Cooling systems are very simple, and determining WHEN you are having heating problems will tell 95% ohe story. First things first. The entire cooling system only consists of a few simple things; a radiator, a water pump, a thermostat, a couple of pulleys, a fan belt, a fan, and a coolant recovery bottle.Keep in mind, horsepower makes heat, because to make power, it requires burning more fuel. With that, lets go with the most common heating problem first. In street rods and performance cars, the single most common overheating problem is when idling in traffic or sitting in the drive-thru waiting for your burger and fries. If a car heats up while sitting, but not when it drives down the road, it is telling you something. What it’s telling you is that when driving down the road, the cooling system is doing it’s job just fine because you aren’t overheating when the vehicle is under a load and burning more fuel. Think of it like running. Standing there you aren’t out of breath, right? Now run down the road. You’ll be out of breath in no time because you are “exerting” yourself. Well, when a car is driving down the road, it is basically “jogging” so it’s a time that is more demanding on the cooling system than when it is just sitting there idling because it is breathing heavier and burning more fuel. Like standing there, you don’t require much air because you aren’t running or jogging. Well, when idling, you aren’t straining the engine or burning much fuel to make much heat, so why is it then that you overheat at this time wne your engine isn’t creating much heat? The main difference with the cooling system between driving down the road and idling in traffic is the amount of air coming into the front of the vehicle and getting through the radiator. When you drive down the road, (after about 25 MPH or so) the air being forced through the grill and radiator exceeds what the fan can pull through it. So in essence, after about 25 or 30 MPH, the fan is pretty much useless and isn’t needed. With that, below 25 or 30 MPH the fan is the ONLY thing pulling air through the radiator. No air coming through the radiator (or at least no ENOUGH air) is the single most common problem with cars running hot in traffic or when idling.
You have to look at a few things. First; is the fan big enough to draw enough air through the radiator at an idle to let the radiator do its job? Many times hot rods come in with tiny little fans because they look cool. Well, they don’t look so cool when they can’t participate in parades or cruise through town because they overheat in traffic. Sometimes these fans are chrome, sometimes they’re stainless, and other times they’re colored fiberglass, but most of the time they’re just too damn small overall to pull the volume of air needed to allow the radiator to do its job like what you see in the picture to the right. A decent sized fan would MUCH bigger blades and would be bigger in diameter, such as 17″ or 18″. Look at how thin and tine the blades are of the “cool looking” fan to the right. That’s NOT what you want on your car if you want it to pull any air volume through your radiator.
Second; How far away is the fan from the radiator? Is it close enough so it only pulls air THROUGH the radiator? Also, is there a shroud to cause that fan to only be able to pull air through the radiator? Pretty much every single factory vehicle has a fan that is engineered to be big enough, and with enough blades, to pull a given amount of air through the radiator to keep that engine cool, and to ensure that air ONLY gets pulled through that radiator, and to do this they are almost always encased inside a shroud. Hot rod guys never want to run a big fan or use a shroud because they don’t look cool. Well, neither does overheating! If your fan is more than about an inch or so away from the radiator, then it’s too far away and you need to get a spacer and get it closer. A shroud may also be needed of you can’t get the fan close enough to the radiator.
Third: are the pulleys the right size to allow ample fan speed at an idle? Power pulley kits spin the fan slower to make the horsepower gains, and slower fan speeds mean less air being sucked through the radiator. I see a LOT of hot rods and performance cars come-in with aftermarket pulleys that are NOT the correct size to spin the fan at a speed fast enough at an idle to pull ample air to keep the engine cool while in traffic. Most likely the engine in your hot rod originally came out of a car that had a bigger fan that was close to the radiator, encased inside a shroud, and with pulleys designed to spin that fan at a speed fast enough to draw enough air through the radiator at an idle to keep that engine cool, and most likely you have taken all of that and thrown it away and replaced it with a smaller, cooler looking fan, you shit-canned the shroud, the fan is “probably” 4 to 6 inches away from the radiator, and is spinning a couple of hundred RPM slower than it needs to be at an idle to pull enough air through your radiator to allow the radiator to do its job, which is why you overheat in traffic. Pretty much, those are the only causes and fixes to cars that run hot in traffic. Saying you make so much horsepower that it causes your engine to overheat in traffic (which a lot of guys claim) is just BS! When your engine is idling, it isn’t burning any fuel, nor is it under any load to create any real heat. We build 1,000 HP street engines quite often and they’ll idle all day long in traffic without ever running hot. It’s because we design the cooling system to be able to do it’s job, which is something most guys just don’t do, which in turn is why a LOT of hot rod guys have overheating problems in traffic.
