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Why is #3 so hot on climb-out???

Doug Rohrer

Well Known Member
I have a major CHT problem with my #3 cylinder that has me "baffled" so I am asking for advice. Problem is getting worse with the hot weather here.

Two year old RV-9A with O-320-D2B 160HP rebuilt engine. Engine was built with all new Lycoming cylinders. Plane has about 140 hours total time. Dual Pmags timed at 2* ATDC. Garmin G3X panel. Catto fix pitch prop.

On climb out, #3 heats up fast and run as much as 100* hotter than the other cylinders. EGT is 150* hotter than the average of the other cylinders as well. I have to stay below 200-300 fpm climb to keep #3 below 425*. Oil temps are fine at 190*. Oil cooler is mounted on the firewall and gets air through a 4" duct behind #4. Even with all this air diverted from #4, it runs relatively cool. These is no air dam in front of #1 and there is a stock full height air dam in front of #2. Any air dam in front of #1 only makes the problem worse. Fuel flow is 13.5 GPH at full rich during climb.

Here is the kicker: once at cruising altitude, all the CHT's level out and #3 is slightly cooler than the other cylinders. If #3 had a vacuum leak, it seems it would be worse at cruise than during climb out.

My baffles fit well and all gaps and holes are sealed. The inlet ramps are sealed internally. I have tried the washer trick between the rear baffle and #3 cylinder, but it seems to make no difference. There are no fins near the screw anyway.

One other piece of info that may or may not be relevant: I have 78/80 compressions on #1,2 and 4, but only 70-72/80 on #3. I has been this way since new. Air leakage during the test seems to come out the crankcase, so it is getting past the rings. I don't see how this would affect my heating issue, but it is suspicious, no?

Has anyone tried to duct air transversely across the top of the engine? Would the ASA electric shutter system solve this problem?

Any suggestions would be appreciated.
 
#3 is a classic.

1- Do you have the 'washer trick' installed?
2- Check at the base of the #3 cylinder where it joins the aft baffle. Frequently overlooked is a tab that is supposed to be bent to seal a hole (around 1" square). If bent, is it RTV'd to be sure it seals?
3- look for flash in the cylinder head fins of #3. Compare to another cylinder so you know what should be open.
4- Is the deflector installed on the face of #1?
 
Scope the #3 and look for hot spots, anything sharp, threads of the plug extending into the cyl.
 
I see you already did the washer trick, so I won’t recommend that.. surprised that the air dam in front of #1 doesn’t help.. that really helped on mine. Check your intake gasket, I wonder if the gasket is broken and leaking, causing a lean condition..
 
others here mentioned to check your baffling and it's the first thing you should do.

if that doesn't help it could be an uneven mixture distribution thing. you climb full rich, right? maybe your #3 is in the vicinity of 50F ROP while all the others running full rich. a to small injector or debris restricting the fuel line to #3 could cause this. in that case #3 would run much hotter as mixture setting has a significant impact on CHT.

how do you cruise? if you cruise slightly ROP the cylinders would run hotter (increased airflow at higher speed counterbalances that) but #3 would be LOP already and cool down. that could explain #3 being slightly cooler in cruise. your engine could still run smooth in that condition.

if you have EGT probes installed i would (during cruise) sweep throug the entire mixture band and check what the EGT's are doing.

regarding the compression test... i don't believe a slightly lower compression could have such a great impact and 70/80 is still ok. in addition to that a lower compression leads to lower compression ratio which leads to lower CHT. at least that's my half know understanding.
 
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To follow up the last post I would flow test each cylinder. Quick and easy to do via the bottle method. 13.5 GPH seems a bit low. EGT is normally a trend item but in this case I think it’s telling you something.
G
 
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Center baffle

I had a hot #3 and did this center baffle trick. Made a huge difference.
 

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One other piece of info that may or may not be relevant: I have 78/80 compressions on #1,2 and 4, but only 70-72/80 on #3. I has been this way since new. Air leakage during the test seems to come out the crankcase, so it is getting past the rings. I don't see how this would affect my heating issue, but it is suspicious, no?
?

Any suggestions would be appreciated.

low compression should reduce CHTs, not increase them relative to cyl's with higher compression.

While #3 can be a challenge to cool without some mod's, it is not typical to see 100+ spreads between 3 and the others without mods. Poor cooling flow also shouldn't cause that cyl to increase in temp at a rate significantly greater than the others. It also wouldn't cause the EGTs to rise above normal. Finally, if it were poor airflow, you would see #3 a good bit higher in cruise also. The higher EGT has me thinking something is going on in that cylinder. Is the #3 EGT also 150 higher than the others in cruise or does it drop down to a different delta? If so, what is that delta.

