Let’s imagine a worst-case scenario: you are pulling a trailer at max GVWR with your 6.7 Cummins up an 8% grade that goes on for 25 miles non-stop, all while trying to maintain an 85mph speed because you’re late for dinner. That’s a solid 20 minutes with your right foot welded to the floorboard, and its torture for your truck. We’ll ignore the fact that you must be driving in the Swiss Alps, and for the purposes of conversation say the temperature gauge is busted so you have no idea how hot your engine is getting. The question is, what really happens to your oil or coolant if it gets too hot? Of course you’ll have the obvious mechanical damage like a warped head or blown head gasket, but for today we’re just going to focus on what happens to the fluids.
Have you ever wondered why the grille of your diesel truck is so large or why there are so many openings in the front for air to pass through? As it turns out, the byproduct of operating a diesel pickup is a lot of heat, especially when its being worked to its max with a trailer, snowplow, or even just climbing a steep hill. Nearly every mechanical system on your truck uses some sort of fluid to provide lubrication and cooling, and that list includes the engine, transmission, power steering, transfer case, differentials, and even the fuel system.
Because heat is transferred into the fluid from whatever component its working in, manufacturers install several radiators on the front of every vehicle and circulate fluid through them. As the truck drives down the road, ambient air is pushed across the coolers to bring the temperature of the fluid back down. The larger and more powerful the vehicle, the more cooling work needs to be done, which explains the massive grilles on late model pickups. Under ideal conditions, the temperature of each fluid will be maintained at the optimal level, but every once in a while, the stock cooling system will be unable to keep up with driver demands. When this occurs, the fluids will continue to get warmer and warmer until the operator lets off the throttle.
Gauging Interest
Before you can identify a heat problem, you need to know how hot something really is, and unless you walk around with a laser thermometer in your pocket, you likely won’t know the temperature of every drivetrain component on your truck. Almost every diesel comes with a transmission temperature gauge in the cluster, and certain newer Ram and Superduty trucks have an option to display engine oil temperature, but what if you have an older truck or just want more information displayed at all times? Often, your engine’s computer knows more than its showing, and if there is a temp sensor installed anywhere in your pickup, chances are the Banks iDash can tell you how warm something is, even if there is no gauge in the cluster. If you must know the temp of your rear axle or have a non-computerized truck like a P-pumped Cummins, then you can go the old fashioned route and install an analog temperature gauge for just about every system imaginable, and yes, they do even offer gauges specifically for a differential.
If You Can’t Stand the Heat
Engine coolant is made from a mixture of water and Ethelene Glycol, and if you pressurize the system to 15psi, it raises the boiling point to 268 degrees F. If you somehow manage to exceed the boiling point, you’re really not going to cause any harm to the coolant, and the only thing which might happen is you will lose a little liquid through the overflow bottle due to expansion. All you need to do is wait for things to cool all the way down, top off the coolant, and you’re good to go. It’s nearly impossible to damage water.
As forgiving as your antifreeze is, the other fluids in the truck are a different story. Petroleum based oils have a certain temperature range they can safely operate in, and if you allow the oil to get too hot, it will start to chemically break down, and a few things will happen. First, its lubricating properties will diminish, but in addition the overheated oil can release acids and other harmful components which over time can damage the internal parts of your engine, and finally, the oil can even change in viscosity. In short, if you overheat your oil long enough, it can turn into a nasty sludge which may harm the moving parts inside your engine. Of any automotive fluid, the oil inside your engine is the most likely to overheat, but the same concepts apply to your automatic transmission and differentials as well, it’s just a lot harder to get those components to the same dangerous temperature.
Cooling System Maintenance
We can all agree that hot oil is bad, so if you are struggling with an overheating truck, what do you do? If you ask 10 different people, you’ll get 10 different answers, but the first thing I always recommend, is to look for some dirt, and I don’t mean gossip around the neighborhood. You can bet your truck was used in a dusty or dirty location at some point in its life, and with enough time and miles, that dirt can build up on the fins of the radiator and intercooler, which blocks airflow and prevents heat transfer, and makes the truck run hotter.
A simple trick you can try before spending any money on upgrades is to wash the cooling stack. A popular trick I’ve seen involves creating an attachment for your garden hose which allows water to be gently sprayed from the back of the radiator. You can pick up a can of radiator cleaning spray for a couple bucks, and in some cases, overheating issues have been cured simply by cleaning the radiator. Another trick I’ve seen many locals do in the South is to install window screen behind the grille to prevent bugs and other wildlife from damaging the delicate fins of the radiator. While it may not seem like a big deal, a couple dirty or bent fins can make a big difference on the operating temperature of your coolant, and ultimately, the engine oil.
Managing Heat
If you increase power output or ask your truck to do more than it was designed to do, the side effect will be more heat generated, and you need a way to deal with the excess heat. Luckily for us, the aftermarket has provided plenty of solutions. On the engine side of things, you can start with a thicker welded aluminum radiator, which has a greater surface area than stock, which will quickly remove any excess heat from your engine and allow the thermostat to regulate the temperature like it was designed to.
