One thing that can cause an overheat that likely would not show up as a leak is a sticking thermostat. These can be tricky to detect. You can often feel the radiator hoses. If the top hose is cool or cold to the touch and you know that you’re at operating temp, you might have a stuck thermostat. It’s a common misconception that both hoses should be hot to the touch when the engine is running. This isn’t necessarily true; the upper hose normally contains the hot coolant exiting the engine to be cooled by the radiator. The lower hose is the return and should be considerably cooler than the upper hose. This means the radiator is doing its job. I get a lot of questions about this very thing and I’d like to set the record straight: With the engine at operating temperature, upper hose hot, lower hose cooler than the upper. This is normal.
Getting back to the thermostat: If you suspect it’s bad, you can prove that. Just remove it from the engine and insert it into boiling water. If it’s working properly, it should open up when it reaches its threshold temperature, usually between 180º F and 220º F; if not, it’s bad, and you should replace it.
Many times you’ll see a temperature rating stamped on the thermostat. As long as the water is at that temperature, the thermostat should open up. As for what to replace it with, I always recommend OE (original equipment) for thermostats. Engine temperature is very important in modern fuel-injected engines, and the best thermostats are the OE ones in my experience. I’m not saying don’t use aftermarket thermostats, as I know many of you are trying to save money and in some cases the OE thermostat might not be available; I just recommend an OE thermostat for best results. The takeaway is to use OE when possible when it comes to thermostats. I’ve always had good results with original equipment thermostats, but not so much with some aftermarket brands. Remember that I mentioned earlier that it can be difficult to bleed some cooling systems because the thermostat didn’t open? Nine times out of 10 it’s an aftermarket thermostat (and sometimes a aftermarket radiator).
Lastly, do not alter the temperature range of your thermostat. As stated, engine temperature is critical to a fuel-injected engine; change the thermostat’s temperature rating and you’re asking for trouble. Put in what the manufacturer calls for; if you don’t, it can compromise fuel economy and performance—so don’t mess around here.
Believe it or not, your radiator cap is a key component of the cooling system. Other than keeping the coolant in the system, it serves some very important functions. For every pound of pressure listed on the cap, it raises the boiling point of the coolant 2º F. So if you have a 15-lb. cap, it raises the boiling point of the coolant mixture 30º F. This means that if you have a bad radiator cap, your cooling system won’t be as efficient as it could be.
The radiator cap serves another function, which is to control the flow of coolant during temperature changes in the cooling system. When the system is at operating temperature, the coolant expands, and it needs a place to go. A valve inside the radiator cap allows the heated coolant to exit the system into the overflow tank. When you shut the engine off and it begins to cool, another valve inside the radiator cap opens up to help draw that coolant back into the system. As the engine cools after you shut it off, the coolant inside the engine contracts and helps create a vacuum that draws the coolant from the overflow back into the system.
If either one of these valves fail, it can cause issues with the cooling system. A bad radiator cap can cause air to enter the system, or it can release coolant too early, causing the overflow tank to overflow and boil over.
In summary: Don’t forget about the radiator cap, as it is an important part of the cooling system. In fact, it’s often considered a maintenance item and is sometimes replaced during a service. I recommend using OE radiator caps whenever possible.
