Resolving overheating 12 valve Cummins turbo diesel

Truck overheated on a cool winter, late afternoon, while crawling up a hill in snarling Los Angeles traffic. Was slowly inching alongside a highway detour to bypass a wreck as it poured cats and dogs from the atmospheric river before Christmas 2021.

As I crested the hilltop, noticed a late model Dodge truck pulled over on the shoulder with its hood up, steam billowing out like a geyser. Ouch, I thought, that’ll slow down their journey. That’s when I glanced down at my own temperature gauge and saw the needle pinned in the read at 230*F !?

Immediately pulled over and shut the 12 valve Cummins down. What’s going on? The cooling system on the relatively new to me truck hadn’t yet been gone through, as it hadn’t exhibited any major problems.

Although, it did once spit out some coolant from the passenger side lower vent and onto the transmission tunnel carpet, months ago when first test driving and taking delivery of the truck from the desert. Coolant would never came out again. That indicated a heater core, so early on procured a replacement unit but was certainly not looking forward to the invasive procedure under the brittle dash to reach it. The original dash lived under a felt pad and was miraculously intact save a single crank.

Along with anticipating a heater core and the imminent day I’d open up the coolant system, had also procured a Cummins 180*F thermostat, Gates water pump, and 6 gallons of Zerex diesel coolant , the premixed red solution.

Prior preventative on the motor was simply a new radiator cap and fan clutch as the original would not spin freely by hand.

Waiting for the truck to cool on the side of the road from an overheat would take ages. So I flipped on my white & amber light bar to strobe over the cab and coasted down the shoulder at engine idle speed to get some air flow over the radiator, merged back into traffic all while willing the head gasket to hold together.

The temperature gauge still would not come out of the red, so in order to ram more air over the radiator, hooked a right at the end of the road and into the direction of reverse traffic. The motor finally started to cool as the truck got up to speed and I dropped the transmission into neutral to bring the RPMs down to idle while coasting.

Likely a sticking thermostat, at the most inopportune time on a trip before the Christmas holiday and hundreds of miles away from home in weather!

Made it home in the middle of the night after avoiding further gridlock traffic and without further temperature spikes. No evidence of water in motor oil, nor oil in coolant. Now to diagnose.

First, started with a coolant flush by uncorking the radiator petcock and draining out the very dark green fluid. Next, added a coolant system detergent and topped off with distilled water to begin flushing process. The cooling system for the 12 valve takes a total of 6 gallons including reservoir capacity, and about 3 gallons come out at a time from the petcock with closed thermostat.

Upon 2 cycles of flushing and topping with distilled water, both test drives on the freeway exhibited erratic temperature where it would climb to 200*F on its way to overheating, then the thermostat would finally open and drop the temp to normal 180*F operating temp. Happened at exactly the same freeway exit both times so exhibiting consistent behavior.

Next, by passed the heater core by taking one of the 5/8″ ID heat hoses to the firewall and creating a u-turn back to the motor. The truck still continued to show erratic temperature.

Finally, time to break the seal on the thermostat housing and get to the likely root cause. Thankfully on the 12 valve Cummins it is relatively easy to get to, requiring removing the accessory belt, unbolting and loosening the alternator bracket to pivot it out of the way, and 3 bolts that fasten the thermostat housing.

The intent now is to isolate and test the thermostat in boiling water, and continue to flush the system now completely open without a thermstat.

Interesting how the flushing drastically changed color once rust tinted water from the cast iron block is drained, as opposed to only from the aluminum radiator.

To test the thermostat and verify it sticking, simply boiled a pot of water on the stove and used a pyrometer to measure when the thermostat was in fact opening. Turns out it indeed didn’t open until over 200*F.

Continued flushing the open coolant system with water and detergent. The diesel motor certainly does not like starting after sitting all night in the 30’s*F temperature, and of course will not warm up without a thermostat even on surface streets.

Took the opportunity to also verify the new thermostat, rated to open at 180*F, indeed operates normally with the stovetop boil test. Final phase of flushing was to use a garden hose and flush the radiator with lower hose disconnected, and also open the petcock drain and poke it clear of any debris; then likewise flush into the motor from the upper hose.

Wrapped up the resolution by installing the new thermostat and seals, reconnected the heater core with new hose, topped off the coolant, and recycled used engine coolant at a proper disposal facility.