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O-360 #2 and #4 high CHT

LeMar

Active Member
Backround:
O-360A1A, 1,000 Hrs since new, RV-7, 1,000 Hrs
Never had high CHTs until about 50 hours ago when #2 and #4 became higher on climb out. Now, at 120 kts cht on those two cylinders climbs to 420 fast and would go higher if I didn't back off power.
Ignition: EFII on bottom plugs, slick mag on top

So far I've changed intake couplings and gaskets, checked the carb for looseness, changed mag and harness. Nothing has had any real effect. Baffle seals aren't perfect but are pretty good and I can't see much change from several years ago. Checked #2 cht probe in hot oil and it tracks with a digital cooking thermometer.

Something has changed but I don't know what. I've read through high CHT posts here on the forums but haven't seen any describing symptoms like this.

Ideas?

Thanks,

LeMar
 
Primer

You don't mention a primer system?
If you have one, check that too.
Also check that the primer ports and injector ports are there and not leaking too.
 
High CHT on 2&4

I would check one of the double duty coil packs on the ignition system. One dead coil pack would leave two cyl's only getting fire from the mag and the cht's on them would go up.
 
I would check one of the double duty coil packs on the ignition system. One dead coil pack would leave two cyl's only getting fire from the mag and the cht's on them would go up.

That would cause cylinder temps to drop, not rise, and then only if both ignitions stopped firing on those two cylinders.

Check you baffles, you might have a crack that is letting the cooling air out. Look around the corner of #4.
 
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Maybe john is on to something..

I know the other guy mentioned chts... but maybe you should look at the exhaust temps.
Exhaust gas temps do go up when you run on only one mag or one side of a dual ignition system. You haven't mentioned your exhaust gas temps.
Did you change your aviation style plugs to new ones or at least confirm the gap? Don't forget to check the resistance too. 1k to 5k is acceptable.

I would also stick a light in the cowling at night and try to observe any egregious air leaks. That side of the cowling gets the least amount of air in it during a climb on a normal day... but 120kt should overcome even the worse leaks.

If the carb got some junk on one side it might alter the performance of the jet negatively.

I hope you get it worked out. Please post what the final resolution is.
 
Is your oil cooler mounted on the back of #4 and did you remove a winterization plate about 50 hrs ago? (sorry to state the obvious).
 
Some good ideas here. I think the next thing to check is CHT vs EGT as I don't remember what the EGTs were doing. Nothing that would have alarmed the system or me but still, maybe something is happening.
The other thought is to remove the primer lines from #2 and #4 and plug the holes. The primer system is the solenoid type and I never actually use it but rather use the accelerator pump for starting. Perhaps there is a leak on that side of the engine allowing air to be sucked in. I have checked for fuel leaks on the primer system but maybe air will leak in where fuel won't leak out. I'll post my findings.

LeMar
 
Primer

Some good ideas here. I think the next thing to check is CHT vs EGT as I don't remember what the EGTs were doing. Nothing that would have alarmed the system or me but still, maybe something is happening.
The other thought is to remove the primer lines from #2 and #4 and plug the holes. The primer system is the solenoid type and I never actually use it but rather use the accelerator pump for starting. Perhaps there is a leak on that side of the engine allowing air to be sucked in. I have checked for fuel leaks on the primer system but maybe air will leak in where fuel won't leak out. I'll post my findings.

LeMar

The solenoid has a "in" and "out"....marked with a 1 and 2....(not sure of the order), if it is reversed it can leak fuel though. The solenoid can go bad as well. The install manual says it can be mounted in any direction but recommended vertical, I think. All things to consider.
The above conditions usually contribute to rich running or stumbling at idle, not lean or higher CHT's....that most likely is a leak in the induction or primer line systems.
Bypassing for troubleshooting seems like a good idea.
 
That would cause cylinder temps to drop, not rise, and then only if both ignitions stopped firing on those two cylinders.

Check you baffles, you might have a crack that is letting the cooling air out. Look around the corner of #4.

Actually losing one sparkplug/ignition source in a cylinder will retard the "effective timing" (the ultimate effect of timing is the occurrence of peak pressure relative to crank rotation and in a dual spark setup it is based on a combination of both spark events) in that cylinder. This should result in decreased CHTs and increased EGTs.

Larry
 
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Is your oil cooler mounted on the back of #4 and did you remove a winterization plate about 50 hrs ago? (sorry to state the obvious).

I had always assumed that the air flow past the engine was based on a pressure difference and not necessarily air flow. Given that the chamber is common, I would expect any leak would equally affect the cylinders. I am hoping an expert can chime in here, as I am struggling with a warmer #4 and would really like to understand if my assumptions were correct or incorrect. I suppose the effectiveness of the dams on #1 and #2 at least partially disproves my thinking.
 
Actually losing one sparkplug/ignition source in a cylinder will retard the "effective timing" (the ultimate effect of timing is the occurrence of peak pressure relative to crank rotation and in a dual spark setup it is based on a combination of both spark events) in that cylinder. This should result in decreased CHTs and increased EGTs.

Larry

I may be a novice but by having a failed ignition timing does effectively retard the timing, as you mentioned; however, retarded timing typically reduces CHT's and increases EGT. Now, if it could advance the timing, causing more pressure in the cylinder, you will see CHT's go up and EGT's go down.
 
I had always assumed that the air flow past the engine was based on a pressure difference and not necessarily air flow. Given that the chamber is common, I would expect any leak would equally affect the cylinders. I am hoping an expert can chime in here, as I am struggling with a warmer #4 and would really like to understand if my assumptions were correct or incorrect. I suppose the effectiveness of the dams on #1 and #2 at least partially disproves my thinking.

This partially makes sense as you mention. I would agree about the common chamber but when people can improve cooling on #3 by adding a spacer or air dam behind #3 also proves that improving airflow in one particular area of the chamber can help. Likewise perhaps improving airflow too much at another area, in the wrong place can also hurt CHT’s.

For example, let’s say instead of adding airflow behind #3 you add airflow behind #4, and so much airflow that instead of going through the cooling fins of #4 its going out through the oil cooler, or a leak in the baffle?? Does path of least resistant rob cooling air from a nearby cylinder?

If so, and your oil temps are low, you can try and close off the backside of the oil filter. Less air through the oil filter should yield more air through the cylinders.

Baffles on the front of #1 and #2 cylinders seem to do two things, 1) raise the temp on the front cylinders 2) deflect more air to the rear cylinders. You should be able to toy with the front baffles enough to level the temps between 1 and 3 equally and 2 and 4 equally. But, if you have FI, there is no reason you shouldn’t be able to level the temps between all 4 cylinders by flow matching the injector orifaces. Is it possible #2 and #4 are currently blocked?

Lastly, a good set of mcfarlane cowl saver baffle material is far superior to the standard engine baffle rubber and if done right will provide a much better seal to the top cowl. http://www.vansairforce.com/community/showthread.php?t=153556&highlight=Baffled
 
I flew the airplane yesterday (air temp was in the 50's) and the problem was not as severe. However, #2 and #4 were still the highest and in a 5,000 foot climb did reach 410. Until this issue started about 50 hours ago the CHTs never went over 390 even in mid-summer. Yesterday I checked the EGTs during that climb and they all stayed about the same, around 1250 which has been normal at full power(carbureted).
I'll cover the oil cooler and see if that makes a change. It's mounted behind #4.
My earlier thought about a primer system leak would seem to not make sense as the EGTs are normal so perhaps it is an air flow issue although I don't know what has changed to cause it.
 
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