Hey All,
I've had my new-to-me 1981 M20J for about 9 months. I'm 2 months away from my first annual and I've noticed a problem that has become increasingly frequent over my past couple flights.... After more than an hour of flying, particularly after stabilizing in cruise and leaning out the engine, my EGT temps on cylinders 2 and 4 have become erratic. It usually starts as small bouncing and the temperature range becomes progressively wider as flight continues. There's no pattern or consistency to the bouncing. The CHT's on those cylinders remain perfectly stable and there is no noticeable vibration, mis-firing, or loss of speed. The engine appears to be purring along as usual. You wouldn't notice it unless you were staring at the JPI (oops). I've had the field mechanic take a look and they were unable to replicate this on the ground. I plan on having whoever does my annual really take a look at the probes and connections on the back of my JPI 700. However, what I find unusual is that this only happens after I've been enroute for over an hour. If I make a 40min flight to another local airport, it'll be good the entire way out and then start acting up on the way back. If I richen up, the bouncing seems to occur less. This has been going on intermittently since I've had the plane, but has only been truly consistent since a long flight from NC to FL over the holidays.
One more oddity that I'm hoping isn't related.... After that longer holiday flight my oil got black pretty quickly, I did run with the Ram Air open (4000ft+) for a period of that flight to try it out. I also came back with low oil and had to add 2 quarts. After a few more hours I had the oil changed in Feb (32 hours on the oil) and they sent a sample out for testing. There was no metal in the filter and the mechanic said that it just smelled like old oil, nothing special. The oil analysis showed higher Cr/Fe/Al/Si. Still in the caution range on everything but I'll be changing out the oil after <=25 hours this time, resending the analysis, and replacing the air filter at that time. My prior oil change in late September, coming off of mineral oil from a top overhaul, was all low/normal levels. I've put 10 hours on the oil since and it has stayed much cleaner, visually.
Some thoughts/opinions here would be appreciated. Are these events most likely unrelated? My initial thought is some cross-talk or bad probes on the EGT sensors. However, given the changes in the oil, I'm wondering if this something bigger that I should be concerned about? At this point I feel like I'm in wait and see mode- for the next oil change and/or my annual- but want to make sure I'm not doing more damage or being unsafe. Thanks in advance for the help.