Another hidden cause of an overheat is a blown head gasket OR as it is better said, “a combustion leak into the cooling system”. The truth is you don’t actually know a head gasket has failed until you remove the cylinder head and inspect it for damage. It could just as easily be a cracked cylinder head or worse, a cracked block that can cause the very same symptoms. A head gasket failure is the most common cause of a combustion leak into the cooling system but NOT the only one so keep that in mind. Come to think of it, a leaking intake gasket can also cause very similar symptoms to a head gasket failure. Things like a LOT of white smoke coming out the tail pipe after the engine has warmed up. I get a lot of people coming to me saying their head gasket is bad because they see white smoke coming out the tail pipe OR they see water dripping from the tail pipe. The truth is that water is a natural byproduct of combustion so seeing white smoke (steam) and water out of the exhaust is normal. You need to make the determination if it’s excessive or not. If the weather is warm (above 50ºF 10ºC) and your engine is warmed up and you’re still seeing an excessive amount of white smoke coming out the tail pipe this could be an indication of a combustion leak into the cooling system. To determine if thats the case you can use the ‘Block Test’ shown in the above video. That test should tell you if you have combustion gasses in the coolant which would indicate a combustion leak into the cooling system. Another test that can prove out a combustion leak is a leak down test. With a leak down test you run compressed air into the suspected cylinder and look for leaks. If you see bubbles in the radiator or expansion tank when you pressurize a cylinder this can also prove a combustion leak. Here’s a link to a video I did on Leak Down testing.
The truth is tracking down internal coolant leaks can prove difficult. The only way to truly know what the issue is is to tear into it and do a visual inspection of the suspected cause. The above tests are a great start but as I said, not always conclusive. When you do get things apart the coolant leak is often evident. You’ll notice that the cylinder coolant was leaking into is often very clean and free of carbon deposits. This is due to the ‘steam cleaning’ effect the coolant has on the combustion chamber.
Another symptom you might run into is milky oil, or oil that looks like frothy chocolate milk. This is usually the result of coolant or water mixing with the oil. This could be caused by a leaking head gasket, a leaking intake gasket, an oil cooler leak, or some other instance where coolant can leak into the crankcase and not directly into the combustion chamber. This is bad, because the oil will not be able to lubricate properly if it’s contaminated with coolant or moisture.
These leaks can be harder to track down because they are internal leaks. The best way to confront these is to start with the easy stuff and take it from there. For instance, start by removing the intake manifold and inspecting the intake gasket for leaks before going for the head gasket. Or, if you have an oil cooler, check it for leaks first before digging deeper into the engine.
One thing to note before I close this topic is that this condition can also be caused by “short tripping.” Short tripping is when the vehicle only gets driven a short distance and rarely gets up to operating temperature. This causes a buildup of condensation inside the crankcase, which can exhibit the exact same symptom of oil that looks like chocolate milk.
To address this issue, take the vehicle on a nice long drive to purge the condensation from the crankcase. If it was an internal coolant leak that caused the problem and you’ve completed the repair, I recommend you change the oil. I do this after just about all major engine work. Then I recommend you take the car for a long drive to make sure the repair took and the problem has been addressed. If you still see signs of coolant in the oil, change the oil again, take it for a drive and recheck. If you still have it then, you might need to keep looking for the source of the leak.
Aside from coolant leaks causing a loss of coolant and air pockets that cause an overheat, a few more things can contribute to your engine overheating. I’m going to call this a breakdown in cooling system efficiency. For this category we’re going to include things like:
- a clogged radiator
- a blocked radiator (airflow)
- a blockage in the cooling system
- a faulty water pump
- a cooling fan issue
I know that’s a lot, but they all cause the same thing: a lack of cooling efficiency. These are probably the toughest of the group to diagnose since they might not cause a full-blown overheat, but they can cause your engine to run hot.
A cooling fan that isn’t working is fairly easy to spot. If you see that you’re overheating and the cooling fan isn’t coming on, this could be the cause of the overheat. The classic symptom of a cooling fan failure is an overheat while you’re sitting in traffic, but when you start moving the temp starts to come back down. There are about a hundred reasons I can think of that can cause a cooling fan not to operate, including air in the cooling system (see the trend here?). Given that and the fact that different manufacturers handle cooling fan operation differently, I’m only going to speak in general terms here.
Say your vehicle is overheating and it has an electric cooling fan(s) that isn’t working. The first thing I often do is check the fuses. A blown fuse is a common problem, and is the easiest thing to fix. If you strike out here, and you’ve bled the air out of the cooling system, then it’s time to do an electrical diagnosis.
