3406 high coolant temps

What makes you believe you have an issue? 205F is not hot. Does is continue to climb while the fan is engaged or does it pull it right down to 180 in short order? Might you be chasing something that just isn't all that bad to begin with?

I know you said thermostats were done. Any housing corrosion present where coolant may be bypassing the thermostats? Does the upper house stay cold till thermostat opening temp?

 

My gut tells me that you are looking in the wrong place. When you are certain adequate air is passing through the grill to cool both the charge air and coolant, I would consider an air/fuel problem is causing the higher temps. I am by no means a mechanic but something just tells me to look elsewhere.

 

Not to be an arse. Move the towel around to be sure it stays everywhere. Solving a problem you gotta keep dotting the i's and crossing the t's.

 

When I check my B models for a boost leak i plum my regulator into my boost gauge line outlet. Then pull the crossover breather tubes leaving rubber elbow on the turbo. Put an expandable pipe plug, Folgers coffee can any thing that will clamp down and I air her up. Spray intake ports every thing. Lots of intake gasket trouble. As well as older tubes give under the clamp a tad.

Something else worth mentioning is pull the exhaust pipe off the back of the turbo. Get a pen size flash light and check the fins up close to the housing. It may have spit something out prior to you buying it and the turbo wasn't replaced

 

I thank there is a middle ground where the pistons are not melted but the excess heat is carried over to the cooling system.

 

I think you're right about 205 being a little hot for a 3406b. Has the timing ever been altered or checked?

 

Ha Rank: Nice picture but I have not been to the part of NY.
I know your truck is not a Freightliner or Cummins but just want to share some problems I have seen with them.
Also some of the exhaust in coolant test kits are for gas burners only. Read the product info real close, that has messed me up.
A FLD ( Detroit ) had an air controlled sleeper heater core valve. It leaked enough air into the cooling system causing the water pump to cavitate a bit. Just enough to heat up under load.
I read a Cummins buletin years ago about checking for air ( combustion, compressor, etc ) in the cooling system. It showed installing a clear hose to the water rail above the head. The other end connected down around the water pump. I would thank on the inlet side. Just sharing the idea but I would want to check another 3406B before you condeemed yours.
Also ran into pumps with the incorrect impeller and or belt pulley. I know you are gear driven but again just sharing.
I have found checking coolant pressure in the block at rated speed to be a valuable tool. Thank your Cat should run at least 30 and steady. There is only one left around here I see a couple of times a year. Cannot check that out for you right away. Maybe someone else can.
That one is an 89 Pet and if it ever gets over 200 degrees something is wrong. It may pull over 80,000 but we have no hills like NY.
Just hope the ideas may help you figure it out.