High EGT, One Cylinder Only, Full Power Climb

[CBogle]

Guys:

I have an Insight GEM Model 602 graphic engine monitor in my plane and have noticed the following for approximately 600 hours on my "old" C145 and the exact same thing is happening on the "new" (different engine) C145 just installed.

At full-power during a climb, the EGT indication for the #2 cylinder (the one next to the fire wall on the left side facing forward) is significantly higher than all other cylinders. The CHT is only 25 degrees hotter on the #2 cylinder during climb, compared with the other cylinders. Once I level off and throttle back, the #2 EGT and CHT indications are in line with all other cylinders. Note - the insight does not indicate absolute EGT temp, only relative EGT temp compared with the other cylinders, so I can't tell what temp is being indicated.

According to Insight, if you see a significantly higher EGT, followed by a significant rise in CHT on the same cylinder, it is likely pre-ignition in that cylinder. However, I am certain that this is not pre-ignition plus there is no significant CHT rise.

One theory that I have is that for some reason, the #2 cylinder is receiving relatively less fuel than the other 5 cylinders at full power, full rich climb, which would be indicated by a higher EGT.

The new engine had its own fuel input manifolds. In fact, the only thing on the new engine from the old is the carb and mags. The mags are Slick and have never indicated any problem and just had their 500 hour inspection. In fact, I think I can eliminate the mags as a possible cause as this same thing happened on the Bendix mags prior to switching to Slick.

One mechanic has opined that the tolerances and fuel distribution of these older engines is not that precise, and, what I'm seeing is probably just normal, but not usually noted due to most of these engines not being monitored by something as precise as an Insight monitor.

Baffling doesn't seem to be the issue either as the baffling is really good and CHT's are all fine.

I don't think it is a faulty EGT probe either as once level and under any other power condition, the EGT is in-line with all other cylinders.

Has anyone else experienced this? Any theories? Is this worth trying to figure out? There are no indications that any abnormal wear or cracking or compression issues exist in this cylinder, either on the old engine or the new.

Thanks,

Curt

[lowNslow]

If you go to the users manual for your unit at http://www.insightavionics.com/pdf%20fi ... 0GUIDE.pdf and go to page 67, it offers some trouble shooting techniques. This may well be a normal indication tho as the fuel/air distribution system on this engine is not exactly "tuned". The other thing to check would be the short section of hose that connects the intake manifold to the cylinder, these can deteriorate or just come loose giving you a over lean mixture.

[CBogle]

Karl:

Thanks for the feedback. The intake hoses are all new and tight. The intake manifold is also tight. Also, this same phenomena happened on the same cylinder on the previous engine, also a C145-2. That is what makes this so strange. Same phenomena, two different, but identical, engines.

I don't think it can be the carb as the other cylinders are apparently getting plenty of fuel, the carb was overhauled 600 hours ago, and there are no other apparent carb related anomalies showing up anywhere else. Plus, in anything other than a full power climb, the #2 cylinder CHT and EGT indications are in-line with all the other cylinders. The anomaly is only at full power, in climb.

Appreciate your input.

Curt

[lowNslow]

Karl:

Thank Same phenomena, two different, but identical, engines.

Curt

I don't have a an analyzer so I can't tell what my indications are but the fact that you had the same reading from two different engines would point to either the indicator or that this is just a normal indication for the C-145. I'm guessing it's the latter.

[johneeb]

Curt,
I'll give you a wildly esoteric theory. As cylinder pressure builds at higher power settings the ignition system has a harder and harder time ionizing the gap at the sparkplug. The ignition always looks for the path of least resistance and goes to ground there. That is a bad sparkplug insulator, bad ignition lead, bad wiring harness block at the mag. Once you throttle back at cruise the pressure in the cylinder reduces and the plug goes back to firing normally. The reason that the EGT goes up under these conditions is that with one plug not firing the timing is off and some of the fuel air mixture is still burning as the exhaust valve opens causing the EGT to indicate high. I notice a similar phenominon on my JPI monitor when ever I would go from 80 octane to 100LL, the 100LL being slower burning would still be burning when the exhaust valves opened causing the EGT's on all cylinders to indicate higher than the did with 80 octane.

