You aren't going to drive your hot rod very far if it has cooling problems. And getting it right the first time may not be enough, as cooling problems can arise anytime during the hot rod's lifetime. Typically these heating problems are the results of less than timely maintenance or a gremlin that manifests itself over time until the end result is: "My hot rod is running hot and I can't figure out why?"

To track down cooling problems there are a number of things that you can do. And by following the steps in this story you will learn some handy tips, as well as potential hiccups you didn't realize could be a problem. There are some basics that are fundamental and shouldn't be overlooked. By checking the obvious you can often solve, or begin to solve, overheating problems. To get answers to some of these questions we visited a number of industry sources; from radiator to fan to overflow canister (tank) to coolants to pulleys to air conditioning to gauges, and so on. Before fixing a cooling problem you have to understand what the problem is and what caused it. Fixing a cooling problem is twofold; find the problem and fix it and also correct the core problem.

Checking the Basics

Sometimes an overheating vehicle just requires a good visual inspection. Debris can obstruct airflow but be difficult to see due to the tight quarters inherent with most hot rods. Don't overlook the obvious, like a collapsed lower radiator hose. A problem that typically arises with longer hoses. On cars with A/C check for debris in between the condenser and radiator, especially after winter storage. (As retired GM hot weather engineer Scot Leon told us, "It's amazing what industrious little critters can store in some unusual places in your hot rod; especially between the radiator and condenser." Make sure your fan(s) are rotating in the right direction and, remember, a high pressure system can run hotter than a low pressure system without boiling. An overflow canister (tank or bottle) and appropriate cap keep the radiator full and "air out" the radiator. A radiator full of coolant is most efficient. (Sometimes it helps to drill an 1/8-inch hole in the thermostat to allow air to bubble to escape from under the thermostat on the initial fill with a new motor.)

To get the lowdown on a wider variety of cooling issues we dropped by U.S. Radiator and spoke with Don Armstrong as he gave us a number of tips for tracking down the cause of an overheating problem.


In a career as a radiator guru Armstrong has identified the two most common scenarios when a car will overheat: one, the engine maintains water temperature at or near thermostat setting while at low speed or idle but gradually heats up as speed and horsepower increases; two, the engine maintains water temperature at or near thermostat setting (10-25 degrees above) while cruising at highway speeds or climbing a hill but overheats at low speed or idle. Scenario two involves airflow and is the most common.

According to Armstrong, "I've found that 40 percent of the time a customer comes to us for a radiator to solve an overheating problem and the radiator was the last thing that needed looking at."

Think about this: Should your hot rod be overheating? At highway speeds, your engine is making horsepower and horsepower is heat. If the car runs within the 10-25 degrees of thermostat while making horsepower you can pretty much rule out a radiator problem because it's handling the heat at the engine's peak output. When the car slows down or gets stuck in traffic and overheats the only thing that has changed is the airflow and coolant flow—airflow being the more important of the two. At low speed or idle you no longer have the luxury of ramming air through the grille and have to rely on fans, either mechanical or electric. The function of either type of fan is to keep airflow through the radiator at such a rate as to maintain temperature drop (approximately 30 to 40 degrees from radiator inlet to outlet). Airflow management is crucial in low speed or idle temperature drop. The most important aspect of airflow management is a shroud and well-placed fan in order to utilize the full area of the radiator core. Fans are circular in design and radiator cores are rectangular so in order to utilize every square inch of the radiator core a shroud is paramount whether the fan is mechanical or electric. According to Armstrong, "With electric fans here at U.S. Radiator we recommend using a shallow box shroud that covers the entire face of the radiator core and becomes the platform for mounting the fan. The shroud is only 3/4-inch thick and will fit in most tight engine-to-radiator applications and promotes airflow through all four corners of the radiator. Without a shroud the fan is only pulling air through an area equal to the circumference of the fan, utilizing only 60 percent of the radiator's cooling capacity. The same holds true for a mechanical fan but the placement of this fan within the shroud becomes more critical. Ideal placement of a mechanical fan within a shroud is a third of the leading edge of the fan in the opening of the shroud and two-thirds of the trailing edge of the fan out of the shroud, which results in approximately a 45 percent angle of airflow out of the shroud. Any other placement of the fan in the shroud will result in the air flowing out of the shroud straight back toward the block and reducing airflow efficiency by approximately 20 percent in most vehicles. Another critical aspect of airflow management at low speed or idle are "air dams" that prevent hot engine compartment air from flowing around to the front of the radiator. Any vehicle whose OE design placed the radiator farther back from the grille will benefit by making and utilizing air dams to ensure only ambient temperature air flowing through the grille to the front of the radiator comes into play. Having hot engine compartment temperatures reaching the front of the radiator over, under, or around the sides is the equivalent to driving on a 200-degree day as opposed to say an 80-degree day and needs to be addressed for low speed or idle overheating problems.