There are few automotive situations that are more frustrating than cooling issues. Heat kills engines, yet an engine needs to run at a certain temperature to operate effectively. You have to maintain the system that balances the proper level of engine heat. When the cooling system lacks maintenance, has excessive wear and tear or the car has been modified, the cooling system can fail, leaving you and your engine steaming mad. Having recently been through a few cooling system issues ourselves, we thought it would be a good idea to share one of these cases with you.
In Oklahoma, we get the worst of both worlds- extreme heat, typically with a couple of months of triple-digits in the summer (at 50%-plus humidity to boot) and single digit temps during the winter, so your cooling system must be in tip-top shape. During the late spring, we had a situation with a 1966 Mustang Fastback. The car had recently been the subject of a late-model 302 engine swap with electronic fuel-injection. The car was originally a 289 V8 pony, so the stock radiator was retained. We had some cooling issues and set out to track down the problem.
The first step in any cooling system issue is to verify there is actually a problem. This may seem a little silly, but temperature gauges, even expensive electronic units can be wrong. In addition, where you install the sending unit can actually affect the temperature by as much as 20 degrees. There are two ways to verify the actual system temperature- use a second temperature gauge, preferably mechanical as they are less inclined to be errant, or an IR thermometer. The simplest method is the IR thermometer. Using the thermometer, target the temp sensor and check the temp. The results on the IR unit should be within 5 degrees of the gauge temperature. You may find that the gauge is wrong. If this is the case, replace the sending unit first. If the temps are within 5 degrees, you need to move on.
- Is the car running hot or full-on overheating? Anything higher than 220 degrees is considered too hot for most older cars; the optimum temperature is between 180 and 200 degrees. Hitting 250 degrees is a big problem, and can cause serious engine damage. Shut the vehicle off immediately and let it cool before doing anything else.
- When is it running hot? This will tell you a lot about the problem. If the car runs hot while sitting at stop lights and in slow traffic, then the problem is most likely an airflow issue. If the engine temp creeps up regardless of driving speed (as in faster than 35 MPH), then you have a more serious problem.
The Mustang was running moderate temps, around 200 in traffic—not a huge deal. The 302, a 1999 model, uses a serpentine system and the distance between the radiator and the engine was not enough to fit a standard clutch fan, besides the fact that a serpentine system just doesn’t look as good with a big fan bolted to it. Driving the car, we noticed that the temperature continued to rise, regardless of vehicle speed. This demonstrates that the cooling system is either blocked, running open or simply lacks the capacity to cool the engine. There are several potential problems here to be narrowed down.
- Water pump– A bad water pump can ruin your day in a hurry. In most cases, a bad water pump goes out all at once and the engine overheats rapidly, no amount of driving cools it down at all, and it heats up very quickly. There are some tell-tale signs of a bad water pump. Most pumps have a small weep hole at the bottom of the unit. If this hole is leaking water, the pump is bad. If the hole is dry, the problem may be internal. There are three ways to test a water pump. The quickest test is to pull the radiator cap, start the engine and let it come to temperature. Once the engine is up to temperature, the thermostat opens, and you should see the coolant flow throw the radiator tank. You can also pull the thermostat which allows the coolant to flow immediately (the thermostat needs to be tested anyway). The next test is to pull the water pump belt and wiggle the shaft. The shaft should not have any play, it should simply rotate. If the rotation feels rough or the shaft moves up and down, the bearings are going out and it should be replaced. The other test involves a pressure test for the entire system. This tool costs around $150-200, but you can often rent them from your local parts store.
- Thermostat– The thermostat is what controls the coolant flow. It is based on temperature, so testing one is quite simple. Pull the thermostat (inside the water neck) and examine it. If it is stuck open, the engine won’t heat up quickly, but once it does, it stays hot for a long time. A thermostat in the stuck-open condition is fairly obvious, the plunger is open. The stuck-closed requires a bit more work. You need a kitchen pot, some water and a thermometer that measures over 200 degrees. Boil some water, bringing it up to at least 200-degrees (higher is OK but not necessary) and take the pan off the heat. Next, place the thermostat carefully into the pan. The plunger on the thermostat should open to the fully-open position. This would allow water to pass though the unit. If it does not open after a couple of minutes, the unit is bad. You can also test the closing. Gradually add cold water, watching the thermometer. You don’t want it to get too cold to fast. By the time the water is below the thermostat’s rating, it should begin to close.
