carbed CHT spread over time

[GeorgeC]

Airplane owners who install EI of JPI engine monitors get alarmed at the modest differences in EGT and CHT between cylinders.

@mondtster @Dan Thomas

As threatened, I collected some data. It's not particularly well controlled for environmental or operating conditions, but it's what I have :-/

Dec 2019: 282/308/290/289
Mar 2021: 290/320/244/265
Oct 2022: 303/352/319/319

It's the #2 CHT that seems to be pulling away from the rest, and today was a modest day where the delta was only 50F; sometimes it's closer to 70 or 80.

So, the question is, what's a normal CHT spread for a carbed (O-200 in this case) engine, and what's big enough to start looking at baffling/induction?

 

[RyanB]

Cylinder #2 is going to generally be the hottest, given its position, so the numbers don’t really surprise me there. That said, a 70F-80F difference in CHT would be something worth investigating. I would at least want to know if it’s normal, rather than an indication of a problem, especially since it appears to be creeping higher. A 50F delta wouldn’t be alarming for me.

 

[Dan Thomas]

If the numbers for a cylinder are creeping like that, both the baffling and induction should be checked to see that they're installed and working as designed. On an O-200, the #2 is the aft cylinder on the pilot's side, and I have frequently found the baffling around the aft side not wrapped and retained against the cylinder like it should be. There are supposed to be wires with springs, or at least some lockwire, between the bottom edges of the front and rear baffles, running under the cylinders, so keep the baffles in contact with the fins so that cooling air has to follow the fins around to the bottom so that it collects as much heat as possible.

I have also found intercylinder baffles loose or missing.

The induction hoses and their clamps need to be in place and secure. The gaskets for the induction elbows at the heads need to be in place. They can get blown out by a backfire, letting a cylinder run lean and hot.

All of this stuff is annual-inspection stuff. All of it. Look at a small part of the inspection checklist for a 150:



Every 50 hours is Cessna's idea of an inspection of that stuff. But FAR 43 Appendix D, the very brief list of the annual stuff to check to meet legal minima, looks like this for the engine section:



Baffles aren't even mentioned. They would be included under (10) All systems, but it seems that too many mechanics just give things a really cursory glance, based on the stuff I've found.

Baffle fit is important. This picture gives an idea, but it mistakenly shows the baffles a little way from the cylinder fins. They're supposed to be right up against the fins.

 

[mondtster]

With a delta that large I’d be doing some investigation to see if I could find something wrong. Everything may be fine, but the time spent would be worthwhile. Out of curiosity, were those temperature measurements made with the throttle wide open (high manifold pressure) or at reduced power? Partial power may magnify a vacuum leak if one exists. As Dan mentioned, the baffling needs to be tight. I oftentimes find baffling that needs help when I’m doing an inspection so if it has been a while since any work has been done on them there might be an opportunity to even things out more.

 

[GeorgeC]

Out of curiosity, were those temperature measurements made with the throttle wide open (high manifold pressure) or at reduced power?

All were in steady state cruise, no more than 25" MAP. FWIW I do ensure that it never goes above 380F. I'll take a close look at the baffling next time I'm out there.

 

[Dan Thomas]

All were in steady state cruise, no more than 25" MAP. FWIW I do ensure that it never goes above 380F. I'll take a close look at the baffling next time I'm out there.

From the 1970-77 150 parts catalog:





In the later serial numbers, #52 was added. That's the interhead baffle. #33 (#26 in the first diagram) is the intercylinder baffle. Both sides of the engine have it. The wire loops that retain the rear baffle against the cylinders are visible in both pictures.

If you have the oil cooler, its airflow will cause a pressure drop right over #2 and that cylinder will run hotter.

 

[mondtster]

All were in steady state cruise, no more than 25" MAP. FWIW I do ensure that it never goes above 380F. I'll take a close look at the baffling next time I'm out there.

Id also look for induction leaks since you’re running at higher vacuum (low manifold pressure) settings. The clamps for one of the induction tube rubbers may have loosened up with time, allowing extra air in the intake without going through the carburetor.

 

[GeorgeC]

In the later serial numbers, #52 was added. That's the interhead baffle.

Thanks. Given that mine's a 1950, I doubt it's there, but I'll look.

If you have the oil cooler, its airflow will cause a pressure drop right over #2 and that cylinder will run hotter.

No oil cooler, my oil is cold enough as it is, maybe 180 in the summer :-/

 

[Dan Thomas]

Thanks. Given that mine's a 1950, I doubt it's there, but I'll look.

Look at the baffle seals and how they fit against the cowling, too. It doesn't take much of a leak to reduce the pressure differential between the upper side of the engine and the lower, which is what drives the cooling air. Even a missing rubber grommet can do it. Baffle seals get old and stiff and frayed. Just like me.

1950? Was this an airplane originally powered by a smaller Continental and upgraded to an O-200? Whoever did the conversion might have messed up the baffling in getting it to fit the slightly larger engine. Might be gaps everywhere. Might be missing some stuff.

 

[GeorgeC]

Yes, it's a 140A converted to O-200.

