[N941WR]

While on the way to Sun and Fun I noticed #3 cylinder?s CHT was running about 50 degrees higher than the other three.

When running on just the right ignition #3?s CHT came down in line with the other three. When running on the left ignition the #3?s CHT went even higher as did its EGT.

I have since replaced the left magneto, the spark plug fired by the left magneto and checked the plug wire for continuity. The left magneto was fine and timed properly, the spark plug looked good but was replaced anyway, and the plug wire checks out fine.

The baffling has not been changed since the Phase 1 period has been completed.

I?m running P-mags but I?m certain they are working fine. P-mag specific issues that I?ve tried: The timing jumper is in place and has been. The vacuum advance is plugged, which limits the timing advance to that of a standard mag. The timing is right on, as I said earlier. I really doubt it is the P-mag. I'm about to swap the P-mags left and right to see if the problem follows the P-mag bug since I've changed out the left one and the problem remained, as I said before, I don't think it is the P-mag.

On the ground the temps of all four cylinders come up together, it is only under load that there seems to be a problem. I now find myself flying by CHT temps rather than RPM. For instance, in yesterday?s flight I was running 64% power at 2500 RPM and the CHT?s were around 360 for 1, 2, & 4 and 400 for #3 while running 120 degrees rich of peak.

It has been suggested I do a compression check on this cylinder but it is my understanding bad valves will lower temps, not raise them.
Any thoughts?

BTW, the engine is an O-290-D2.

 

[Ironflight]

Bill,

Did you swap CHT sensors between cylinders, just to make sure you don't have flaky instrumentation?

 

[AlexPeterson]

Bill, when you say you are 120 ROP, on which cylinder do you base it?

There is a fair bit in the archives about a spacer between #3 and the aft baffle. If you don't have this spacer, you can expect a noticeable improvement in the cooling of that cylinder after adding it.

Regarding your observation on things balancing out with only one of the ignitions might not be too important. Combustion dynamics have a lot of effect on cylinder temps, and they are influenced by how the mixture is ignited. The cht mis-match you have (which really isn't that unusual at all), may just be indicative of mixture imbalances that are also typical.

If you have a fuel flow sensor, I would suggest getting the following data: Set up some throttle setting that you typically cruise at. Set the mixture where you feel it is very rich (a couple gallons rich of where you might normally cruise). Let things stabilize for a minute, and record the fuel flow, all four cht's and all four egt's. Lean about .1 to .2 gph, wait one minute and record the same data. Keep doing this until you are so lean the engine is running fairly rough. This data can then be plotted (let me know if you need help with this) to see how each cylinder's mixture varies with overall fuel flows. There is a lot of information on this topic in the archives.

Also, you can have a look at: http://www.n66ap.alexap.com/Mixture_article.htm

Good luck and keep us posted!

 

[N941WR]

Did you swap CHT sensors between cylinders, just to make sure you don't have flaky instrumentation?

I did swap #1 w/ #3 and the high CHT stayed with the cylinder and not the probe.

I suspect your mags are giving you trouble.

Very unlikely. As I said, I changed out the mag, replaced the plug, and tested the plug wire. Oh, and timing was spot on. The other data point regarding the P-mags is they fire two cylinders at the same time. One on the compression stroke and the other on the exhaust. Same as a car. If it was a timing issue then both #3 & #4 would have problem. However, you just gave me an idea. See below.

Bill, when you say you are 120 ROP, on which cylinder do you base it?

It is based on #2, the cylinder that goes rich first. #3 was around 90 ROP.

There is a fair bit in the archives about a spacer between #3 and the aft baffle. If you don't have this spacer, you can expect a noticeable improvement in the cooling of that cylinder after adding it.

The spacer is in there.

Regarding your observation on things balancing out with only one of the ignitions might not be too important. Combustion dynamics have a lot of effect on cylinder temps, and they are influenced by how the mixture is ignited. The cht mis-match you have (which really isn't that unusual at all), may just be indicative of mixture imbalances that are also typical.

If you have a fuel flow sensor, I would suggest getting the following data: Set up some throttle setting that you typically cruise at. Set the mixture where you feel it is very rich (a couple gallons rich of where you might normally cruise). Let things stabilize for a minute, and record the fuel flow, all four cht's and all four egt's. Lean about .1 to .2 gph, wait one minute and record the same data. Keep doing this until you are so lean the engine is running fairly rough. This data can then be plotted (let me know if you need help with this) to see how each cylinder's mixture varies with overall fuel flows. There is a lot of information on this topic in the archives.

Also, you can have a look at: http://www.n66ap.alexap.com/Mixture_article.htm

Good luck and keep us posted!

Good advice. Thanks I'll give that a try on Sunday. Saturday we have a builder's workshop I'm helping teach.

