Q I have a 2002 Dodge Durango, with a 4.7-liter V8 engine, that overheats. I changed the fan clutch and thermostat, and the coolant level is fine. The only thing that fixed it was changing the radiator cap. That works for a few months, and then it starts getting hotter and hotter again. I change the radiator cap and it fixes it again, but only for a few months. I have been through five radiator caps in the past few years. Any ideas what this could be?

A First, let's review the function of the radiator cap. This cap seals and maintains pressure in the cooling system up to its rated release pressure -- usually 16 to 20 pounds per square inch (psi). By keeping the system under pressure, the coolant can operate at temperatures of 200 of 240 degrees, well above water's boiling point of 212 degrees.

What happens if the cap fails to maintain pressure in the system? If temperatures are above 212 degrees, the coolant boils. This creates air bubbles, stalling coolant flow at the water pump, increasing temperature and spiraling into uncontrolled overheating where coolant is forced past the radiator cap into the overflow reservoir.

Is coolant actually boiling somewhere in the system? If so, you might hear, feel or see a vibration in the radiator or upper hose and/or see the coolant boiling in the coolant reservoir after being released from the pressurized system through the radiator cap.

If coolant is only boiling in the recovery tank but not in the engine/radiator, the cap is working but may not be maintaining its rated pressure. Have the cap tested to determine the actual pressure it will hold until releasing. The cap should open to relieve pressure at about 21 psi and should hold pressure at 18.8 psi. If the cap is still good, check the filler neck on the radiator carefully to make sure the cap seals properly.

And don't forget the basics: coolant flush and fill every 30,000 to 50,000 miles, cleaning any dirt, debris and bugs from the front of the air-conditioning condenser and radiator and the thermostat itself, which should begin opening at 195 degrees, and fully open by about 210 degrees.

The lack of coolant loss tends to point away from exhaust gas pressurizing the cooling system from a head gasket or similar compression leak, but it might be worth testing the coolant for the presence of exhaust hydrocarbons just to make sure.

Q I have a 1984 Nissan 4x4. When it's damp out, it's hard to start. I noticed moisture on the wires and distributor cap. What can I do to eliminate the problem? I was told to spray the wires with silicone.

A Rain, moisture and high humidity can provide an easier path to the ground for high-voltage current than being forced to jump the gap between a spark plug's electrode and ground, especially when there's excess electrical resistance through aged coils, distributors and plug wires.

Try starting the engine on a dark wet night with the hood open. Once running, watch the high-voltage secondary ignition components carefully. If you see arcing or a bluish haze, you've identified excess electrical resistance. Spraying these components with an aerosol silicone to help keep out moisture can help, but the long-term solution is to replace the suspect ignition components.

Q I had to replace one of my tires on my front-wheel-drive SUV. The other three tires have 30,000 miles and lots of tread left on them. Will it hurt anything to get just one new tire? If not, where would you put the new tire, front or rear?

A On a front-wheel-drive vehicle -- not four-wheel drive or all-wheel drive -- it's OK to replace just one tire with a new tire of the same size and model. Because of the difference in rolling diameter between the new tire and those with 30,000 miles, put it on the rear.