Overheating: Five Reasons You

Overheating: Five Reasons You’re Not Running Cooler And What To Do

They say, the more things change, the more they stay the same. This even holds true for the cooling system in your vehicle. Cooling systems are far more sophisticated today than they were decades ago. But the problems we had back then — like overheating — can still ruin a weekend.

Cooling systems during the muscle car era were basic. If the pressure inside the radiator got too high, the system would vent to the atmosphere (i.e., the ground). When your car overheated, you were left with a lime-green puddle beneath it. During normal operating conditions, the cooling system would find its proper level based on the expansion occurring during heat cycles. No matter how often you added coolant to it, that extra coolant would get squeezed out, again and again.

Reservoirs operate by atmospheric pressure, not gravity. They don’t need to be higher or lower than the filler neck, but should be close to the radiator. It should not be full when the engine is cold, so don’t add coolant unless it’s needed when the engine gets hot.

Eventually, cooling systems incorporated an overflow reservoir (recovery tank). As the cooling system expands, the coolant is pushed out through the vent and collects in a reservoir. As the cooling system cools, and the pressure decreases, the extra coolant is siphoned back into the radiator through the center of the cap, maintaining your coolant level.

So why does your classic overheat? Generally, the lack of proper coolant flow or insufficient airflow causes overheating. We’re going to look at how to combat these problems, plus a couple of others that can cause your engine to run hot.

What Coolant, And What Mixture?

Many modern vehicles incorporate fancy coolant colors: pink, red, orange, yellow, etc. While these might be good for specific applications, they’re probably not going to perform very well in your classic muscle car. Most of these coolants are designed for closed systems that don’t vent to the atmosphere. Your typical overflow reservoir is part of a “sealed” system, and although it stores coolant, it still vents to the atmosphere in some way.

The best coolant to use for your classic is the standard green stuff that’s been around for ages. A glycol-based coolant, such as Prestone concentrate antifreeze/coolant works well. Other brands are sufficient to use as well, but we’ll always recommend a name brand when it comes to the fluids that protect your car.

There are a lot of choices when it comes to coolant, but don’t overthink it. That same green stuff we used 40 years ago is still the best coolant for a classic car or truck. Leave the fancy coolants to modern vehicles.

David Kruse, with Prestone, tells us, “The recommended mixture should be between 40- and 70-percent coolant-to-water. In this range, the corrosion inhibitors are designed to protect the system.” We’re told this range is based on two factors: the desired freezing point, and desired heat transfer. “The higher the coolant concentration, the lower the freezing point — and heat transfer. But, it provides a higher boiling point,” David said.

“Distilled or deionized water does not have the additives that make water more drinkable,” he added. These additives lead to the quicker formulation of corrosion, whereas distilled or deionized water allows for vital engine parts to operate at optimal levels without the corrosion building up as quick.”

A 50/50 mix of coolant and distilled water is better for your engine and radiator than tap water. This mixture will raise the boiling point, which helps to decrease pressure.

Waterless Coolant

Waterless coolants, like Evans, is a bit more expensive than regular coolant and raises the boiling point to roughly 375 degrees, compared to approximately 233 degrees for standard coolants. Straight water boils at 212 degrees, and by raising the boiling point, system pressures are reduced. Waterless coolants have higher boiling points, and therefore generate less pressure and are less prone to leaks.

Does using waterless coolant sound too good to be true? To find the right answer, we asked Mike Tourville, director of sales and marketing for Evans. He said, “For those who plan to keep their vehicle a long time and only put a few miles on it each summer, Evans Waterless Coolant is a great product.”

Evans tells us that its waterless coolant will last much longer than conventional coolants, which should be changed every few years or when it’s dirty. Of course, dirty coolant should always be changed, but Evans won’t break down over time.

“Waterless coolant can help prolong engine life because of the lower pressures achieved in the cooling system,” Mike said. “Also, because of its non-corrosive properties, it can last for decades without needing replacement.” A typical glycol-based coolant should be replaced every three to five years.

For a daily driver, though, Mike told us that upgrading to a waterless coolant might not be necessary. The bottom line is, a glycol-based coolant is more cost-effective, and with a well-maintained cooling system, it can perform rather well.

But, there are some drawbacks to the use of waterless coolant. In addition to the expense, you must thoroughly flush your system before adding it. Mike reminded us, “You cannot have more than three-percent water/conventional coolant in your system with Evans waterless coolant.” He told us that you can mix water with it should you have a leak and need to fill the radiator to get home, but advised that you would need to flush your system again, as the water would diminish its effectiveness.

Your Radiator: Size Does Matter

It’s no surprise that size does matter when it comes to radiators. The small, OE radiator from your 1967 small-block with its anemic 185 horsepower is not going to be the right radiator for your 450 horsepower LS-swapped Camaro. But keep in mind, a four-row radiator is going to require a much stronger cooling fan.

While we typically try to discourage the “bigger is better” mentality, you should consider a larger radiator when you’ve increased the power output of your engine. Horsepower generates heat, and although that small, 22-inch radiator cut it back in the 1970s, it might not be enough for today’s performance engines.

That doesn’t always mean a four-row core is best if it’s still dimensionally too small. Adding a condenser for the A/C or an auxiliary transmission cooler could create an airflow problem. The goal is to have enough radiator to cool the engine properly. Many times that means a larger-than-stock radiator or a double- or triple-pass radiator to get more coolant flow.

For many moderately-built engines, a two- or three-row aluminum radiator will suffice. Many three-row radiators will support up to 700 horsepower.

