Does Your Radiator Pass Inspection?

Today’s plastic-tank, aluminum core design radiators are more dependable than ever, but when there is a problem, the problem generally makes itself quite apparent.

Ordinary leaks are easy enough to diagnose because they’re hard to miss. A leak of any size will weep, drip or spray coolant. The resulting loss of coolant usually leads to engine overheating, which can cause more damage if the leak isn’t found and fixed.

Leak-inhibiting additives can usually seal small leaks. But sealers are a temporary fix and more of a do-it-yourself product. Even so, some professionals recommend using a sealer for preventative maintenance and to prevent or seal porosity leaks in aluminum cylinder heads.

Some types of sealer, though, may increase the risk of deposits forming in the cooling system, which could cause radiator clogging, especially if the cooling system is overdosed with such a product.

Causes of Leaks
Leaks caused by internal corrosion may be found almost anywhere on a radiator. The most vulnerable points are usually the seams and where the tubes are joined to the headers. The underlying cause is almost always cooling system neglect, but it may also be due to bad ground connections between the engine, charging system and vehicle body. If the coolant has been changed regularly and tests good, check the engine’s ground connections and clean and retighten as needed.

Solder bloom is a type of internal corrosion that can form when neglected coolant, rust and some types of leak-inhibitor additives react with the soldered joints in a copper/brass radiator. White-to-green crumbly deposits begin to grow, which can block tubes and restrict the flow of coolant. A radiator with this kind of problem must be recored or replaced. The cooling system also should be cleaned and flushed to remove deposits and sediment.

Leaks caused by punctures in an otherwise healthy copper/brass radiator usually can be patched by soldering, brazing or even sealing with epoxy or specially formulated high-temperature hot-melt adhesive (which is not the same kind of adhesive used in a home hot-glue gun!). If the radiator is full of corrosion, though, a patch is not going to last. The radiator will have to be recored or replaced.

Leaks also can be the result of fatigue cracks from vibration, mechanical stress or collision damage. This type of failure is most often found where inlet and outlet fittings connect to end tanks, along tank/tube header connections, or where the radiator support brackets attach to the radiator. This type of damage can be repaired as long as the radiator is otherwise in relatively good condition.

Radiator Failure
Splits in radiator seams or between the tubes and header may be the result of too much pressure in the cooling system. This may be caused by using a pressure cap with the wrong rating (a 15-lb. cap on a system that’s rated at 5 lbs.), or by combustion chamber leaks that allow exhaust to enter the cooling system. Pressure testing the cooling system will tell you if there are any internal coolant leaks, and pressure testing the cap will tell you if it has the correct rating.

Excessive heat can be another cause of radiator failure. Radiators with plastic end tanks are very vulnerable to steam erosion. If the coolant level is low and the engine starts to run hot, steam can erode and melt a hole right through an end tank. White deposits on the inside of the plastic are evidence that hot steam rather than coolant was flowing into the tank. If you find this kind of damage, check the thermostat and pressure test the system for leaks after the radiator has been repaired or replaced.

A clogged radiator may look fine on the outside but allow little coolant flow and heat transfer because of plugged tubes. Coolant neglect is the underlying cause.

Replacement radiators in copper/brass or aluminum are available for most vehicles and in some instances cost about the same as a recored radiator. Some provide increased cooling capacity and/or more efficient fin designs than the original radiator for better cooling.

The single most important factor that leads to radiator failure is lack of cooling system maintenance. The corrosion inhibitors in conventional antifreeze are gradually depleted over time, so the recommended coolant change interval has traditionally been every two years or 24,000 to 30,000 miles for preventative maintenance. The new long-life antifreeze formulas that can go five years or 100,000 miles between changes reduce the need for cooling system maintenance and can reduce the risk of premature radiator failure.

But most vehicles still have antifreeze with conventional additives in their cooling systems. So when regular coolant changes are neglected, corrosion goes to work.