Just about every modern diesel has some sort of engine oil cooler mounted to the block, and a popular style is a liquid to liquid heat exchanger because it’s compact, and the OE’s have more flexibility when choosing mounting locations. As oil runs through the cooler, heat is transferred into the engine coolant where ultimately the radiator is responsible for rejecting the heat. This is why a properly functioning radiator is so important. However, in order to work properly, the tiny passageways in the heat exchanger must remain clear. One engine in particular has an ultra-sensitive oil cooler, and that’s the 6.0 Powerstroke. Tiny particles or debris in the coolant can plug up the oil cooler on the water side, and when it fails, the first indication will be the temperature of the oil. Ideally, it will be within 10 degrees F of the engine coolant temp, but if it gets significantly warmer, it’s a sure sign the oil cooler is clogged, and a replacement is needed, although a coolant filter kit could have prevented this from happening.
Generally, a liquid to liquid style heat exchanger can keep your engine oil temperature under control, but if you have an oddball diesel without an oil cooler, or you regularly work your truck beyond its limits, it is possible to add an external oil cooler into the system to supplement its cooling power. There are some ready-made kits out there for popular applications, or you can install a universal cooler from BD Diesel which can remove 40,000 BTU’s per hour from your oil.
Finally, consider where the heat is coming from: combustion. By increasing the efficiency of your engine, you lower the temperature of the exhaust gasses, which means there is less heat to be transferred into the coolant and the engine oil. It may not sound like a direct correlation, but parts which increase airflow can actually help control temperature as well. Just look at the LLY’s with their restrictive turbo inlet: by simply replacing it with a less restrictive LBZ style elbow, you could cure some of the overheating problems, and the same concept applies to parts like a free-flowing exhaust, cold air intake, and even a larger intercooler. The less heat you generate, the less heat you will have to reject.
Transmission Temps
Your automatic transmission isn’t affected by how hot your engine coolant is, the only thing it cares about is how much power and torque the engine puts out and how much weight is behind the truck. As the transmission works harder, the heat level goes up, and just like the radiator for the engine, your stock trans cooler can only reject so much heat. Plus, if you have a “built” automatic transmission with higher line pressure and a higher stall converter, both of these will put more heat into the fluid than a stock trans would. An oversized transmission cooler can solve those problems by improving heat rejection by up to 60%, which keeps your transmission fluid up to 30 degrees cooler than stock, and that goes a long way toward keeping your ATF nice and fresh between changes.
Fourth gen Rams with the 68RFE or AS69RC have one more complication when it comes to managing the transmission temp, and that’s the recirculating thermostatic valve FCA installed between the transmission and the fluid cooler. It was designed to operate like the thermostat in your engine, so it would help the transmission warm up quicker and maintain a minimum temperature, but all too often the valve just sticks, and the fluid never makes it to the cooler so the trans gets way too hot. While you can install a replacement it’s best eliminated altogether, and the ATS Thermal Bypass Valve Delete is an inexpensive way to do just that. It installs in your transmission cooler lines in the same exact place as the stock thermostatic valve, but it’s essentially just a hollow block of aluminum which allows fluid to pass through freely.
Cool Down Your Differential
Everyone knows the importance of keeping the engine and transmission temps under control, but your differential often gets forgotten. A quick drive around town or blast up the freeway might only get the gear oil up to 130 degrees F, but when you throw a trailer on the back and have to use a higher sustained power level, the temperature of the rear gear oil can rapidly climb and get as high as 300 degrees F, which is a danger zone for the fluid. While it is possible to install a scavenging pump and a fluid cooler into a differential, that’s usually reserved for a competition environment, but the next best thing is the new differential cover from Banks. Its finned aluminum construction increases the surface area which improves heat transfer over the stock steel cover, but its engineering genius has to do with the small air scoops which sit on the bottom of the cover, and direct airflow up and over the back to pull away even more heat. Their testing has shown a decrease in fluid temperature of the rear end by a staggering 61 degrees which is simply mind blowing and has totally revolutionized the differential cover for the better.
Clean on The Inside
Sludge and sticky varnish can build up over time inside any engine especially if its overheated, and short of disassembling it for a full rebuild, there aren’t a lot of ways to clean the insides. HotShot’s Secret Stiction Eliminator is the next best thing, however. It’s an oil additive you use on every third oil change, and it will remove the nasty build up from critical surfaces like HEUI injectors, turbo bearings, piston rings, the oil pump, and more. And finally, consider selecting a premium oil next time you do your change, because synthetics have a much higher tolerance to heat, and will degrade less in a harsh environment. I’ve always said basic maintenance is key to a long and happy life from your truck, and regular oil changes are still top of the list but making sure the temperature of every lubricated system is kept under control is just as important. So, if you have to install a few extra gauges to keep an eye on your temperature, it’s a smart investment because they will let you know if you need to upgrade some hard parts to keep your fluid temps regulated. It all comes down to this: if you take care of your truck it will take care of you, and a cool truck is a happy truck.