Cold cylinder - 200F in cruise

bflynn

bflynn

Final Approach
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Brian Flynn
BLUF - anything else I could have done different?

Flying on Sunday, I saw the #2 cylinder indicating 200F in cruise, when the others were in the 1200+ range. The engine seemed to be running fine and performance wasn't low so I suspect this was just a sensor problem. I was maybe 5 miles north, so I announced the cold cylinder and asked for priority, flew a straight in, and landed with no problem. I noted on final that #2 was up to 700, but the gauge only shows one at a time. I wasn't scanning all the cylinders anymore, so I don't know what the others were doing.

Before flying, the mag check ran rough on that side. Usually that's because I didn't lean enough during a long taxi, I leaned, ran it up, and did it again with success - left drop was 75 rpms, right was 50. After flying, the mag check was normal.

Anything else I could have done? I didn't consider an inflight mag check, but I don't think I would have tried it because the engine was running ok and why risk breaking it. Probably should have flown direct to the airport at altitude rather than do a normal approach.

Thoughts?
 
Pull the plugs would be my first suggestion. Given the 1,200 for the others, assume you are referring to EGT? Given carb engine, spark would be guess #1, stuck valve guess #2. Could be as simple as a lead clinker but unusual for both to not fire (you'd be above 200 on one only). Intake leak would be the one other possibility. All this assumes sensor is reading correctly.
 
I’d trace the wire leads from that sensor and disconnect-reconnect line connections. Connection issues are common.
 
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Swap sensors? See if the problem follows the sensor or the cylinder? I assume that much of a temp difference would be obvious on a ground runup?
 
A cold cylinder on a 6 does not vibrate like a 4. Given he reported some mag issues it’s possible the cylinder was cold. Since it bumped up to 700 at some point I suspect as others mentioned it’s an indication issue. it could be easily verified if you have a CHT on that cylinder. Be aware also that when most reference a cylinder temp they mean CHT. Your discussion based on the numbers has to be the exhaust or EGT.
 
bflynn said:
BLUF - anything else I could have done different?

Flying on Sunday, I saw the #2 cylinder indicating 200F in cruise, when the others were in the 1200+ range. The engine seemed to be running fine and performance wasn't low so I suspect this was just a sensor problem. I was maybe 5 miles north, so I announced the cold cylinder and asked for priority, flew a straight in, and landed with no problem. I noted on final that #2 was up to 700, but the gauge only shows one at a time. I wasn't scanning all the cylinders anymore, so I don't know what the others were doing.

Before flying, the mag check ran rough on that side. Usually that's because I didn't lean enough during a long taxi, I leaned, ran it up, and did it again with success - left drop was 75 rpms, right was 50. After flying, the mag check was normal.

Anything else I could have done? I didn't consider an inflight mag check, but I don't think I would have tried it because the engine was running ok and why risk breaking it. Probably should have flown direct to the airport at altitude rather than do a normal approach.

Thoughts?
Click to expand...
Like Hang 4 said, it sounds like you are seeing #2 reading 200F EGT if the others are all 1200+F

What is the CHT reading on that cylinder? I assume you have CHT probes on all cylinders if you do for EGT?

If the CHT is normal and the EGT is 200F then I would say it is obviously a sensor issue. If both CHT and EGT are abnormal, it would be a dead cylinder.

I recently had a similar issue on my T6. For a while one cylinder CHT had been reading unusually cool in the climb (250-280F while the others were all 360F+). Then it was reading less than 200F in flight. BUT, EGT was behaving completely normally just like the other 8 cylinders. Bad sensor.
 
There's only EGT, an older Warrior II. The cylinder felt just as hot as the others, but metal conducts heat well.

I haven't been back with the mechanic to see what it was, but the airplane is back online so I don't think it was the cylinder, probably the sensor wire.
 
Did you try a mag check in flight?

If this EGT symptom occurs under all combinations of L R and Both, then it’s unlikely to be a bad spark plug or a bad magneto — more likely a bad sensor (or bad connector for that sensor, especially if you’re seeing rapid random fluctuations in that EGT.)
 
bflynn said:
I didn't consider an inflight mag check, but I don't think I would have tried it because the engine was running ok and why risk breaking it. Probably should have flown direct to the airport at altitude rather than do a normal approach
Click to expand...
 
I’m with others on this. If the cylinder was producing power AT ALL the EGT would be well above 200. If it wasn’t producing power at all the engine would be pretty rough, esp for a 4-cylinder. The EGT cannot be 200 if the CHT is 300+ (i.e. combusting at all).

I wouldn’t be afraid of an in-flight mag check, although I’m not sure it would actually help with the diagnosis here. The check is useful for finding a bad spark plug/lead but you either had BOTH plugs for that cylinder not working (unlikely) or a valve issue causing the cylinder to be dead (and the engine running rough) if the EGT was truly 200. But the check may temporarily cause the engine to run rough but turning it back to “both” immediately fixes that.

Your situation reminds me of a recent experience. On my Warrior, the EGT probes on the right (#s 1 and 3) interfere with taking out the bottom plugs so I need to remove the probes to pull the plugs. I fired up the other day after cleaning the plugs and the #1 probe kept reading basically ambient temperature with a very slow rise. Yep - I had forgotten to put the probe back in (and it was picking up the slow rise in temp of the engine compartment). That’s the type of thing your story seems most consistent with, in my non-A&P experience. BTW: what does your EGT read first thing when you get to the plane after sitting overnight? It should be ambient temperature and the same as any other probe (EGT,CHT, OAT) you may have.
 
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