You are correct that at WOT, any intake leaks should be minimal in terms of mixture deviations and would be VERY noticeable at idle and other low MAP situations. Advanced timing will raise CHTs and drop EGTs, though rising combustion temps raise absolute EGT even though relative EGT is dropping, so worth considering if it is possible for only one cyl to advance - probably not, especially given that it all comes back in line at cruise.

If you cannot find any airflow problems, you might consider looking at sources of mild detonation or pre-ignition. That raises CHT very fast, but if it is only mild, they won't run out of control. As mentioned above, hot spots in the cyl can create pre-ignition, which typically is destructive. Possible yours is mild. Also, pre-ignition can, depending upon the location of the source, have symptoms similar to excess timing advance. Basically, the hot spot is igniting the charge earlier than the plug (more advance), but not early enough to be causing the damage often seen with pre ignition. This would present as too much advance - your symptoms before cruise. Lean mixtures are harder to light, therefore possible that the pre ignition stops once you get to cruise and lean the mixture. Leaner mixtures also burn cooler and therefore the hot spot could become benign once the temps come down. Hot spots tend to respond to combustion temp and not head temp. Some time with a borescope in #3 is probably time well spent.

Do you have an air dam in front of #1?

Larry
 
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It is seemingly a strangely acting measurement. Do you trust the instrumentation?

I'd strongly consider applying Occam's razor here. Consider swapping two TCs and see if the output characteristic follows the probe or changes on number three. If that can't be done, just get a new one for #3 and verify the lead integrity/clean, secure junctions/clear of heat sources while you're at it. They're only making millivolts so they can be sensitive.

Could save you a whole lot of time consuming and unnecessary work. Been there, done that.
 
I was told when I installed my sensors to be exact in how far from the flange to place them. If they are, I would try what has been suggested, swap sensors and see what happens.
 
#3 cylinder heat

I’ve had a similar issue in my 9A, only it was the #1 EGT running wild at climb out, then settling back to normal temp at cruise. Talked to Mike Busch, who suggested inspecting ignition lead to that cylinder. He felt fuel was not igniting at proper time, causing higher temp at full throttle. Waiting for my A&P to conduct the wiring check to confirm, but it sounds reasonable. You may want to do a similar wiring check.
Best of luck.:)
 
Is that the recommended timing?

Have you checked and compared plugs?
(I was surprised years ago when I watched my aviation plugs being tested under "pressure".)

If any deviation in the plugs, new one(s) are only $2 each and a few minutes to eliminate as a problem.

Just another set of things to consider.
 
I’ve had a similar issue in my 9A, only it was the #1 EGT running wild at climb out, then settling back to normal temp at cruise. Talked to Mike Busch, who suggested inspecting ignition lead to that cylinder. He felt fuel was not igniting at proper time, causing higher temp at full throttle. Waiting for my A&P to conduct the wiring check to confirm, but it sounds reasonable. You may want to do a similar wiring check.
Best of luck.:)

Excess ignition advance will cause CHTs to rise and EGTs to fall. If #1 CHTs are similar and EGT is climbing 150-200* above normal, I would be looking at retarded timing, not advanced. When one plug stops firing, effective timing is retarded and EGTs rise while CHTs fall a modest amount. It is possible that the plug could not fire during the high MAP involved in the climb and start firing at the lower MAP cruise. High MAP environments make it more difficult for the plug to light the mixture, showcasing any weakness in the ignition system. A classic problem back in the days of distributors and single coils was rough running only at WOT or heavy acceleration - classic failing coil.

You can observe this yourself in flight. Just go to WOT and drop one mag and watch the EGT/CHT gauges. Runups do not produce high MAPs, so won't show the problem.

Larry
 
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Thanks to all who responded. I will try to answer the questions and comments below.

The washer trick did not work because the fins on my Lycoming cylinders are deep and go all the way down with no casting flash. Angle valve engines must be different.

The baffle tab behind #3 is bent properly and sealed.

Installing an air dam in front of #1 only increases #1 CHTs but does not help #3.

I used copper intake gaskets that are more robust than stock gaskets. Besides, any intake leak would show up during cruise with higher vacuum, and it does not.

I typically lean to 50* ROP during cruise. #3 cools down to match the other cylinders in cruise.

I tried swapping TC's and confirmed the problem is the cylinder, not the TC.

I installed new spark plugs 15 hours ago. Made no difference.

Timing was originally set at TDC per Pmag instructions. I retarded it a couple of degrees in a feeble attempt to reduce CHT's. There is no way a Pmag can fire a single plug early or late once it is locked down. All plug wires are virtually new.

I never heard of a cylinder flow test. How is this done?

I am intrigued with the idea of a baffle running down the spine of the engine. I am thinking about trying this. I have also ordered a borescope since I am a toolhound. I will also try to do a more comprehensive CHT vs. fuel flow sweep the next time I fly.

Thanks again for all the suggestions. I will report back when I have new info. I know others have similar issues.
 
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