Attention: TDR Forum Junkies 
Just to start: I am intimately familiar with coolant temperature cycling due to the thermostat and large cooling system. I see it all the time. However, I saw something new tonight and I'm not sure what to make of it.
It is cool here, about 60 degrees, with an unloaded truck. While in stop and go traffic, I lock out the O/D and stay in 3rd gear, particularly where I'm back and forth between 35 and 50 mph.
So there I was in 3rd gear and 45-50 and I noticed my temp gauge at 190, but it didn't cycle down (like it normally does) until I slowed down or stopped. EGTs were about 600 post-turbo, boost was negligible with no load at 2000 rpm.
When I stopped, the gauge slowly went down to about 150 and stayed there until I started moving again.
Once on the interstate at 65-70 mph, everything stabilized normal (hardly any fluctuation) until I got into stop & go traffic again. Then it came back.
The gauge seems to respond (slowly) to engine rpm and speed. The $64K question: t-stat gone bad? Head gasket gone bad? bad ground? Anything else?
It is cool here, about 60 degrees, with an unloaded truck. While in stop and go traffic, I lock out the O/D and stay in 3rd gear, particularly where I'm back and forth between 35 and 50 mph.
So there I was in 3rd gear and 45-50 and I noticed my temp gauge at 190, but it didn't cycle down (like it normally does) until I slowed down or stopped. EGTs were about 600 post-turbo, boost was negligible with no load at 2000 rpm.
When I stopped, the gauge slowly went down to about 150 and stayed there until I started moving again.
Once on the interstate at 65-70 mph, everything stabilized normal (hardly any fluctuation) until I got into stop & go traffic again. Then it came back.
The gauge seems to respond (slowly) to engine rpm and speed. The $64K question: t-stat gone bad? Head gasket gone bad? bad ground? Anything else?
. I wouldn't be overly concerned about it. If it starts running over 190 and staying there, then you might have a problem.
