Auto Repair Q&A



Cooling System










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Diagnosing Engine Overheating

Cooling Fan (mechanical)

Engines should normally be operating in the 195-220 Fahrenheit temperature range. Hotter than that and they become less efficient in every way. Too hot and they can overheat and shut down, potentially causing serious damage in the process.

There are many reasons for an engine to overheat. Following on our recent articles regarding cooling system maintenance and finding and fixing coolant leaks, we now turn to overheating and its root causes. An engine overheat is the worst case scenario and can be avoided if the maintenance has been kept up and an astute driver pays attention to the warning signs.

Engine Overheating Warning Signs

Most vehicles have a temperature gauge in the instrument cluster. All vehicles have an engine coolant warning lamp that will illuminate if engine temperatures reach extremes. Both can be used to keep track of the engine's potential overheating. If the temperature of your vehicle's engine begins to rise unexpectedly and goes past 220F, pull the car to a safe place and park it. Shut off the engine and let it cool.

Warning signs of potential overheat include sudden sluggish response, vapor coming from under the hood, engine knock or missing, and heat in the cabin despite having the A/C or cold vents open.

Causes of Overheating

Many things can cause an engine to overheat. Most engine coolant (ehtylene glycol antifreeze and water) can handle heat up to 265F, but if your engine leaves the 220+ Fahrenheit zone, you should stop the vehicle and avoid potential overheat. A heating engine can become very hot very quickly.

Note: never touch the radiator cap or other engine components when the engine is hot!

Possible causes of overheating include:

  • Low coolant levels.
  • Broken/defective radiator cap.
  • Defective thermostat.
  • Poor airflow through the radiator.
  • Broken fan or fan clutch.
  • Loose or broken water pump.
  • Blocked hoses or radiator.
  • Deposit accumulation within water ducts.
  • Restricted exhaust.

Note that an overheated engine can cause warped heads, damaged piston rings or rod bearings, and sticking or broken exhaust valves. Among many other things.

Diagnosing the Problem

I there are no cooling system leaks and the coolant levels are good, then you should begin checking other systems. If you have access to the tools, check exhaust back pressure to check for restrictions. This is an easy check and does not require taking parts off or draining fluids.

Next, check the radiator for airflow restrictions. Bugs, dirt, grime, etc. in the vents at front or back of the radiator can restrict airflow and reduce its ability to cool the engine's coolant. Clean it thoroughly with pressurized water (be careful not to over-pressure and break components).

Check the fan to be sure it's operating correctly. If the fan and fan clutch are working, then your issue will require some hands-on work.

The thermostat is your most likely suspect, as is the radiator cap. Remove both and check, starting with the cap as it does not require draining fluids. If these check out, then you may have a more serious problem that will require major dissembly to check/repair. If the water pump is not leaking, it is not likely the cause of your problem, but once you begin checking coolant ducts and other items, you'll have dug deeply enough into the engine to remove that water pump as well.

Most of the time, engine overheating problems can be traced to the simpler issues on our list. It is relatively rare for more serious problems to be at hand.