Now, let’s say your car runs cool at an idle but runs hot when driving down the road, or when pulling hills. Well, the first thing to check for is if the radiator is full. Radiators don’t work when they are partially full. The second thing would be to make sure it isn’t clogged. A radiator can’t do it’s job if it can’t flow enough water through it so the air flowing through it can wick away the heat. It also may be too small of a radiator for your engine. You can’t expect a stock radiator, that was designed for a stock engine, to keep a serious performance engine, or a big stroker cool, so make sure your radiator is large enough to do it’s job. Another thing to look for is the water pump. It may not have the water flow capacity to keep your engine cool, so a new high flow pump may be needed, OR, possibly one or more of the impeller blades have rusted off, or the impeller itself is slipping on the shaft and not moving the water it needs to move. Again, if you have “power pulleys” or some sort of aftermarket pulleys, chances are the water pump and/or the crank pulley is the wrong size which can slow down the speed at which the water pump is spinning and pushing water.
Another thing to look for is the lower radiator hose. The lower hose should have a spring inside of it. This is because the bottom hose is the suction side where the water pump sucks water out of the radiator and pushes it through the engine. When the spring is removed or it simply rusts and disintegrates, this leaves nothing to prevent the hose from collapsing under the suction of the pump. If the hose sucks closed, it cuts-off the water supply to the engine. This is easy to check. All you have to do (with the engine OFF) is reach down and squeeze the hose. You should not be able to squeeze the hose closed very easily. Sometimes the spring inside gets sucked-up inside the water pump inlet a few inches, which exposes a few inches of lower hose to be vulnerable to being sucked closed, especially after the engine has warmed-up and the hoses become even more soft and supple. This whole issue is made worse when high flow water pumps are used and if your radiator is a little bit clogged because the pump is trying to pull more water volume through the radiator than the radiator can flow so the hose (being the soft / weak link) gets sucked closed.
If your engine overheats on occasion OR all of the sudden just started heating-up, this indicates a sticky or stuck thermostat. The fastest way to check for this problem is to start the engine, and let it warm-up and observe the temperature of the upper hose with your hand. WATCH OUT FOR THE FAN!!! Keep your fingers away from the fan or you might end up with a couple of less fingers! If the engine gets warm BUT the upper hose stays much cooler than the engine, it is telling you that no water is passing through that hose. If the engine is hot, then the water in that hose should also be hot. If it ain’t, it’s because the T-stat is closed and it isn’t allowing any hot water to get out of the engine and into the radiator.
The second easiest test is to simply remove the thermostat and test drive the car. It will most likely run cooler, and if it does, then it’s telling you it was the T-stat stuck shut. In text books, you’ll read that by removing the T-stat, the engine will run hotter because the water passes through the radiator so fast that the radiator doesn’t have time to cool it down. I have to tell you… this isn’t my first day. I’ve been doing this for about 30 years, and to this day, I have NEVER seen an engine run warmer when the T-stat was removed… NEVER. They have always ran cooler for me, and this goes for EVERY Chevy, Ford, Chrysler, flathead, in-line, pancake, stock, performance, race or whatever kind of engine you can think of, and we’re talking hundreds and hundreds of cars and trucks, not a just a couple.
Now; for you guys that want to “believe” an engine will run warmer when the T-stat is removed because you think water flows through the radiator too fast and won’t have time to cool, I have a few things to say. First: You’re smoking crack or something! Think about it for a moment. What happens when you remove the T-stat? You increase the water flow. If water is flowing too fast through the radiator to get rid of the heat, remember that the cooling system a closed loop system which means the water is also passing by the heat source inside the block too fast to pick-up too much heat. Just remember, for every action there is an equal reaction. Second; If increasing water flow through the cooling system is SO bad and causes an engine to run hotter, then please go ahead and enlighten Edelbrock, Stewart, Weiand, and ALL of the companies that make HIGH FLOW water pumps to cool engines down MORE than standard flow pumps do, because obviously they don’t know what they are doing then according to your logic. If decreasing the flow & speed through the radiator was the key to cooling an engine, then don’t ya think that maybe all of the companies that make after market water pumps would make LOW flow pumps instead of HIGH flow pumps? But they don’t now do they? No, they make HIGH FLOW pumps that INCREASE the water flow, speed, & volume. Well, by removing the T-stat, you are removing a restriction in the system which will now make the water flow faster and with more volume. Just remember; ALL a T-stat does is keep the water from flowing so the engine will warm-up faster so your heater will work quicker, and so the engine will warm-up and run more efficiently. Once that T-stat is open, if your engine wants to run warmer, it will continue to rise until the engine reaches it’s normal running temperature. All it means is the engine can not drop below what the T-stat is calibrated at, so if it is a 160 degree T-stat and your engine wants to run at 200, then it’s going to run at 200 degrees no matter what. All the 160 degree rating means is that the T-stat won’t open to let water flow through the engine until it reaches 160 degrees. Remove the T-stat and you remove a restriction, which WILL cause the engine to run cooler just like a high flow water pump will when it “increases” the flow. But that’s right.. I forgot… doing that makes the engine run warmer, at least according to what a LOT of guys believe anyway, so please… all of you guys that say an engine will run warmer when you increase the flow by removing the T-stat, make those calls to Edelbrock, Stewart and Weiand and let them know that their high flow pumps are causing people’s engines to run hotter because they increase the flow, LOL!!
