I hqve a 99 24V 3/4 and pull a fifth wheel approx 9600 lbs loaded. When I first got the truck the guage very seldom went over 90c (Canadian model) but recently it goes to 100c and stays there on flat runs. Is this a thermostat problem, I just changed all fluids in October 2002. <br><br>Thanks

 

Doesnt to me. thermostats usually stick open or closed, or they are intermittant. I've not known one to stick partially open but maybe someone else has. Here's some things to check:<br><br>1. watch the gauge carefuly as the engine warms up, during moderate to light driving while empty. does the gauge rise steadily to 100 degrees or does it stop where is used to (90). I don't know that a 'stat would suddenly change its opening temperature... <br><br>2. in very warm weather (we hit 105 yesterday in Boise) let the engine idle and establish a stable temperature. the 24v engines idle cooler than the 12vers, so this test might differentiate between a guage problem, a 'stat problem, or a cooling system problems. does it want to run at 100* still? In cooler weather and with a good cooling system, the idle will not achieve thermostat temperature so be careful how you interpret this step. <br><br>3. run the engine hard, up a long hill. does it want to stay at 100*?<br><br>In general, here's how I interpret experiments such as these:<br><br>If when working lightly or idling in hot weather the engine stabalizes at the same temperature that it does when working hard, then your cooling system is good, your 'stat is good, and your gauge is bad (if it reads the wrong temperature).<br><br>if when working hard, the needle drifts above the above mark, and then settles back down when you stop working it hard, then your stat is working but your cooling system is not sufficient there are a number of things that can cause that of course, including the fan clutch. bwt, three years w/o a fluid change is not good ;D<br><br>if the needle seems to follow or track how hard you work the engine, then your 'stat is stuck open. If it warms up normally and then keeps warming up when you drive, then the 'stat is stuck closed. <br><br>hope that helps. -Doug

 

Another thing to look at is your radiator, I chased a overheating problem round and round with 30,000 miles on my truck and found the radiator stopped up with road crap preventing proper air flow across the cooling fins... I had to pull the Radiator to clean it properly and have done it once again since then. Another thing when they stop up like that the fan clutch will not come on because the air does not blow thru the radiator and will not heat the thermo coupler in the fan clutch. In other words the fan clutch does not get the hot air that should be coming thru the radiator and it will not engage....

And like Doug said 3 years without a coolant service is not good, I would rec at least a drain and fill once a year or every 15000 miles...Diesels are hard on coolant.

Take Care,
FreightTrain Wayneo,

 

Thanks very much for your replies, I changed the stat yesterday, but will not be pulling my fifth wheel until August 1st that's when I'll know more about the temperature. Normally without pulling it stays just below 90c. just bothered me when all of a sudden it went to 100c. I did change the coolant in October 2002 and only put about 5000 miles on the truck since then.<br><br>This has to be the best site ever when great people like you folks take the time to respond to my situation, many thanks and I'll let you know after my pull in August.<br><br>Thanks again.