More Data Points
This evening (Thursday) I performed the following:
1) Verified the left mag switch controlled the left mag and the right switch controlled the right mag.
2) Verified the timing again. It was spot on.
3) Tested the plug lead in question with an ohm meter. It was fine.
4) Turned the prop over by hand while a friend and fellow builder listened to the engine through the filler tube for a week cylinder. No noticeable difference after 12 compressions cycles. I would still like to perform a compression check.
5) Separated the plug leads from #1 & #3 so they didn't touch anywhere and went flying. No change.
6) Plugged the hole for the cabin heat which is behind #3 and went flying. No change.

Additional Tests
1) Inspect the plugs on #4.
2) Remove air dam in front of #1 and test fly. (We were playing with this when the problem first came up and I want to reduce it to the size I had at the end of phase 1.)
3) Swap P-mags left and right and test fly.

Thanks and please keep the ideas coming. We will track this down and we will all be more knowledgeable as a result.

 

[N941WR]

Problem Solved!

As I suspected it was not an ignition problem. This is good news for all of us P-mag fans.

This morning I started out with the idea of changing the remaining seven spark plugs and doing a test flight. What the heck, they had 135 hours on them and this is a low compression engine so they could be fouled with lead. They had been checked at 75 hours and all looked good and since there was a full set of new plugs sitting in my hangar, why not change them all out and not just the one on the suspect cylinder.

As I was looking at my #3 cylinder from underneath I realized the plastic tubing that protects the safety wire used to pull the cylinder baffles together was touching the side of the CHT probe and cylinder head. It had melted and actually burned some. I'm lucky it didn't go up in flames. The tubing had to be pulled off the cylinder head with some pliers, it was that stuck.

The third thing I did while working on the engine was to tighten the clamps on the intake tube. They weren't loose but were easy enough to tighten a turn or so.

A quick test flight before the storm showers moved in showed my CHT's were back in line with where they had been in the past.

Again, there was no issue with the P-mags and I would like to thank Brad and Tom at E-mag Air for their help and support. They gave up a good deal of their valuable time on the phone, via email, and at SnF in trying to figure out what the issue was.

With 100% confidence I can recommend their products to anyone who is looking for a good and reliable electronic ignition system for their airplane!

 

[rv8180]

Way to go, Bill.
Glad it wasn't anything..."costly"....

 

[N941WR]

Update: CHT issues solved

As I said early in this thread, I thought I was having an ignition problem, this was not the case. The P-mags were not at fault and continue to provide trouble free service.

After two plus months of testing, my #3 CHT is down within 20 degrees of the other three on 90+ degree days and all are well below 400. My fear now is will it run too cold in the winter.

Here is a quick rundown of the changes. Each change was made one at a time so the impact on the errant CHT could be assessed.

Swapped out a P-mag: No change
Tested Spark Plug lead: No problem, thus no change
Changed all spark plugs: No change
Smoothed out the inside of the top cowling per Van’s: 5 to 10 degree reduction on #3
Replaced airseal fabric in the corner around #3: 5 degree change
Changed the intake gaskets on #3: No change
Changed #1 and #3 CHT probe: No change
Tried different combinations of aluminum tape over the front of #1 & #2 cylinders: Still playing with this to balance them.
Closed off a gap in the baffles behind #3’s barrel. (This problem is unique to the O-290. See my 3rd engine page for pictures.): Change 4 to 6 degrees.

The following changes all brought the temp down a degree or two so the net change added up.
Trimmed the opening at the bottom of the cowling to be even w/ the firewall: All CHT’s dropped equally about five degrees.
Cleaned out the flashing between the cooling fins on #3.
Put RTV on any air leak I could find.
Added a 2nd washer behind #3.

Now, with 90+ degree OAT’s I can run it w/o worrying about any of the cylinders going over 400. Climb temps are down as well. All around, it was well worth the effort.

 

[frankh]

Was the plastic tube nylon?

I didn't think you could melt the nylon tube (unless it was touching the exhaust pipe of course)

Frank

 

[N941WR]

I didn't think you could melt the nylon tube (unless it was touching the exhaust pipe of course)

Frank

Frank,

At first I thought it was the tube that was causing the high CHT's as it melted against the CHT probe but that was not the case. I did move the safety wire over so the tube wouldn't touch the CHT and the temp didn't change one degree.

As you read, lots of little changes made the difference. I'm going to continue to work on it and see if there is any chance of getting them to within five degrees of each other. I know, I'm dreaming on this one.

 

[N941WR]

It has been almost a year since I first noticed the problem with my high CHT's and I finally resolved.

All the changes I made helped reduce the temps but with all the work we have been doing developing a controller for the E & P-mags I realized one of my P-mags had the timing shift set higher than that other.

This is the reason why the CHT's dropped when turning of the right mag. Why I didn't think to check the settings before is beyond me. At the time I had taken the units out of the box and installed them and never thought to verify that they were set the same, which they were not.

My CHT's are now running 360, 359, 359, & 371 and this was with 81 degree OAT's. It now looks like I should start looking at #4 to see if I can bring it down another 10 degrees.

Thanks for the help.