, I noticed CHT reached about 470 (yikes). Now, I watch the monitor extra closely during all climbs and #4 reaches 400 rather quickly and will keep going if I keep climbing. #1and #2 seem fine. #3 is ususally about 20 degrees higher during climb and #4 can range from 30 to 50 degrees or more higher than 1 and 2. During level flight running somewhat lean all is well. The only other thing I noticed is that after long taxis on hot days I tend to find a fouled plug at run-up, but that has been mostly on #2. I now lean on taxi and that has not happend again. My fuel usage tends to run about 8gph. Sometimes less, rarely more.

I will have my mechanic check it out but I was wondering if anyone could tell me what to expect or what to look at or change first. This is my first plane.
John

 

[Anthony]

Wow 470F is very high and not good to do. I am sure the mechs will chime in but on my plane high CHT's were due to poor baffling condition, but my Tiger is tightly cowled, much more so than your Cessna. You shouldn't be seeing that kind of CHT. Do you trust your engine monitor and probes? What kind is it. I have an EDM 700 and I trust it, but I have heard that improper probe placement etc can give false readings.

Once I got my baffles in shape the engine runs nice and cool now, even in summer climb outs. YMMV since its a Cessna.

 

[RotorAndWing]

I bought a 1977 Cessna 172N with a carburetted PenYan 180 upgrade in April. The engine had about 1180 hours and now has 1225. At first it was fine, although it used/uses a of oil (about 1 qt every 1.5 hours). I am learning that the more I top it off the more it uses to lubricate the belly. After I put about 25 hours on it, the EDM began flashing high CHT on #4 during climbs. This started about the time I had the oil changed, but I am not certain it didn't do it once prior. The first time it did it, I noticed CHT reached about 470 (yikes). Now, I watch the monitor extra closely during all climbs and #4 reaches 400 rather quickly and will keep going if I keep climbing. #1and #2 seem fine. #3 is ususally about 20 degrees higher during climb and #4 can range from 30 to 50 degrees or more higher than 1 and 2. During level flight running somewhat lean all is well. The only other thing I noticed is that after long taxis on hot days I tend to find a fouled plug at run-up, but that has been mostly on #2. I now lean on taxi and that has not happend again. My fuel usage tends to run about 8gph. Sometimes less, rarely more.

I will have my mechanic check it out but I was wondering if anyone could tell me what to expect or what to look at or change first. This is my first plane.
John

The reason for the "high oil usage" is that you are "topping" off the oil before each flight and most of that oil is going out the engine breather onto the belly. Lycomings (320's and 360's) typically like to run at about 6 quarts. Pour any more into them and out the breather it goes. When doing the oil change only put 7 in, not 8.

On the other problem, check the baffling and also the cht probe.

 

[JOhnH]

Wow 470F is very high and not good to do. I am sure the mechs will chime in but on my plane high CHT's were due to poor baffling condition, but my Tiger is tightly cowled, much more so than your Cessna. You shouldn't be seeing that kind of CHT. Do you trust your engine monitor and probes? What kind is it. I have an EDM 700 and I trust it, but I have heard that improper probe placement etc can give false readings.

Once I got my baffles in shape the engine runs nice and cool now, even in summer climb outs. YMMV since its a Cessna.

I too have an EDM700 and it seems reliable and consistent. I spoke to my mechanic and he also said to check my baffles. I am a little embarrassed that I had to ask how I do that, and he just said use a flashlight. I am mechanically inclined and have done a lot of work on cars in the distant past, but I am new to airplanes, and at this stage of my life, I am happy to leave that work to the pros. But I am still not sure exactly what to check. He said I wouldn't need to pull the cowling, but there is not much to see otherwise. There is something that looks like an air intake up front and down low and it seems to be secured by some sort of tar paper looking sealant and this sealant has cracks and gaps that air could get around. Could this be the problem? There is another opening low center under the prop that has what looks like a foam air filter that seems to be oil soaked. Is that the way it is supposed to be? Going from memory, the foam is about 5in x 6in. Sorry if I sound like a stupid air plane owning dufus. This is really my wife's airplane since she is the family pilot, due to medical reasons.

