What Causes a Car to Overheat?

Updated on July 13, 2016
Overheat can destroy your engine block and cylinder head.
Overheat can destroy your engine block and cylinder head. | Source

What causes a car to overheat may be something as simple as low system coolant or more complicated as an intermittent radiator-fan failure. Often, though, the reasons vary around a few common failures.

Vehicle engines need to operate at temperatures that range around the 180 and 210 F (80 and 100 C) degrees. This allows for better engine combustion, low emissions, extended components service life and better fuel efficiency. But let the temperature climb out of control and it'll destroy your engine in minutes if you fail to notice.

So, whenever your engine begins to overheat, don't wait. Here, you'll find some of the most common — and not so common — reasons for your car engine to overheat to help you fix the problem as soon as possible.

A car thermostat may fail in the closed positions, causing the engine to overheat.
A car thermostat may fail in the closed positions, causing the engine to overheat. | Source

I. Inspecting the Cooling System

Start with the most obvious reason: Check the coolant level and condition (the mixture of antifreeze and water inside the radiator).

* Low or Worn Out Coolant:

If you've neglected the cooling system for quite some time, it might be running low on fluid or the coolant mixture has worn out.

Antifreeze looses its preventive properties over time. That's why car manufacturers recommend replacing the coolant every two or five years, depending on the type of coolant you're using.

Check your owner's manual or repair service manual for the recommended change interval for your particular vehicle model. If you don't have your manual, buy an aftermarket service manual. They are inexpensive and full of maintenance and repair how to's you'll use frequently on your car. Besides, It'll pay for itself on your first repair or maintenance job. Buy a copy at most auto parts stores or from a retailer on the Web.

* Coolant Leak:

On the other hand, if you find yourself adding coolant to the system on a regular basis, suspect a leak.

Check for wet spots under your car, around the radiator and heater core, radiator and heater hoses, engine block, water pump, and thermostat housing. Your car repair manual will help you locate the different cooling system components.

Do you suspect a cooling system leak but can't find the source? Then your engine may have an internal leak. Take in your car for a cooling system pressure test to locate the leak.

Why Do Cars Overheat
* Failed Thermostat
* Bad cooling fan or wiring
* Worn out or damaged radiator cap
* Bad water pump
* Worn out or damaged water pump drive belt
* Clogged up radiator
* Insufficient or worn out coolant in the system

Troubleshooting an Overheating Engine

II. Checking for Potential Faulty Components

Bad components in the cooling system and engine can also lead to overheating problems.

* Inspect the radiator and heater core for collapsed or damaged hoses restricting coolant flow.

* Check for poor air flow through the radiator and A/C condenser. Look for debris and bugs covering the grill on the radiator and condenser — the radiator-like device mounted in front of the radiator. A blocked radiator or condenser will cause the engine to overheat. Also, bent fins will block air flow through the radiator. You can straighten out the fins using an inexpensive radiator-fin comb kit tool.

* Inspect the radiator cap. A failing or loose cap will cause the coolant's boiling point to fall and allow coolant to leak. Replace it if the cap or its seal show signs of wear or damage. Have the radiator cap pressure tested, if necessary.

* A thermostat stuck in the closed position, blocking the flow of coolant between the engine and radiator will cause the engine to overheat too. With the engine running and unable to carry heat away, heat will practically cook and melt the engine, if you fail to notice. Check the thermostat yourself following the simple procedure described in the article Car Thermostat Problems.

* If you haven't serviced the cooling system at the recommended interval, rust may be clogging up radiator passages, affecting water pump performance, choking water jackets inside the engine, and causing the temperature to rise. In mild cases of rust buildup, you may have a chance to save the radiator by flushing the cooling system and adding fresh coolant.

* On the other hand, if engine overheating started right after you replaced the old coolant, did you forget to bleed the system? On Modern vehicles — specially those with low hood lines — the cooling system tends to form air pockets whenever you refill it with coolant. Cooling systems — specially on vehicles with low hood lines — tend to form air pockets whenever you refill it with coolant.

Air pockets create hot spots that cause overheating. And they'll crack the engine if the temperature rises well beyond its ideal range. Check your repair manual for the proper procedure to purge the cooling system in your vehicle.

An over stretched timing belt will cause an engine to oeverheat as well.
An over stretched timing belt will cause an engine to oeverheat as well. | Source

* An old, worn out water pump will cause cooling system problems as well, preventing proper coolant circulation between the radiator and engine. A noisy water pump is often a sign of a failing pump shaft or bearing. Also, look for signs of leaks around the pump.

* Look for problems with the cooling fan. The electric fan should cycle on and off as the temperature rises and falls. And the fan should run when you turn on the air conditioner; otherwise, replace it. Check the fan blades for damage, the fan motor and radiator fan switch for proper operation. Connect the fan directly to battery power and see if the fan's motor runs. Also, bypass the fan switch by connecting a jumper wire on the switch wiring harness to test the fan and the switch for operation.

* Check the operation of the fan clutch, if equipped. Look for signs of leakage around the clutch. Then, with the engine off, try rotating the fan manually. If it spins without much effort, you need to replace the clutch. Now, slightly push and pull on the fan. If the fan wobbles, replace the clutch.

* Verify that the fan shroud is in place and without damage, otherwise it won't help direct air flow around the engine as it comes through the radiator to cool the engine.

* Check for a loose fan belt or water pump belt. A loose drive belt or slipping serpentine belt that runs the fan or water pump won't operate either device properly, causing the engine to overheat. Visually inspect the belt for cracks, missing chunks and rough edges. Then, check the belt tensioner for proper operation. Consult your owner's manual or repair manual for the service interval and replace the belt and tensioner if necessary.

* Check for a clogged catalytic converter. If the catalytic converter blocks exhaust flow, it'll cause the engine to overheat as well. Usually, you can check for intake vacuum at idle using a vacuum gauge. If low, you'll probably need to replace the converter. Consult your vehicle repair manual.

* Sometimes, the temperature gauge will warn you about the engine overheating even if there's nothing wrong with the cooling system. These situations usually come from temporary driving conditions. For example, when you are towing a trailer, moving through slow traffic in the middle of the summer under high temperatures with your air conditioner on, going up a steep hill. Other times, it just means the temperature gauge or sensor has gone crazy and you'll have to replace it.

Replacing a Failed Car Thermostat

What causes a car to overheat. More often than not, a common source of engine overheating problems is little or no cooling system maintenance. Still, an overheating condition will happen at any time, unexpectedly, even if you maintain the system at the recommended intervals. And, at that point, any system component remains a suspect until you do some troubleshooting and find the culprit. This guide highlights the most common sources of engine overheating problems to help you inspect, locate and fix the issue as fast as possible. Just remember, look for the obvious suspects first. Applying some common sense will help you fix your car faster, save you some money in repairs, and, of course, save your engine.

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