Spark Plug no sparky ??? then do ???

[MARZ]

So here's the setup IO360a1a Flying home from OKC around 7,500 settled in after the climb and look to my EGT to adjust the lean - number 2 is zero'd out the rest were reporting normally

Slight panic - lost a cylinder? no - making good power, air speed seems a little bit slower than normal but not more than 4 knots and I haven't got things quite to cruise.

No obvious change in the performance no undo vibrations..... I surmise I've lost a probe or have a bad connection

I land at home, turn on the taxiway and notice that all four cylinders are reporting normal

Has anyone had something similar happen? Will spark plugs come and go at altitude ? I cant imagine the likelyhood of two plugs at the same time reacting like this but stranger things have happened.

I've set aside some time with the mechanic but thought I'd ask.

[fantom]

It's very likely a bad probe. How old are they?

[sreid]

If one plug was not firing, that cylinder would have an increase in EGT, not drop to zero.

I've never personally had a dead cylinder on a four cylinder 360, but if two plugs were not firing in one cylinder, I doubt you would have any question if there was a problem or not!

So I agree likely a bad probe.

[KSMooniac]

Sounds like a probe, but plugs will intermittently fail sometimes, and if I noticed that indication on the engine monitor I would do a mag check *right then* to see if one mag or the other gives an abnormal indication. If you didn't get massive vibration than you didn't lose both on the same jug at the same time.

I'd take the plugs out and measure the resistance and see where you're at.

[N201MKTurbo]

Definitely sounds like a probe.

I have had a cylinder completely quit and had to fly about 20 miles on three cylinders. It runs a bit rough, but not plane destroying rough. It made enough power to fly OK.

I had an exhaust valve stick open.

[DonMuncy]

I would suspect a poor connection on the probe wire first. If yours are like mine, the probe has two wires somewhere around 18 to 20 inches long with ring connectors on the ends. These are connected to wires from the panel with very small machine screws and nuts. I would take those loose, clean them up and reconnect before I did anything else.

[Alan Fox]

Likely a probe, These fail quite often.....

[Marauder]

Probe is my guess as well. The ONLY time I saw this happen when it wasn't the probe, was I lost a complete exhaust stack off of one cylinder. I had a metal fatigue failure of 1 stud and the other worked its way free after a bit. The stack fell into the cowling conveniently held by the probe wire.

What was interesting was that I saw a few minutes of fluctuating EGT temps that gave me a little warning something was amiss.

[Greg_D]

If you had completely lost a cylinder, you would know it. There would be a ton of vibration and a speed loss much greater than 4 knots.

[Marauder]

One other thought came to mind. I am not sure how much power loss you would experience running on 3 cylinders (I suspect a lot) but I guess it would be theoretically possible that complete loss of fuel flow to the cylinder would also drop the EGT off completely. If on a fuel injection system the injector was plugged, it could be why you saw no EGT temp. But since you did not notice a significant speed loss, it's doubtful that is the cause.

Probe is still my guess.

[Greg_D]

Again, the complete loss of power on any cylinder, whether it be from lack of fuel or spark would be so noticeable that you wouldn't need an engine monitor to know it. The monitor would only tell you which cylinder.

[MARZ]

I'll be checking the wiring for connections, The probes only have about 75 hours on them. if it crops up again I'll change the probe to see if the issue follows. Thanks all.

[Marauder]

Mike, if you have access to an ohm meter, you may want to compare resistance with the other probes. Unfortunately, it sounds like if the probe is the problem, it was only failing in flight, i.e. at a higher temp.

That said, I would expect the probes to be within a range for resistance at one temp. The other option you have, if you suspect a higher temp related problem is to switch probes around between cylinders. Hopefully you have enough lead wire to do that.

[MARZ]

Good idea on the ohm meter.

[jetdriven]

Thermocouples generate millivolts based on a temperature. I'm not sure what an ohm-meter will tell you but if you have an open circuit on the probe.

[N201MKTurbo]

Thermocouples fail when the wires come apart, an ohm meter will tell you that.