- Blockages- Your system may be plugged up, not allowing the coolant to flow easily through the system. Blockages naturally occur from calcification and rust. Over time, calcium and rust build up and get trapped in the tubes of the heater core and radiator. The easiest test for this is to remove a heater hose from the engine, and using a high-pressure nozzle on a garden hose, flush the system. Once the system is full of water, it will come out of the fitting from which you removed the hose. If there is a significant blockage, there will be little to no flow. Consider the flow level coming out of the hose, and the level coming out of the engine. If the radiator tank and tubes are heavily calcified, the radiator should be cleaned or replaced regardless. Having a radiator rod cleaned by a professional shop typically starts at around $100.
- Radiator too small- If the car stays fairly cool under idle conditions, but heats up during driving, then the likely culprit is too little radiator capacity. At idle, the engine is not producing as much heat, so the smaller radiator can manage, but as the engine spins faster, more heat is generated, which eventually overcomes the radiator’s capacity to cool because the coolant is not staying in the radiator long enough before flowing back to the engine.
After going through these tests, we determined that the stock straight-six radiator simply was not enough to support the 302. Replacing the radiator is a simple thing. There are several options: stock V8, direct-fit aluminum, or custom fit. The stock radiator for the ’66 is a down-flow style, meaning that the tanks are at the top and the bottom, and the coolant flows from top to bottom. The inline-6 cylinder radiator is a 3-core brass unit that covers both the a\c and non-a\c cars. The V8 radiator is also a 3-core unit, but has larger tubes to accommodate more coolant and transfers more heat.
Electric fan vs clutch fan
There has long been a debate between the electric fan and the clutch fan. Proponents of the electric say that a mechanical fan (engine driven) eats up too much horsepower (about 20hp in most cases), and point to the fact that almost all modern vehicles use electric fans. Mechanical fan believers say that electric fans just don’t work well enough to cool a muscle car engine and that mechanical fans just plain work better.
While there are truths in each argument, the reality is that either one is an acceptable solution, as long as the proper components are used. A mechanical fan eats up horsepower, but with the addition of a clutch, the parasitic loss can be reduced to less than 10 horsepower, not enough to notice. There are two types of fan clutch: centrifugal and thermostatic.
- Centrifugal clutches are marked by a smooth front cover. These clutches are tight at low rpm, but as the engine spins, the fluid inside the clutch allows the fan to slip, gradually reducing the drag on the engine at higher rpms. These were commonly used on small-block V8s and smaller engines. They only draw air while the engine is spinning at lower rpm.
- Thermostatic clutches have a tell-tale coil on the front of the fan. As this spring gets hot, the fan’s slippage decreases, so that by the time the coil is up to 180-degrees, the fan is fully engaged. This means that the fan will help draw air under any rpm while it is hot. These were commonly used on larger engines. This is the most common style you will find in parts stores. For any high-performance engine, you want one of these. They simply work better and draw air when the engine is hot.
You can tell that the fan clutch is going out by three signs. These are fluid streaks from the center out, the fan spins noticeably slower than the engine at any speed, and there is no suction to the front of the radiator. If you place a piece of paper over the radiator, it should stick.
Electric fans get a bad rap because they are so often misused. The key to a proper electric fan installation is surface area coverage. Most radiators for the early Mustang can be covered with a 16” fan, as you move into the later model Mustangs, the radiators get wider, requiring a different solution. Installing a single 16” electric fan to a 30-inch radiator is simply not going to yield enough coverage or air flow to properly cool the engine. This is the most common mistake made with electric fans, not enough coverage and\or airflow. Another issue that plagues the electric fan is the on\off control. You don’t want the fan to operate all the time, as this can wreak havoc on the cooling system. As the coolant temp drops, the thermostat closes, if the fan runs all the time (or comes on at the wrong time, the fan will be fighting the thermostat, and the engine may run too cool, which will hinder performance. Installing a thermostatic control device, like the one shown here from Zirgo, allows you to set the point that the fan turns on. This should typically be 10-15 degrees above the thermostat rating.
Any fan, whether it is an electric or a mechanical fan, needs a shroud. For the vintage Mustangs, a 16” fan covers the entire finned surface, so you don’t need to build a full shroud, as you have full coverage. Any time you use a smaller fan than the finned surface, you must have a shroud. To cover a 30-inch radiator, you can use two 12” electric fans with an aluminum shroud. This will force air to come through the entire radiator surface, and not just the two 12” sections covered by the fans.
With the proper installation, both electric and mechanical fans work, but a poor install will always result in poor cooling.