I just put in a new thermostat from cummins (part # 3928499), and I'm getting huge temperature fluctuations. I read that this was a problem for unloaded trucks because of the large radiator, but I thought the new thermostats from cummins would fix the problem?

When the truck is warmed up and I'm cruising down the hi-way the gauge jumps from the halfway mark down to under a quarter and then climbs back up and repeats. I drive most of the time unloaded or very lightly loaded so I would like to fix this problem if possible

 

You may have gotten a bad t-stat, or there is some air still in the system.

 

Ok thanks, what's the best way to bleed the cooling system on these trucks? Might as well try that first before I try to get warranty on the thermostat

 

Drain about one gallon from the radiator. Remove the upper hose and neck together, then pull the t-stat out. Pour coolant into the block where the t-stat sits until the block is full. Tighten the neck back down, then fill the radiator, squeezing the upper hose until you purge all the air, and the radiator is completely full of coolant. Also, be sure the recovery tank is at or just above the max mark. Drive the truck and see if the problem still persists. If it does, need to get a new t-stat.

 

i dont think you necessarily have any problems the thermostats are notorious for doing that from cummins. mine did the same thing. replaced thermostat with 4 different ones and same thing every time. i just gave up and dealt with it

 

its normal!!

 

I believe so. You are seeing the actual temp. Be thankful - the later trucks have a dummy guage.

 

I read that it was normal, but I also read that it had been fixed by an updated thermostat. I guess I just don't like the thought of the engine being heated and cooled so drastically all the time. But I do like having a real gauge

Quote from geno's garage:

So according to this cummins sold me the "old" thermostat, I guess it's my fault for not asking for the new part number specifically, I assumed they didn't sell the old one anymore . Has anyone tried the "new" thermostat, how does it work? I would think it just opens slower?

 

The temp changes used to drive me crazy to the point of watching the guage paranoid something was wrong but after 2 t stats and a full coolant flush ive learned to live with it.

 

Haven't had an issue since I changed over to a NAPA heavy duty.