I've chosen a general maintenance problem I've encountered many times in the course of my more than 24 year career as a Datsun/Nissan mechanic. I'll call it "How come my Z/ZX is still overheating?" Typically, a customer will bring a Z/ZX to my shop having recently replaced one or more cooling system components in an effort to cure a persistent overheating condition by first replacing the thermostat. This can frequently be a good starting point, especially if the car was purchased used and/or the service history is unknown. If after replacing the thermostat the overheating continues, a good place to look for a solution could be the radiator.

As we all know, the purpose of a radiator is to dissipate heat. In the case of an automobile, heat is generated by the combustion that takes place in the cylinders of the engine. Water jackets surround the cylinders and these jackets, hollow cavities really, are full of coolant. The water pump circulates the coolant through the engine and into the radiator. The radiator of a Z/ZX is made up o 50 or more vertical, slightly flattened, hollow tubes approximately 3mm in diameter. Between the tubes and arranged in a zigzag pattern are the thin metal heat sinks. In a perfect world, the coolant is circulated by the water pump through the tubes and the heat caused, as you will remember, by combustion is dissipated through the heat sinks, called cooling fins, and into the atmosphere. If these tubes become plugged, the ability of the radiator to transfer heat to the atmosphere will be reduced in direct proportion to the percentage of blockage. The solid material that causes the blockage is heavier than the coolant used in the system so it will collect in the bottom of the tubes. Except for extreme cases, the blockage is largely invisible to a casual observer looking into the radiator though the filler neck.

"Where does the solid material that causes the blockage come from?" The solid material is the corrosion that results when lead, steel and aluminum comes into contact with depleted anti-freeze. This depletion of the corrosion resisting additives present in anti-freeze occurs after about one year. Lead corrosion comes from the lead solder used in the manufacture of your radiator. Steel corrosion takes place in the water jackets of the engine block. Aluminum corrosion can occur in the water jackets of the cylinder head or anywhere else in the cooling system where aluminum is present. Have you ever removed the upper radiator hose of your Z/ZX and seen a white/yellow crust on the thermostat housing neck? That crust is aluminum corrosion.

"How can I tell if my radiator is blocked and if so, what can I do about it?" The most comprehensive way to find out if your radiator is causing overheating due to blockage involves removing it from the car and having a radiator shop disassemble and inspect it. However, I'll pass along a couple of rough guidelines that I employ. If a radiator is more than eight years old and the coolant has never been changed or if the history of the cooling system is cloudy (no pun intended), you may assume that the radiator is at least 50 percent blocked. If you're sure that the components of your cooling system are operating properly, yet your car exhibits the symptoms described at the start of this piece, you may safely assume that the radiator is operating at no more than 50 percent capacity. With the supply of relatively inexpensive radiators available today, I've found that the most cost effective way to deal with a plugged radiator is replacement.

Another option would be to have the radiator rodded. Rodding is a procedure whereby a radiator shop disassembles the radiator and passes, with the aid of strong chemicals, a rod through the tubes to unplug them. In defense of option #1, I've found that rodding causes so much stress to the radiator that when you're looking for a reliable long term fix, rodding is more often than not unsatisfactory.

"Okay, Mike, I've installed a new radiator. I sure don't want to have to go through that again. What preventative maintenance for my cooling system can you recommend?" The best way to maintain your cooling system is with yearly replacement of the coolant as per the procedure described in the service manual.

"Do I need to use any of the cooling system additives I see advertised?" Modern anti-freeze already contains all of the additives your car needs – save your money!

Mike McGinnis

Banzai Motorworks