When you have an engine that doesn’t warm-up as fast as it used to in the mornings, then it’s telling you the T-stat is probably stuck open, and isn’t allowing the water in the engine to warm-up fast enough anymore. With thermostats always remember to install it correctly! The side with the spring goes into the motor. If you install one upside down, (with the spring facing up, or towards the radiator) it will take too long for it to open and the engine will overheat before the T-stat opens.
Let’s say your engine runs cool for a few days, and then starts running warmer and warmer until it finally starts running hot and you have to add water. This is usually caused by not having a water recovery bottle or by not having a proper radiator cap for a recovery system. On cars equipped with nothing more than a puke tube for the water to just dump-out, onto the ground, here’s what happens; each time you run the engine, the water expands, which causes the radiator cap to open at its given pressure setting and puke some water on the ground. Lets say your radiator holds 3 gallons of water. The first day you run it, you puke-out a cup or so of water. The next day, you puke a couple of more cups. After a few days of this, you have puked-out a half gallon or so of water from the radiator. Now you only have 2 1/2 gallons (or less) of water in there. With less water, the engine will start to run a little warmer, which in turn causes even more heat and expansion, and even more water to puke out of the radiator. Now you are down a gallon or so of water, and guess what, the engine starts running warmer each day you drive it until you simply over heat and have to add more water.
A coolant recovery bottle saves the water being puked out of the radiator and when the engine cools-down, it sucks all of the water back into the radiator again, so you don’t actually LOSE any water. It just passes back and forth instead of being dumped and gone forever. If you don’t have a coolant recover bottle (aka a radiator overflow bottle), then you should get one.
Another cause for an engine to run warm is if the timing is too far retarded. Every 10 degrees the timing is retarded can cause the engine to run around 10 to 20 degrees warmer than it should. When the timing is too late (retarded), it can also cause the headers to glow cherry red because the burn in the cylinder is happening to late and it can’t expend itself or finish by the time the exhaust valve opens, so that still burning fuel will often times cause the headers to glow. There’s more on where and how to properly set the timing on a performance engine on this tech tips page under “how to set the timing on a performance engine”.
If you just built an engine (especially a Ford or a late model Chevy), and you are having overheating problems right from the start, you have to make sure you installed the head gaskets pointing in the right direction. Some head gaskets only have water passages on one end of the gasket. If one (or both) of these gaskets get installed facing the wrong way, it blocks-off the water flow through the engine and you’ll have an instant overheating problem and the only way to solve it is to remove the heads and replace the gaskets with new one’s, facing the right way.
For you Early Mustang people out there, there is a wives tale about 289’s and 302’s running hot after being bored out for a rebuild. Factory spec is up to .060” and 99% of the time that is just fine and will not present any heating problems. In 65 and 68 Mustangs though, it is taboo. It isn’t the engine’s fault though, it’s the crappy radiator’s design fault. Have you ever noticed that 3 and 4 row radiators are extremely common in aftermarket ads for early Mustangs? You don’t see Chevy Camaro’s or Chevelle’s needing 4 row radiators, or 69 and up Mustangs needing 4 row radiators. You also don’t see any 5.0 Mustangs needing 4 row radiators. 5.0L engines are pretty much the exact same engine and have the same water passages as any early 289/302 in any 68 and earlier Mustang, so why do those early cars have overheating problems? The reason for this is crappy design of the early Mustang radiators with the location of the radiator hose inlets and outlets. If you look at one of these cars, you’ll see that the upper and lower radiator hoses are right in-line with each other, (one right over the top of the other on one side). This is about the worst thing you can have. The water only flows down the one side of the radiator and leaves the rest just sitting there. I mean, think of it like this, why should the water flow through the rest of the radiator when it is being pumped-in at the top and sucked-out of the bottom on the same side? The radiator will only flow up to what the upper and lower hose diameters will flow. In other words, if it only requires 8 or 10 inches of radiator to meet what those hoses are flowing-in, and flowing-out, then there’s no reason for the rest of the radiator to flow anything, which is exactly what happens and why pre 69 Mustangs have overheating issues. Look at the image to the right. That is what a typical pre ’69 radiator looks like. I marked in red lines about how much of that radiator is being utilized to meet the flow of the upper and lower hose diameters.
