For starters Ive done a lot of reading and testing different parts. Im at a stand still and I dont want to be a parts changer..I bought a 02 3500 4x4 6 speed cummins 4.10 gears with 270k miles a week ago. All stock except for k&n intake and straight piped. I am experiencing a few problems. I noticed that the heat was barly warm. With a hint of antifreeze smell with blower on and selected on warm. So first problem is heater core. Ive driven the truck around town 55-60 mph. Coolant temp stays around 190 degrees. Once I hit the interstate at 70-75 with the 4.10 gears im turning about 2300-2400 rpm. The coolant temp starts to climb as the speed increase. Upwards of 220+ when im running 80. As soon as I slow back to 65-70 the temps go back to 190-200. Everytime I go to 80 and back down to 65 the temps go up and down every time. Keep in mind that its only around 50 degrees out. Moving on I drove truck for a 1 hr yesterday morning and it was 40 degrees out. temps did same as stated above but When I got home the radiator was cold to the touch and the radiator cap was not under pressure. Upper radiator hose was hot and lower hose was warm/hot. Coolant was full in both radiator and overflow tank. Both heater core hoses were hot. I read a lot about cooling issues and the radiator / intercooler/ conderser are clean from debris, the fan clutch was replaced by previous owner a month ago and has resistance when engine is cold. When spun by hand it only spins freely about a inch. When engine is normal temp 190ish and I cut off motor, the fan stops imediatlly. I first thought fan clutch was bad because temp was increasing with motor speed/vehicle speed. I dont hear it enguaged while the engine is upwards of 220..but i do have stright pipe and turbo sliencer ring is MIA. So its loud already. Second the thermostat could be bad, but cooling does perfect around town as long as im below 60mph. I put water through heater core and it wasnt plugged. Refilled coolant and tried to bleed all air from system. Used allen plug near t-stat. Filled overflow bootle and got engine warm and let it cool down a few times. No coolant was used. both overflow and rad are still both full. Still same symtoms. Oil looks good with no water residue. No exterior coolant leaks. no play or leaking water pump, coolant is nice and green. So either a faulty sensor, plugged radiator, broken water pump impeller, bad thermostat, bad clutch or air in system. Again systoms are semi warm heat with antifreeze smell inside cab. Coolant temp increases with vehicle speed above 70mph and continues to increase until i slow down but is normal around town doing 55-60 even when I accel hard. Sorry for the long story but Any help will be great.

The fan has little bearing on cooling at highway speeds, it is primarily needed at lower driving speeds/idling. I would replace the thermostat, it is too cheap not to especially given it's a fairly common part to fail. If that doesn't do it I would pull the water pump and inspect the impeller for any damage or corrosion. Also check your radiator for oil/dirt contamination. The factory blow by hose tends to cover the radiator with oil and will plug it off over time.Sent from my DROID4 using Tapatalk

Just like diesel4life said, my suspects would be thermostat, water pump, or radiator plugged with oil

I did a little test today.radiator was clean of oil and such. Water pump looked new inside. Tight with no shaft play. Thermostat had made in Germany on it. I boiled it in water. It started to open at 180ish. But only opened 1/4 in. I boiled water to 210 and it stayed 1/4 in. Open. Is this normal? I figured it would open more. Also what is the stock rad cap pressure? Mine says 15lbs

If you have the thermostat out, replace it, and my cap is 15. Manual says 14-18. Both are cheap replacements and are common to go bad.

I had a customer's car that drove me nuts. The only time the temp gauge would go above halve way and climb into the red was at freeway speeds, 70mph or higher. I'd replace the radiator about 1 1/2 yr before, so I checked fanclutch, temp sender, temp gauge, voltage stabilizer, and thermostat. I then checked the radiator, found it plunged over 50%. I had a rod out done at the radiator shop and no more problems. The higher the speed the more fuel burn, the more heat, and a blocked radiator just can't get rid of it.

^^^ this^^^ good info same symptoms. I'll check also!

I think I suggested that the thermostat might be worn & not opening fully. Time to replace.

I did a little test today.radiator was clean of oil and such. Water pump looked new inside. Tight with no shaft play. Thermostat had made in Germany on it. I boiled it in water. It started to open at 180ish. But only opened 1/4 in. I boiled water to 210 and it stayed 1/4 in. Open. Is this normal? I figured it would open more. Also what is the stock rad cap pressure? Mine says 15lbs

Another one I've seen before is the lower hose collapsing under higher engine loads. I don't believe it's possible with the Cummins all the lower hoses I've seen are reinforced but on smaller engines where it is a straight hose when they get thin the water pump will actually suck the hose in partially at higher loads which causes aenough of a restriction to overheat an engine, esp in hot weather JR now Free

I've seen old hoses fail internally. Had one on my 83?? Fird Ranger with 2.2 Diesel. Lower hose separated & innards obstructed coolant flow while exterior looked normal. It was hard to feel in that location.

Well I replaced tstat with a cummins replacement. Flushed cooling system and washed out radiator. My problem is fixed. I still have no heat but that's a different problem. Thanks for they help guys

Back flush the heater core, I wonder if somebody didn't use some coolant leak fix and that does wonders on the cooling system

Yes I did flush the heater core. And the blend door moves. I had a vac leak but I fixed it. The hvac has low volume of air and a whistle sound with fan on high. The sound goes quite when I reduce blower speed. I took out blower and it works fine and quite. Could cracked heater core cause noise. Mainly when I'm at cruising speeds. I do have coolant smell in cab. Tested all vac lines and they all hold 8ins vac

I guess if its plugged with dirt it would reduce the airflow and could cause a whisyle. Did the recirc door look good? It sits right above the blower motor. On 3rd gens when it breaks, it causes airflow restrictions