[CBogle]

John:

Would a brief mag check at full power, full rich, be likely to harm anything? I would be able to tell very quickly if one of the plugs weren't firing on the #2 cylinder via the GEM monitor, and which one at that.

My concern would be that if there were no spark in the cylinder, even for a brief period of time, when I turned the good plug "back on" the ignition of excess fuel in the cylinder would do harm to the exhaust system or something else.

Your theory is very plausible, and would explain why this is happening on two separate engines. The plugs are new and different so if this is the problem, it has to be upstream from the plug. in the ignition wire or mag.

In summary, I need opinions on whether a full power, full rich, mag check will hurt anything assuming one plug isn't firing and the cylinder will have no combustion for a short period of time?

Curt

[lowNslow]

Curt, if you look at manual for your indicator (web address above) page 67 it walks you thru basically the same procedure to determine if it is a bad plug.

[CBogle]

Karl:

I just need to make sure that it would be O.K. at full power, full rich. I've read in other places cautions against mag checks in the air at high power settings. I don't recall exactly the situation, but, I'm under the impression that if ignition is lost to cylinders for a brief period of time, the excess fuel built up in the cylinders can cause exhaust system damage. What I don't know is if this applies to all cylinders being cold for a moment or to just one.

Thanks,

Curt

[blueldr]

Personally, I don't think you have a problem. The mixture on these stock engines is notoriously irregular. Precision EGT and CHT instruments installed on this type of a fuel distribution system generally only provide pilot worry. The cylinder with the high EGT is probably just running a little closer to best power mixture and there is nothing you can really do about it. The C-145 is anything but a high performance engine. After all, 145 HP from 300 cu. in. isn't very much, and the carburetion system was stolen from John Deere in about 1933.
Please don't misunderstand me. I consider the C-145 to be one of the best and most reliable airplane engines ever built. It just aint very sophisticated and shouldn't be considered "high tech".

[CBogle]

BL:

I hope you are right. I'm really not looking for problems and you make a good point that I've already considered...that is, would I even be trying to run this down if I didn't have the monitor? Answer - no, as there is no indication of anything being wrong.

That said, I'd know for sure if one plug wasn't firing under high pressure if I could do the mag check. But, before I do, I want some expert opinions on the wisdom of this.

Experts...weigh in!

Thanks,

Curt

[ronjenx]

How about switching two EGT probes to see if the indications follow?

[CBogle]

Ron:

Swapping EGT probes is going to be some work. Wires aren't long enough on the #2 cylinder to reach any others and wiring is all tie wrapped, heat shielded, etc. Would be no small job to swap probes. Installing a new probe would be easier but, a mag check would be much easier than that.

Would like to start with mag check if enough people think it is O.K. to do.

Thanks,

Curt

[cessna170bdriver]

Curt,

I don't think you have a problem. I installed an Electronics International UBG-16 6-cylinder CHT/EGT on my O-300 at overhaul, and I notice that both #2 and #5 EGT's run 100-150 degrees hotter than the other four. In a rich climb, #2 edges out #5 by a few 10's of degrees, then in lean cruise, #5 is the first to peak, and is the hottest by a few 10's of degrees. Even in lean cruise, #'s 2 and 5 are running 100 degrees or more hotter than the others. I suspect that, due to the intake configuration, the manifold pressure is slightly higher at those two cylinders than the rest.

Just get used what your "normal" readings are, then be on the lookout for something out of that "norm".

Miles

[CBogle]

Miles:

I'm leaning strongly towards concluding that high EGT on the #2 cylinder at full pwer, full rich is the norm. I flew early this morning and did the mag check at full power, full rich in flight and both plugs in the cylinder fired fine and the EGT increased even more on one plug, which would be expected.

I also pulled the plugs for a visual inspection and they are fine.

Thanks to all for the input.

Regards,

Curt

[GAHorn]

Well...sorry to be late catching up on the forums...been busy with convention stuff lately.
There should be no problem checking mags at full power on this engine. But I doubt you have a mag problem at high power that doesn't show up at lower power (like on pretakeoff checks.)
No. 2 cylinder is the leanest on this installation according to Cessna manuals, and is why Cessna put the CHT probe on #2...it runs hottest on most original 170 engines.
But now you already know all this.