EGT/RPM fluctuations

[ohdub]

While flying today I noticed occasional rpm fluctuations, rpm would drop by about 25 and bounce back. I don't believe it's an indication error as I heard it first which drew my attention to the tach.

I then noticed that when these fluctuations were occurring, the #2 EGT was fluctuating as well. It would rapidly rise by about 100F and drop back down. 

These fluctuations would happen over a minute or two and then everything would be normal for a while before happening again.

I eventually switched from LOP (my normal cruise setting) to ROP and I didn't notice anymore rpm fluctuations, but I didn't watch the egt too closely so I'm not sure if they stopped too.

I'm heading back out to the hangar to download the engine data and upload to Savvy.

Any ideas what might be going on?

Edited June 5, 2018 by ohdub

[N201MKTurbo]

Sounds like a bad plug. If one plug quits the EGT will rise on that cylinder. Did you do a mag check while it was acting up?

[ohdub]

I did try a couple of mag checks, but each time I just missed the "event".

[jlunseth]

I have also seen a failing connector at the mag show the same symptoms.  It does the same thing Rich is talking about, stops the spark.  The connector was corroded inside the socket where it attaches to the mag.  Eventually I just replaced the harness, because it was getting old.  It was intermittent failure at first, then constant.

[carusoam]

Expect the JPI download to show some interesting lines...

Start looking in the logs for what plugs you have...

If they say Champion on them, expect to measure their resistance...

If the resistance on one is approaching a number nearing infinity... and it matches the plug that is selected via the JPI graph... from the mag test...  You have a winner!

 

Plan B... check the logs again. This time search for what mags you have and how long they have been in service...

Engine challenges that are intermittent are funky...

PP thoughts only... not a mechanic.

Best regards,

-a-

[tony]

Mechanical or electronic rpm gauge?

[ohdub]

Mechanical

[ohdub]

So I'm looking at the engine data from yesterday's flight and all the egt's seem to be fluctuating:

Where as my flight the day before looked like this:

I've asked Savvy to do an analysis, I'll be interested to hear what they say.

[tony]

As the speedometer cable ages, they are known to have a bit of friction.  The cable stops (binds), then frees up goes and  kinda acts like a spring.  I would replace the cable to see if it stops before doing much digging into the engine. 

You can try to drop a light oil in the cable to see if it helps for a little while. 

Edited June 6, 2018 by tony

[N201MKTurbo]

My guess is one side of the mag is intermittent. I would check the points. You can diagnose by running on one mag at a time while this is happening. You should have smooth operation on one mag and the engine will quit on the other one.

[ohdub]

The issue was a bit of grease in the left mag. M20Doc fixed while I waited this morning.