water temp will slowly build to 190 then fall off to 150 then slowly build up and fall off. we changed the thermostat and water pump. the water pump did show signs of age the shaft had excessive end play. the problem is still present. what next? is this a common problem?[

 

Need to update your sig so we know what you have. I am assuming a 94-98, 2nd gen truck. If so, the fluctuation sounds normal. These trucks have a large cooling system, and seeing fluctuations between 140-190 is not unusual. Mine does this as well. One possible solution is the newest Cummins 190 thermostat, which is suppose to reduce the fluctuations a lot. Don't remember the pn, but do a forum search and you will find it.

 

Yes, according to the owner's manual, that is normal operation. The cooling system is large enough to maintain temperature under extended heavy load, which means that its 'oversized' during normal work. The newest t-stat is supposed to fix that issue (the one direct from cummins), but its not a *problem* really.

 

It's just the thermostat opening and closing.
If it stops fluctuating then you should be concerned.

 

the constant change of temp by that many degrees will make more stress on the block and heads than throttle up on a cold engine. we warm up an engine before placeing it under load for a reason?

 

I've noticed with mine that as long as i'm working it, the temp stays constant. Also - I wonder that as the coolant temp shifts by ~30 degrees, how much the actual casting temperatures swing? I can't imagine its a 1:1 relationship, there has to be some heat retained in the casting.

I do agree though - it seems like it would cause more stress. I think the real-world evidence of a number of CTD's running around with insanely high mileage would point to the fact that we're overthinking it...

 

I had an oil temp gauge for awhile but got rid of it because it was boring, read about 180° all the time after the engine was warmed up.
This indicates to me the block temp doesn't vary much.

Cummins isn't concerned about the coolant temp fluctuation at all but got tired of people complaining about it and moved the sensor in '98.
No more complaints.
You put a load on the truck like it was designed to be used and you'll be glad the cooling system is oversized enough that the temp fluctuates when the truck is empty.