Ford made a change in ’69 to the water pumps and radiators by moving the lower hose outlet to the driver’s side and leaving the upper hose inlet on the passenger side, thus pretty much forcing the entire radiator to be used. No more overheating issues. See any 3 or 4 row radiators advertised for ’69 and up Mustangs? How about 5.0 Mustangs? Nope, because there’s no need to because the flawed design was fixed in 1969 when staggered outlet radiators were invented. 5.0 Mustangs never had overheating issues, yet they’re pretty much the same engine as any early Mustang had so if there was a problem with the engine itself, these cars would have over heating issues too, so don’t blame the early 289’s and 302’s or listen to wives tales created by people who don’t know what they’re talking about. If early 289s and 302s and had that bad of a heating problem, then 1: Ford would have re-designed the cooling systems on these engines for the later model 302’s and 351W‘s, (which they didn’t because there is no problem to begin with), and 2; Larger, heavier cars with the same engines (like big old Galaxies or trucks) would have even worse problems, and that simply isn’t the case. It’s only the early Mustang’s that had the cooling problems, so it obviously isn’t the engine’s fault for being bored up to .060″ over size.
If your radiator is doing its job, no matter what kind of car it is, you should see at least a 25 – 30 degree difference in temperature between where the water goes into the radiator (the upper hose inlet) and where the water comes out of it (the lower hose outlet). In other words, water coming out of the engine at the thermostat housing and heading into the radiator might be at say, 190 degrees. If the radiator is working properly and is cooling that water down, you should see about a 30 degree temperature drop at the lower hose outlet, so it should be about 160 degree temperature at the bottom hose outlet if the top was 190. If it is anything less than a 25-30 degree difference, then your radiator isn’t doing its job, OR something is preventing it from doing its job, such as the lack of air flow going through it. No air flow across all of those tubes and fins equals a radiator that simply cannot function the way it is supposed to. It can’t cool if A) water can’t flow through the entire inside, and/or B) if air can’t flow through it to wick away the heat.
Another thing to consider, so you aren’t chasing your tail, is WHERE your water temperature gauge sending unit is located. It should be near the water outlet on the intake manifold so it is reading the water temperature coming out of the engine just before it heads to the radiator.
A lot of guys with Chevys like to put their sending units in the side of the cylinder head, (where many factory Chevys had them), and this is perfectly fine as long as you understand that the head temperature, or at least the actual sending unit temperature itself, IS going to be about 25 – 30 degrees hotter than if it were located up on the intake manifold, simply because head temperature runs a hotter than manifold temperature, AND you have the exhaust right there which radiates 400 – 500 degree heat (at idle) directly onto that sending unit, so it is going to read hotter. Cars that came from the factory with sending units in the side of the head, were calibrated to read “normal” at higher temperatures, but if you’re expecting your aftermarket gauge to read a typical 180 – 190 degrees, like the majority of cars run when the sending unit is in the intake manifold, you are in for a rude awakening because it is going to read more like 210 – 220 or so down there.
One of THE best ways of finding issues and diagnosing where, and why, your cooling system is having troubles is to use an infra red temperature gun and actually SEE what the temperatures are at various locations. You can check the actual temperature at the front of the intake manifold, at the water pump, the top of the radiator vs. the bottom to see the temperature difference and how well the radiator is working, the head temperature, your exhaust temperature, (which by the way can tell you if you’re too lean, too rich, or whether you have a miss in a given cylinder because that cylinder’s temperature will be much lower than the one next to it), and so on. If you don’t have an infra red temperature gun and you like to tinker with cars, or at least want to KNOW what the actual temperature is, or how to diagnose an ongoing or sudden cooling system issue, then you need to get yourself one.