Hi Louis:

 

Be cautious. 

 

Runaway temperatures that are consistent (smoothly changing) and within reason can indicate detonation or preignition, dangerous conditions that warrant immediate attention.

 

Since you are burning oil, it may be that you have deposits that are glowing and causing detonation.  

 

The giveaway is the lowering of temperature upon a major power reduction.   Once started, detonation or preignition can continue even with modest power reductions, particularly if caused by a glowing deposit, cracked spark plug insulator, etc.  The combustion chamber has to be immediately and substantially cooled to stop these bad news combustion processes.

 

Usual failure mode on an EGT is the thermocouple or a fault in the brittle thermocouple wiring which is prone to work hardening and cracking or breaking under vibration.  If the reading is erratic, up and down or open circuit and then ok, then a fault in the wiring is most likely.  A consistent reading that moves up and down smoothly usually suggests instrumentation is OK, at least from thermocouple through wiring and connector to the black box where the voltage reading is converted to a temperature reading.

 

So if I were in your shoes, I would pull the plug, inspect closely, find a boroscope if possible and check the cylinder for damage, look at the exhaust valve for unequal heating, and finding none, switch TC probes to confirm that it is cylinder  and not probe. 

 

And I would make the next flight very short with a rapid reduction in power and full rich immediately if there is another runaway temperature episode.   

 

Runaway temperatures are emergency events requiring fast and forceful corrections.

 

Fred

 

 

 

 

Hello all!

I have a bizarre CHT issue that I'm currently troubleshooting, and I could use some advice and perhaps a source for a part.

I have a LNC2 with an IO-360-C1C (angle valve 200 hp).  The engine has 365 hours SMOH (which was about 10 years ago), a history of good compressions, and clean oil analysis results.  It is burning about half a quart an hour, but I suspect most of that is through the No. 1 cylinder.  I'm currently running mineral oil for 25 hours in an attempt to reseat the oil ring in that cylinder.  That issue is most likely unrelated, but I figured I'd mention it in case I am overlooking something.  I have experienced slightly higher oil temps because of the oil choice, but otherwise no issues.

All four EGTs and CHTs are monitored by a Vision Microsystems VM1000.  I fly from a 5000' MSL field, and on a normal summer day on a 140 mph climb, I usually see CHTs between 350 and 380.

Two flights ago, I took off on a standard climb profile and the No. 2 CHT ran away.  Climbing through pattern altitude it was already pushing 400, and after slowing my climb rate, accelerating to 160, and adding fuel, it reached a peak around 420.  All the other cylinders were around 350 initially and cooled down to 300-330 while the No. 2 stayed hot.  Even downhill on the way back to the airport with the power around 15", the No. 2 stayed high (around 400) while the other three cylinders were in the high 200s / low 300s.  The highest oil temp I saw was 212.

Upon inspection, there were no obstructions to the airflow, and everything in the cowling looked unchanged--leading me to suspect instrumentation.

I was unable to duplicate the problem on the ground; I ran for 15 minutes, got all four cylinders up above 400, and they stayed mostly matched.

After cleaning and reseating the connections to the No. 2 CHT probe and double checking the baffling, I went for a test flight...and the problem came back.  Once again, the No. 2 went up to around 420 and the others stayed in the mid 300s.  The No. 2 stayed hot even after pulling the power.  However, after some delay getting into the pattern, they all came down to around 300 before landing and were fairly well matched on the taxi in.

At this point, I'm still suspecting a faulty CHT indication, as the engine runs quite smoothly and the cylinder is getting plenty of air.

I was wondering if anyone knows the failure mode of VM1000 CHT probes--does this sound like it's on the fritz?  Also...does anyone know where I can get a new CHT probe for the system?  Am I on the right track with troubleshooting?

My next step is to swap the probes from the No. 2 and No. 4 cylinders and see if the problem moves.

Thanks for your time everyone - 

Louis Gabriel

LNC2, KRTS