So my 06 5.9 w/G56 is well maintained, with about 120k on it, stock everything.

Recently while towing across country I noted coolant temps a bit higher than normal (maybe 215) when pulling grades, but it was hot out and I was towing heavier than normal. I also noted a vibration at idle, not terrible or a dead miss, seems worse when hot, and some vibration can be felt while on the road. Power I can't say for sure, maybe down slightly. At first I was wondering about a vibration caused by the (original) dual mass flywheel because when you allow the engine rpm to drop while stopped there is a almost wobble feel as it passes by 1,100 rpm, but now I'm not sure.

When I got home I ran cylinder contribution with #1 97%, #5 100%, #3 102% #6 100%, #2 96%, #4 100%. There is no unusual smoke, no blow by, no injector knock or other bad noises. I did note the coolant bottle slightly down. Not sure if more than normal evaporation after 4000 miles towing in high heat. No codes or sign of external leak. As I understand that test all of them at 100 would be ideal, but being within 3 or 4% is considered acceptable?

My thinking was that those numbers are pretty close, and maybe it's OK, but two adjacent cylinders being slightly down, with possible coolant use, could be head gasket slow leak? I did the cold start, see if the hoses get immediately hot thing, and it seems normal- pressurizing once it's got a good bit of heat in it as you'd expect. I also did a block test which passed.

Anybody have any thoughts about what I should do next?

 

Low mileage! I have 210K on my 2003. If your serious about towing, your tranny will create a lot of heat, which in turn will heat up your radiator. I went to the double deep pan on the 48RE tranny and replaced the stock radiator with a Mishimoto and it definitely helped. The double deep pan allowed me to install a tranny temp gage which is super important. Send me a PM if you want to discuss further.

 

It’s a G56 manual transmission, so not an issue with that.

Turns out my vibration/shudder was the dual mass flywheel.

Has a Valair quiet dual disk organic clutch in it now, and it’s smooth as silk.

The quiet dual disk is quieter than most dual disks I’ve heard, but still makes noise when disengaged. For my use if I had to do it again I might have gone single disk, but overall I’m quite pleased.