What to check if your car is overheating at idle. DIY

Blocked radiator hose
Is your Car Overheating? Check these simple issues
Overheating car fix

Do you have an overheating car or truck after a short drive or at idle? Here are some quick and easy DIY checks and fixes for you to do. Try to diagnose the problem yourself and hopefully save some money with a simple fix rather than going to the dealer workshop.

Quick story, my Toyota Corolla was overheating and the local garage wanted to replace the radiator for $1000. My radiator looked fine with no leaks and no blocked cores. Time to investigate this myself.

Wait for the engine to cool down (Over an hour) and use a rag and slowly remove the cap.

A cars cooling system is very simple in its design, it comprises:

  • Radiator
  • Fluid
  • Fan
  • Thermostat
  • Piping
  • Radiator Low speed resistor

If any one of the above points fails you will have overheating problems. But sometimes diagnosing an overheating car can be a little tricky as the symptoms may be erratic.

Car overheating at idle

car engine lots of smoke steam

If your car is overheating at idle then this could be a number of reasons. Most commonly it is a failed radiator fan, low coolant level, or a failed thermostat.

Sometimes the low-speed radiator fan resistor has failed and will need to be replaced. Most radiator fans have duel speeds for when the air conditioner kicks in.

Car overheating when coolant is full.

If your car is still overheating when the coolant is full check the radiator cors for blockages or the low-speed resistor for the radiator fans. These sometimes burn out due to old age and need to be replaced.

The best way to check the radiator fans is to start the car in the driveway and warm it up. Keep an eye on the internal temp gauge and see if the car radiator fans kick in. Don’t let the radiator temp gauge get into the red zone. If the fans don’t kick in there is a problem either with a fuse, the fans, or the slow-speed fan resistor.

Radiator Problems:

car radiator leaking overheating

Some of the common issues with radiators are due to low radiator fluid. If your radiator has no or low water then wait till the engine has cooled down usually for 1 hour and top it up.

The problem with topping up an overheating engine is you suddenly introduce cold fluid which can warp your engine head or block. (Bad news and expensive) Always use radiator fluid not tap water. Radiator fluid has special anti-corrosion elements that will stop the corrosion of your engine. It also has antifreeze properties to stop the fluid from freezing in the winter. You can use tap water in an emergency but flush it out when you are home.

All vehicles have the same 3 components to cool the engine, coolant, pump, and heat exchange. Air is usually used to cool the fluid in the radiator. Some marine craft uses seawater to cool the motor.

In the case of Sea-Doo, they use internal coolant which after it is hot from taking the heat out of the 4tec motor is passed over the ride plate. This takes the heat out of the cooling fluid and transfers it to the seawater underneath the jet-ski. Quite remarkable. Check this post to see how to replace the coolant in a Sea-Doo Jetski.

Blocked Radiator Core:

Other problems with the Radiator are blocked cores. As the coolant gets old and some corrosion set is the small cores of the radiator can become blocked with what looks like dirt particles. These can sometimes be flushed away with a radiator flush but usually, it requires a new radiator. Blocked cores mean an overheating car engine or truck engine.

Always read your car service manual for the recommended replacement of your radiator coolant. To be on the safe side every 2 years is usually fine. If you notice dirt around your radiator cap flush and change your coolant straight away. Your coolant is now past being a use full anti-corrosion additive and is slowly eating away the internals of the engine.

What Radiator Coolant Color To Use Green or Red?

Never mix Red and Green coolant, they will gum up your cores in the radiator. If you need to swap coolant colors please flush out the old coolant and refill it with the new.

Genuine Toyota and Lexus long-life red coolant can be brought from here. It’s pre-mixed and ready to go. But you can use the cheaper generic coolant if you are out of warranty.

If your radiator cap is leaking or not sealing properly, you will have overheating issues as the radiator circuit can not build up the pressure required for optimum pressure. Replace your radiator cap. Another issue can be a blocked overflow pipe. The Overflow pipe and Tank is designed to act as an overflow container.

Blocked overflow hose pipe radiator

As the coolant gets hot it expands and overflows into the container. As it cools it is redrawn back into the Radiator thus always maintaining a full radiator. If this pipe is blocked it will cause excess pressure to build up in the radiator and the water pump won’t be able to pump coolant around effectively. To clear this blockage take off the overflow pipe and use a piece of wire to poke through the blockage, rinse with fresh water, and blow through the pipe to check it is clear.

I have seen these blocked pipe issues with a few cars now complaining about overheating problems and once the pipe is clear all the problems disappear.

Vehicle Thermostat:

A Thermostat is a very reliable piece of hardware. The function of the thermostat is to restrict the amount of coolant flow that can move around the engine block thus maintaining the optimum temperature range for the motor.

Car thermostats can fail but it’s rare. They mainly just get old and fail either closed or open at the seal, spring or just wear out. Defiantly will cause an overheating car if the thermostat jams.

To check your thermostat, drain all the coolant from the radiator then remove the thermostat. Check your workshop manual on how to remove it but it’s usually only 2 bolts for the cover and you can pull it out.

Once out you can put it in a pan of cold water and put it on the stove heat up the pan until it is boiling. You should see the thermostat open. If it doesn’t replace it.

See Video here on how it’s done: Thanks michaelovitch

Radiator Cooling Fan:

A Car engine radiator cooling fan is there to help cool the engine when the car is stationary. Usually when a car is moving the air passes through the radiator and thus cools the coolant which cools the engine. When a car is stopped no air can cool the coolant so when the temperature starts to rise, a fan will auto kick in forcing air through the radiator. 

To check the fan start the car open the bonnet and wait a few minutes till the engine starts to heat up. Watch the gauge to make sure we don’t overheat the engine. If the needle is starting to get up near the red and the fan is not starting this is your problem. You should be able to see and hear the fan or fan’s start-up.

If not check the fuse for the fan or replace the fan. Older cars may have a fan belt that can break or slip and cause an overheating car or truck.

Cooling Hoses:

Thanks, BleepinJeep

Your radiator will be connected by a few hoses, over time these can get brittle and leak. If you are losing fluid check your hoses for leaks. You will see either a white powder on dried leaks or a wet oil-like mark if leaking, Replace the hose.

Internal Head Gasket:

If your internal head gasket has blown it can cause the overheating car symptom. This is a major job and will need to be taken to a garage to check and replace the head gasket, not a quick fix, and certainly not cheap.

When a gasket fails, the exhaust gasses can flow into the radiator circuit causing accessible carbon/build-up in the coolant and causing fluid loss. As the coolant can also be entering the cylinder it can cause a lot of other problems like rust or a seized cylinder.

A quick way to check is to take the radiator cap off a cooled engine. Start up the car and watch for bubbles in the radiator. You may see a few in the beginning as trapped air bubbles make their way through the system but if they don’t stop or you see a lot you could have a failed head gasket.

I have used the pour-in head gasket sealer in the past that worked great. So check it out.

Good luck and hopefully you can fix your car overheating problem. It may be an easy fix.

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