Aluminum is more vulnerable to electrolytic corrosion than either copper/brass or cast iron because aluminum is a highly reactive metal. When the corrosion inhibitors are used up and the pH of the coolant drops to 7 or below, aluminum becomes a sacrificial anode and is eaten away.

This same type of corrosion can also occur even when the coolant is in good condition if the engine does not have a good ground connection. Voltage from the charging system will flow through the coolant to ground, creating electrolysis corrosion that attacks the components in the cooling system.

Checking the pH of the coolant with chemically treated test strips can help you determine if the coolant is overdue for a change. The alkalinity of a typical antifreeze/water mixture will vary depending on the additives in the antifreeze and the ratio of ingredients, but is usually somewhere between 8 and 11. The average for most antifreezes is around 10.5, but when diluted 50/50 with water and added to the cooling system, the pH drops to the 8.5 to 9 range. Higher is not necessarily better, though, because some of the new long-life coolants have a pH of only 8.3. Staying power is what counts.

Replacement Tips
Whenever a radiator is replaced, hoses should be checked and the entire system flushed and refilled. It also is important to check the radiator mounts to assure they are okay.

A leaky heater core will drip coolant and leave a wet carpet on the passenger side and/or blow steam out of the defroster/heater ducts.

Replacing heater cores is not easy because they are buried inside the HVAC plenum under the dash.

When replacing a radiator, the width, height and thickness of the old and new units should be fairly close but may not always be an exact match because of consolidation (especially if you’re replacing an aluminum radiator with one made of copper/brass or vice versa). Even so, the cooling capacity of the replacement should be the same or greater than the original.

When bolting the radiator in place, check for misalignment between the mounting brackets and radiator support. Misalignment can cause stress that may lead to fatigue cracking and radiator failure.

At the same time, inspect the condition of all belts and hoses. After four years of service, the incidence of failure goes up dramatically. So if these parts have not been replaced, recommend new belts and hoses for preventative maintenance. Also, make sure hoses are properly positioned and supported to minimize stress on the radiator connections.

Make sure you replace the fan shroud or any other ducting that was originally on the vehicle. Leaving off a fan shroud can significantly reduce the radiator’s ability to cool the engine at low speeds when most of the airflow is generated by the fan.

One item that’s often overlooked when replacing a radiator is the radiator cap. The cap holds pressure in the system, which actually raises the boiling temperature of the coolant. Pressurization also prevents the formation of steam bubbles inside the engine, which improves cooling efficiency. Cap pressures can range from 4 lbs. to 18 lbs. The replacement cap must have the correct rating for the application because too little pressure can allow coolant loss and overheating, while too much pressure may damage the radiator or water pump seals. Radiator caps can be pressure tested to check their condition. If a cap can’t hold the rated pressure for two minutes, it should be replaced.

Other Overheating Causes
It is also possible that a suspected radiator problem may not be a radiator problem at all. Sometimes overheating can be caused by not getting all the air out of the cooling system when refilling the radiator after a coolant change (that’s why the cooling systems on many late-model cars have bleed valves). Other possibilities include a fan clutch that is slipping to the point where it pulls little or no air through the radiator; an electric cooling fan that never turns on because of a defective temperature sensor, relay, fan motor or wiring; a weak radiator cap that can’t hold pressure and allows coolant to escape out the overflow tube; or an internal coolant leak due to a leaky head gasket or cracks in the cylinder head or block that allow coolant to "disappear."

Less obvious causes of overheating may include a plugged catalytic converter, over advanced ignition timing or a detonation/pre-ignition problem in the engine itself.

Radiators are relatively straightforward in most applications since new radiators are normally equipped with automatic transmission coolers to reduce SKUs. A few caveats do apply since some applications may vary from model to model on transmission cooler coupling design, cooling capacity and hose connection size.

On rare occasions, for example, some heavy-duty and emergency vehicles may be delivered from the factory with an under-capacity radiator.

Some passenger car and light truck applications may also require extra cooling capacity for law enforcement work, high altitude and off-road use, so keep an eye on that when ordering a new unit for these vehicles.

Larry Carley, Underhood Service, June 2001