Sigh, I think I figured it out. See the hole to nowhere above the spark plug wires?
My old alternator had a blast tube aimed at it. New alternator doesn't need it. Guess that needs to be removed and covered up properly at next annual.

 

[mondtster]

That would do it. For the time being you could cover it up with aluminum tape and see if the CHT comes down. Assuming it does, I’d get it blocked off now instead of waiting for annual time.

 

[Dan Thomas]

Yup. Big leak there.

Looks like you have the interhead baffles. I drew an arrow pointing to the support bracket for it.



Now, what's the yellowish thing the other arrow is indicating? Is that under a big hole where more air escapes?

 

[Domenick]

Awesome stuff. I use it often. Peel off backing. Have a roll of this and fabric duct tape in the plane, truck, and house.
https://www.homedepot.com/p/Nashua-...Purpose-HVAC-Foil-Duct-Tape-1541239/100030120

 

[GeorgeC]

Now, what's the yellowish thing the other arrow is indicating? Is that under a big hole where more air escapes?

Not sure. Looked at it again today, all I could see is the top of the muffler. The inside of the cowling is painted cream, and the sun was right over my shoulder yesterday when I took that picture, so it might have been lighting something up in a weird way. Tried to take a better pic with the flash, not sure it will help:


In any case, after taping up the hole, #2 was still warmer than the rest today, but cooler overall at 293/335/253/273.

 

[Dan Thomas]

That baffle piece I circled looks like it's bent down. It might be supposed to be level, to close that gap. What does the other side look like?

 

[GeorgeC]

What does the other side look like?

Looks the same to me:

 

[GeorgeC]

Dec 2019: 282/308/290/289
Mar 2021: 290/320/244/265
Oct 2022: 303/352/319/319

After taping up the hole to nowhere,
Nov 2022: 302/338/266/275. That's at 3400', 54F OAT, 2400 rpm, 23" MP or so, leaned, a fairly typical configuration.

An A&P friend didn't see anything obviously wrong with the baffling. He did notice a couple of places where zipties had been used with little slices of rubber hose as standoffs, which is cute until the ziptie cuts through the rubber and the whole thing falls apart. Sigh. Need to rework them all to protect the mounts.

 

[Dan Thomas]

Need to rework them all to protect the mounts.

You need to get those zipties off the mount altogether. Grit gets under them, embeds in them, and vibration then eats that thinwall mount tubing. It's only .035" thick, and a 10% loss trashes it. That's .0035", the thickness of a sheet of paper. I have spent good money having eroded mounts rebuilt on the flight school airplanes, almost all of the damage being from zipties. At least get some electrical tape layered up under them.



Some metal gone off this one above already. Adel clamps cost more, for sure, but they're still a lot cheaper than a rebuilt mount.

 

[Lndwarrior]

Yes, it's a 140A converted to O-200.

Sigh, I think I figured it out. See the hole to nowhere above the spark plug wires?
My old alternator had a blast tube aimed at it. New alternator doesn't need it. Guess that needs to be removed and covered up properly at next annual.
View attachment 111853

Kind of amazing you went through 3 annuals and no mechanic noticed the huge hole in the high pressure plenum? If you used the same mechanic I would be looking for a new one.

 

[GeorgeC]

At least get some electrical tape layered up under them.

That's what my A&P friend said, but won't the zipties go through the tape as well? If we're doing poor man's Adel clamps, why not throw some 1/2" (or whatever) ID hose between the mount and the ziptie?

 

[Dan Thomas]

That's what my A&P friend said, but won't the zipties go through the tape as well? If we're doing poor man's Adel clamps, why not throw some 1/2" (or whatever) ID hose between the mount and the ziptie?

Adel clamps, correctly sized, will grab the tubing firmly enough to prevent any movement. It's tough to get any ziptie tight enough to stop movement, even over a chunk of hose. Grit gets under hose, too, and can abrade the steel. The electrical tape is soft and resists abrasion, and the tie can embed into it somewhat and resist movement. You'd want three or four layers of it. It sticks to the tubing and prevents grit entry and movement.

 

[GeorgeC]

Dang. Turns out that was a 1.5" scat tube.

 

[GeorgeC]

I guess new baffle seal is in my future:

 

[Domenick]

Nice photo.
I agree.
The soft baffling should flex inward (like right-hand piece) so the positive pressure above the cylinders presses it against the cowling.

There is some wear to the underside of the cowling. Cover the wear points and soft baffling contact areas with metal foil tape.

I would also put a grommet or wool around those plug wires to protect from vibration against the sheet metal edge. That big hole and the one on the other side will also reduce airbox positive pressure. If that gap at the bottom goes all the way through, pressure is leaking there as well.

 

[Mikey52]

There are 2 inter cylinder baffles on 0 200's. I see the support for the outside baffle but not the baffle or spring itself.

In pictures I see #53, but no #52 or #54. We lost number 54 one time and subsequently the number 2 cylinder.

 

[nrpetersen]

Notice the orientation of baffles 52 and 33. They are not symmetrical.

 

[GeorgeC]

I guess new baffle seal is in my future

That helped the CHT spread a lot; 335 +/- 15 in the climb today with a OAT of 55.