Troubleshooting Issue on M20J, running rough and low EGT/CHT on one cylinder.

[carusoam]

Swapping sensors around can be helpful in diagnosing a sensor issue.

But sensors don't cause a drop in rpm...

Best regards,

-a-

[ArtVandelay]

When I had my engine monitor installed, the first flight to be, it ran rough, #3 was showing no EGT, turned out to be a plugged injector, I've had other problems that were found just after maintenance, now every time I get it back from the shop I do so with some trepidation.

[Bob - S50]

We had Champion massive plugs with about 200 hours on them.  It just came out of annual in August.  A couple times in the last couple weeks we had one cylinder go cold during the mag check.  Running it for a while fixed the problem.  We decided to have our A&P check for fouled plugs.  He inspected, cleaned, gapped, and tested for resistence.  Six out of 8 failed (over 5000 ohms).  We replaced them with Tempest fine wire as recommended by our A&P.  Glad we did.

 

If a cylinder is going cold it can only be one of a few things.  No spark.  No fuel.  No pressure.  Mixture so rich or lean that it won't burn.  Bad indicator.

 

My money is on the spark plugs.

 

Spark could be magneto, wires, or plugs.  Fuel would have to be at the spider or later.  No pressure could be a sticky or failing valve.

 

Plugged injector would most likely make the EGT high, not low.  Failed distributor seems unlikely.  If you download your insight monitor history, look at the EGT curves.  An EGT that fluctuates up and down about once each minute is supposed to be a sign of a failing exhaust valve.

 

Last maintenance performed is always a good place to start looking, including the recently overhauled mags.

 

Good luck,

 

Bob

[ArtVandelay]

Plugged injector would most likely make the EGT high, not low.

Why would you say that? No fuel to burn, no EGT

[FloridaMan]

A partially clogged injector will make that cylinder run lean and all others to run rich. The end result is that that cylinder will have peak EGT way earlier than the others. And it may stop firing altogether, making the EGT drop to nothing. All other cylinders will get more fuel than they're supposed to since the fuel going to the cylinder with the obstructed injector will go to the others. 

[Jsavage3]

Possible sticky valve?

[M016576]

If it's a single bad plug, your egt will be *higher* than the rest, and you'll get a rough running engine when you turn off the mag with the good plug (and CHT and egt drop dramatically). If it's 2 bad plugs, CHT and egt will be dramatically lower, and the motor will run very rough.

Like I mentioned before, I don't think its a bad plug based on your description. Most likely a (partially) plugged injector. or worst case, a warped or sticky valve.

A "minor" plugged injector will cause an egt rise. A "medium" plugged injector will cause an egt drop as the cylinder goes past peak prior to the other three, which like Antares mentioned, are now getting a higher fuel pressure. A fully plugged injector will snuff out combustion in a he cylinder (obviously). There is no "one size fits all" type of symptom for a plugged injector. But there is for a spark plug. Either 1 is firing, 2 are firing, or none are firing.

I guess you can have an intermittently firing plug, too... That will cause surges in your egt: CHT will drop slightly.

[FloridaMan]

Another thing to note in regards to valves. "Morning sickness" is supposedly a symptom of a potentially failing exhaust valve and, to my understanding, is more likely to occur on higher time cylinders. The symptom, as described to me, is a rough running engine at startup that gets better and the roughness goes away. A broken valve strikes me as the biggest risk of catastrophic failure of a high time engine. If memory serves me correctly, Mooneygirl had an off airport landing that resulted in substantial damage as the result of a failed valve. 

[pieterdev]

I would agree with previous posts re the plugs. There is a bad batch of new spark plugs doing the rounds. Our M20J came back from an annual with a sporadic miss on the engine. Turns out that the resistance of four of the brand new replaced plugs were out of spec. They were replaced with another batch with the correct resistance, and the engine is running very smoothly now.