Why would a car only overheat when driving on highway not when driving around town?

Answer 1

Each of these things can be checked independently of the others without spending any money on parts.

1-Low coolant level.

2-Restricted airflow to the radiator as from leaves or plastic bags in fron of it, or the plastic under-skirting being broken away (as from running up onto curb-stops.)

3-Restricted flow of within the radiator or hoses. Restricted flow in hoses is rarer, but rev the engine to 2000 to 2500 RPM for several seconds after the engine is fully warmed and look for either radiator hose to collapse. To check for restricted radiator, you can take the temperature of the radiator with a non-contact thermometer in several places or you can open the cap (if so equipped) and look for white deposits. White deposits mean you need a radiator. If your radiator has no cap, remove the upper hose and look in through that opening.

4-Poor heat transfer. Poor heat transfer can be radiator to air (look for missing or greenish radiator fins) or internal due to a high ratio of anti-freeze to water (use a hydrometer) or internal due to scale deposits (with an iron engine your coolant will be brownish, aluminum engines can be checked by removing the thermostat or water pump and looking inside for whitish deposits).

5-Insufficient coolant pumping. Metal water pump impellers can erode over time, especially with excessively old coolant. Plastic water pump impellers can break randomly, and are prone to break if the engine is a Ford Duratec or Volkswagen or if the water pump hasn't been replaced after an overheat. Cheap substandard $5 replacement water pumps have often have a poorly designed impeller that can become dislodged. To check, remove the thermostat(s) and fill the system with coolant or water. Remove the radiator cap (if so equipped, otherwise surge tank cap) and observe the coolant level while rapidly snapping the throttle from closed to wide open. The coolant level should change drastically (more than 1/4 inch) if the pump is good.

6-Exhaust gases leaking into radiator. This is usually caused by a leaky head gasket, but can also be caused by a cracked or eroded engine. Note that GM Quad-4 engines are particularly prone to cracking, and four cylinder Chrysler engines and cars owned by youthful males or Jewish women are prone to head gasket leakage. An electroneic gas analyzer can be used to check this condition, but for the do-it-yourselfer, check with the local parts store or tool dealer for a disposable chemical based detection kit.

7-Boiling. Boiling can be caused by too much water in the coolant, insufficient pressure due to a leak or a bad cap, Partially restricted water jackets, or overheating due to another cause. Presumably, any other overheating has already been ruled out so you might as well check for leaks, then replace the coolant with a correct mix and put a new cap on it. If you have a restricted water jacket, you're going to need the engine rebuilt.

***Fan related problems will _not_ cause overheating at highway speed.***

AnswerThere are many possibilities. But if you are traveling locally a very short distance, possibly the engine does not reach temperature to open the thermostat. The water is circulating in the engine. On the highway, the car may reach temperature, opening the thermostat. Now the water is also going through the upper and lower radiator hoses and radiator. If there is a small leak in the hoses or radiator, that would cause overheating, now that the water is leaving the engine, (and is under pressure).

A pressure test of the cooling system usually will show a leak.

Especially if you notice fluid under the car after driving.

AnswerA weak lower radiator hose could collapse at highway speeds but not lesser speeds around town. AnswerThe most likely cause is a plugged radiator. Over time sediment build up in a radiator. Every time a radiator is heated and then cools the sediment collects at the bottom of the radiator slowing reducing it's efficiency. I have seen this particular overheating problem numerous times and traced it to a plugged radiator. Take it to a shop and have the radiator rodded and repaired and it will cure your problem. AnswerHave your clutch fan checked. the tension of the clutch can weaken causing the fan to turn too slow at higher rpm's. AnswerI had a similar problem. If the car has sit for any length of time a mouse or rat may have built a nest in the muffler or exhaust pipe. This causes back pressure to build up at highway speeds but not at short low speed in town driving. If one of the other ans did not fix the problem this is worth a shot. Answeranother possibility is a blown or leaking head gasket. A cracked head/block will also cause a vehicle to overheat when operated at higher rpm, the crack doesn't expand until temperature is excessive. typically the vehicle will produce a smoky emission from the exhaust, but it is actually steam from the antifreeze/water mixture leaking into the combustion chamber.