I normally start with the fan itself by running jumper wires to the connector to check its operation. Most cooling fans are simple motors and just need a power and ground to run. 12 volts straight from the battery should do it along with a wire run to battery negative or a good body ground. Just touch the wires to the terminals inside the cooling fan connector to see if it runs. In fact, you can often hook them up backward and the fan will still run, but in the opposite direction. Just know that the black wire is normally the ground side of the circuit, so try to hook that one to battery negative or a good body ground. Body grounds are any bare metal spots on the chassis. Sometimes you’ll see wires that are just bolted to the body; these are body grounds and a good place to use as a ground if you need one. For diagnostic purposes, you’re just trying to see if it works or not. If it doesn’t work, it’s likely that the fan motor is bad and needs to be replaced. If the fan does run when you supply it with power and ground, you need to keep looking. From there, you would need an electrical schematic for the cooling fan circuit and some basic electrical testing equipment. See the electrical section for more information about that.
If you have a thermostatic fan, which is a fan that’s not electric but driven by a drive belt on the front of the engine, you handle things a little differently. This is a thermocouple device that is supposed to lock up when it gets hot enough and run the fan at full power.
These types of fans came about to help improve fuel economy. By design, they are only supposed to work when they heat up, and when not needed they kind of free wheel, which allows the engine to work less, thus saving fuel. The symptoms of a failed thermostatic fan are the same as an electric fan failure: You’ll start to overheat while you’re sitting in traffic, but when you start moving, things will start to cool down again.
An easy test is to spin it. Obviously don’t do this while the engine is running. With the engine off, just grab one of the blades and give it a spin. You should feel some resistance and it should only spin about a half a turn or so. If it spins freely, or keeps spinning after you spin it, then replace it. Also, if you see fluid on the outside of it, this could indicate that the fluid inside the assembly has leaked out; if that’s the case, replace it.
Another old-school test is to hold a piece of paper in front of the radiator when the engine is running and up to temperature. The paper should be pulled toward the radiator if the fan is working properly. If not, it might not be moving enough air through the radiator to cool the system. This one is a bit more difficult to nail down because it’s a mechanical device, but if you use the above information, it should guide you through it.
As long as we’re talking about cooling fan operation, I think the fan shroud is at least worth noting. For the most part, it might seem like a hunk of plastic that gets in the way when you’re trying to work on the engine, but it does serve a particular function. That function is to channel the air through the radiator to maximize the cooling fans effects. Without it, the fan cannot properly pull air through the radiator. A vehicle that doesn’t have a fan shroud and should is likely to overheat in traffic in the same way it would if the fan was not working. So if your vehicle is supposed to have a fan shroud and doesn’t, find one, replace it, and recheck for your overheat problem.
Clogs in the radiator or somewhere else in the cooling system can be a bit harder to nail down. The best way to find problems like this is with an infrared thermometer.
With this tool, you can check localized temperatures within the cooling system. One example is using this tool to check the temperature of a radiator across its surface. If you’re checking the temperature and you come across a sudden change in temperature, this could indicate a blockage in the radiator. On a normal radiator, you should see a gradual change from hot temperatures at the top to cooler temperatures at the bottom of the radiator. I don’t see radiator blockages often, but when I do, this is a good way to find them. When I see this problem, it’s often associated with improper use of stop leak products. I’ve also come across it on vehicles that have been sitting for long periods of time. Say the vehicle has a salvage yard engine; that engine might have been sitting at the salvage yard for some time and, as a result, accumulated a lot of rust buildup inside the block. Once the engine is running again this rust and scale breaks up and clogs the small passages inside the cooling system.
I’ve also seen this same issue arise from not running coolant in the cooling system. I’ve had viewers from tropical climates tell me that they just run straight water in their cooling systems. This is a very bad idea because coolant also lubricates the system and prevents corrosion. It also helps raise the boiling point of the mixture and assists in cooling system efficiency. Running straight water instead of coolant can cause the entire system to corrode from the inside and possibly cause the above-mentioned blockages. So don’t run straight water in your cooling system, no matter what your climate is.