Many companies, like Champion Cooling Systems, have found that a three-row — or even a two-row radiator with one-inch tubes — will perform very well. The trick is to get the coolant to flow through the radiator and spend enough time in the core to be cooled, either by the fan(s) or the air forced through it while driving.

We asked Mike Petralia from Hardcore Horsepower about electric water pumps, and he had some good advice. “A good mechanical, belt-driven water pump can flow 150-plus gallons of water per minute. The best electric pumps flow about 55 GPM,” he said. “The power you save is typically less than 10hp, and is not worth the possibility of overheating. But, for guys who stay near home and just cruise the local streets, a good electric pump can be ok.”

Mike also warned us that overdrive pulleys can cause all sorts of problems, as they’re primarily designed for use at low RPM. Underdrive pulleys are for race cars, as they keep from overdriving other components at very high RPM, and can help your components last longer. Neither pulleys is necessary for a street car.

An inline coolant filter, like this one from Champion Cooling, will help keep debris out of your radiator. It also allows you to inspect the condition of your coolant without removing the cap. Remember, products designed to plug leaks are bad for your cooling system and should only be used as a temporary fix to get you home. After all, those products plug small holes – and a radiator is a series of small holes to flow coolant.

Will Aluminum Radiators Prevent Overheating?

While aluminum radiators can lower temperatures up to 15 degrees, they are not necessarily a solution for overheating. A sufficient copper/brass radiator should cool your engine, providing you have sufficient airflow and proper coolant flow. If you’re boiling over, you’ll need to solve the problem and figure out why, before you start throwing money at parts.

Even though your friend’s car might run at 180 degrees, that’s not always the target temperature. Many big-blocks are comfortable running at 205 to 210 degrees, and it won’t hurt your engine at all. In fact, modern cars operate at 220-plus degrees. Unless you’re climbing past that, let your engine find its norm by providing a proper-flowing cooling system.

Aluminum radiators look cool under the hood, but if you have a problem, they are not an automatic solution.

Proper Airflow

Insufficient airflow is another issue that can cause overheating. If your car is overheating at freeway speeds and also around town at slower speeds, coolant flow might be the first place to look. But, if you’re overheating during only one of those situations, then airflow is most likely going to be your problem. When a radiator cools at freeway speeds, that is an indication the radiator is doing its job and performing well. Overheating at slower speeds, like stop light to stop light, could mean your cooling fan isn’t pulling enough air.

Decades ago, we used to test airflow by holding a magazine to the radiator to see if the fan held it in place. If it did, we had enough airflow at idle. If it didn’t, we would move the fan closer to the radiator. If it still didn’t pull enough air, it might not have enough blades, or the fan clutch was failing to lock up the fan and needed to be replaced.

If you’re going to run electric fans, puller fans work best, especially with a shroud. Never run a shroud on the grille side of the radiator, as it will block airflow. Also, keep in mind that OE alternators from the 1960s may not provide enough juice to run electric fans.

Timing And Air/Fuel Ratio

Poor timing and poor air/fuel (A/F) ratio can also cause your engine to run warmer. We should also state that if your timing or A/F ratio is so far off that you’re overheating, you should have already noticed the engine was running poorly and possibly knocking and/or pinging. However, these conditions can contribute to running hot. Today’s fuels aren’t like our leaded fuels of the 1960s. That means five to eight degrees of initial timing might not cut it these days — especially in California, where the fuel has more additives.

We asked Petralia about timing issues, and he had this to say, “There’s not a general number to shoot for, it’s simply a matter of giving the engine what it wants. If you have a healthy cam and aluminum heads with efficient combustion chambers, you can run more initial advance. But, you might have to recurve the distributor because more initial also means more total advance. While your engine might start and idle better at 15- to 20-degrees initial timing, most street engines on pump gas don’t need more than 35- to 40-degrees total timing.”

He also suggests a low, initial advance and a slow distributor curve that doesn’t bring all the timing in until after 3,000 rpm. “Most street cars with overdrive transmissions cruise at lower than 3,000 rpm. In this case, you want your total advance to be in at around your typical cruise RPM. Vacuum advance will increase timing even further at cruise RPM, which can help drivability,” he said.


It’s all about the timing. Many muscle cars today run better with 15-to 20-degrees initial timing and a total of 34 degrees.

When it comes to your A/F ratio, you can run a little lean at freeway speeds at light throttle. “At cruise RPM, a leaner mixture (13.5 to 14.5) can be good for drivability and won’t affect engine coolant temps as long as you have the correct cooling system components,” stated Mike. “But under load or at WOT, you’ll want to richen up (12.0 to 13.0).”

When your coolant starts turning brown, that’s an indication of rust or grime in the cooling system, and it should be refreshed. We don’t recommend flush kits on older engines; the chemicals can eat away at old gasket edges or seals. A straight water flush is perfectly fine.

Again, these issues can contribute to overheating, but they will also let you know in a discouraging way that your timing or A/F ratio is way off. If you’re overheating, first determine why and then fix the problem instead of trying to buy a solution. With a properly functioning cooling system, you should be able to cruise all summer long without any issues.

If your car has been giving you grief and getting a little hot under the collar, hopefully, these tips can help overcome the issue and get you back behind the wheel of a “cool” cruiser.

Article Sources

About the author

Michael Harding

Michael is a Power Automedia contributor and automotive enthusiast who doesn’t discriminate. Although Mopar is in his blood, he loves any car that looks great and drives even faster.
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