 

[Anthony]

I am not familiar with Cessnas as I have only owned Pipers and now a Grumman. I believe my Cherokee's air filter was soaked in oil then drained out to leave a small residue film.

Baffling can "Look" good to me or you but IMHO it takes a mechanic familiar with your engine compartment to really get the baffling in tip top shape. Mine looked OK to me, but was really not doing its job until I had it redone. Now all my CHT's are below 400F even in climb in mid summer.

 

[Dan Thomas]

I too have an EDM700 and it seems reliable and consistent. I spoke to my mechanic and he also said to check my baffles. I am a little embarrassed that I had to ask how I do that, and he just said use a flashlight. I am mechanically inclined and have done a lot of work on cars in the distant past, but I am new to airplanes, and at this stage of my life, I am happy to leave that work to the pros. But I am still not sure exactly what to check. He said I wouldn't need to pull the cowling, but there is not much to see otherwise. There is something that looks like an air intake up front and down low and it seems to be secured by some sort of tar paper looking sealant and this sealant has cracks and gaps that air could get around. Could this be the problem? There is another opening low center under the prop that has what looks like a foam air filter that seems to be oil soaked. Is that the way it is supposed to be? Going from memory, the foam is about 5in x 6in. Sorry if I sound like a stupid air plane owning dufus. This is really my wife's airplane since she is the family pilot, due to medical reasons.

The oily foam thing is the air cleaner. Not related to CHTs. Is there a bunch of leaves or dead bird or bid's nest junk over the rear cylinders? #3 is the rear cylinder on the copilot's side, #4 the rear on the pilot's side. Any junk blowing in there will go to the back and block the airflow over the cooling fins. Even enough dead grasshoppers will do it.

Dan

 

[gismo]

I too have an EDM700 and it seems reliable and consistent. I spoke to my mechanic and he also said to check my baffles. I am a little embarrassed that I had to ask how I do that, and he just said use a flashlight. I am mechanically inclined and have done a lot of work on cars in the distant past, but I am new to airplanes, and at this stage of my life, I am happy to leave that work to the pros. But I am still not sure exactly what to check. He said I wouldn't need to pull the cowling, but there is not much to see otherwise. There is something that looks like an air intake up front and down low and it seems to be secured by some sort of tar paper looking sealant and this sealant has cracks and gaps that air could get around. Could this be the problem? There is another opening low center under the prop that has what looks like a foam air filter that seems to be oil soaked. Is that the way it is supposed to be? Going from memory, the foam is about 5in x 6in. Sorry if I sound like a stupid air plane owning dufus. This is really my wife's airplane since she is the family pilot, due to medical reasons.

The foam air filter looking thing is likely exactly that. Assuming for the moment that it's a Bracket (brand name) filter it's supposed to have a sticky oil coating although many airplanes use a paper filter which AFaIK shouldn't be oily.

Chances are that your mechanic meant you should put a light (a trouble light works better than a flashlight) on one side of the baffling and look for leaks by observing the other side. Any light you can see there indicates a passage that air can travel as well as photons unless (not likely) there are transparent substances involved. If you don't already know this, most aircraft engines are "pressure cooled" from top to bottom meaning that air is forced into the area above the crankcase and cylinder heads by ram pressure and that air then flows around/through the cylinder and head fins then exits out the lower rear of the cowling. This is far more effective if the high pressure air above the engine isn't allowed to pass through to below the engine without passing through the cooling fins and extracting some heat.

So your goal is to find all the leaks and plug them, keeping in mind that the pressure is higher above the engine than the pressure below and behind it so the soft baffle seal material needs to be oriented to "blow" closed by that pressure difference.

Other possible causes of high cylinder head temps are improper leaning and/or unbalanced mixtures between cylinders. For instance if your engine is injected, the injector for the hot cylinder may be partially blocked (assuming you're running ROP).

Finally, the JPI-700 has programmable alarms including one for CHT. I strongly recommend you set that alarm limit to somewhere between 380 and 400 F as 380 F is as hot as you should continuously operate the engine and 400 F is as hot as you should ever let the cylinders get briefly. Keep in mind that aluminum loses something like half it's strength at a little over 400 F and that some portions of your cylinders will generally be as much as 50 F hotter than the probe location.