cm2250 rough idle

[JimT]

Ok, guess it's time for a new problem.

2012 pete 587, cm2250 537,000mi

Started the truck up the other day, cold, and after a couple minutes at idle it started to run rough, like a cylinder is cut out or missing. At the same time the engine had entered SCR catalyst warmup and the turbo spooled up accordingly. Once the SCR warmup was complete the engine returned to a normal smooth idle (set at 720-725 rpm).

Since that first occurrence it has happened 2 more times. They all occurred while the engine was still cold (first morning cold start, outside temps around the freezing point), fairly soon after starting the engine. Once the engine is warmed up it does not happen at all, regardless of operating state.

When it occurs the rpm's will slowly start to fluctuate. At the peak the gauge shows that it'll drop down to 710 rpm then up to 730 rpm in about an audible 1.5-2 second cycle. Never lasts more than a few minutes.

Less than a week prior to this I had performed a cylinder cutout test with no issues. Each cylinder ran by itself and they all leveled out roughly the same. Injector performance test passed 2 times in a row and fuel pressure test held at 30,000psi for over 5 minutes before I restarted the engine.

I did manage to get Insite up and running during one of the occurrences and tried another cylinder cutout test while it was running rough, all cylinders operated independently with no noticeable difference between them.

Last oil and filter change was about 1,000 miles ago. davco filter is not clogged up. Recent EGR tune-up, EGR valve replacement and DPF/DOC cleaning. Crankcase pressure reads pretty steady at about 1.5 inHg through insite. DPF has been operating normally since the repair. No fault codes.

I have not tried taking anything apart yet as I'm not sure where to look. My initial thought was an injector or something with the turbo, but other then these isolated incidents the engine has been operating normally.

Suggestions?