How to Diagnose a Cooling System Problem

Co-authored by wikiHow Staff

Last Updated: April 13, 2019 References Approved

Issues with the coolant system in your vehicle can be difficult to diagnose. If your vehicle’s engine has been running too hot, overheating, or running too cold it could mean there is a leak in the system or one of the components of the system has failed to function. By paying attention to the symptoms your vehicle exhibits and inspecting the coolant system itself, you can often identify the source of the problem fairly easily.

Method 1

Method 1 of 3:

Determining There is a Problem

1
Keep an eye on the temperature gauge. The first sign that there is an issue with the cooling system in your vehicle will often come from the temperature gauge. If your vehicle has been running consistently warmer or has recently begun having issues with overheating, there is likely an issue with the cooling system.^{[1]
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- The temperature gauge in your vehicle should have an acceptable operating range. Even if your vehicle is not overheating but has been running at the higher end of the operating range, there may be an issue.
- If your vehicle is running with the temperature gauge in the red, it is overheating. Pull over and shut off the vehicle.
- Issues with the cooling system can also cause the engine to run cold. In that case, the temperature gauge will remain in the blue.
2
Look for the check engine light. Another early indicator of a problem with your cooling system may be the check engine light on your dashboard. Your check engine light comes on when one of the sensors in your car sends a message to the ECU (Electronic Control Unit) telling it there is an issue. If something isn’t performing properly, the check engine light is how the ECU notifies you.^{[2]
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- The error codes that prompt the check engine light to come on are the same codes that are read by an OBDII scanner.
- Many vehicles have dash board lights that will notify you of low coolant levels or extreme engine temperatures as well.
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3
Identify puddles of coolant below the car. Leaks are a common issue in coolant systems. If you spot a puddle beneath your vehicle, there’s a chance that it may be the result of a coolant leak. Touch the puddle with a finger, then smear it onto a blank white sheet of paper. Oil is often brown or black, water from the air conditioner will be clear, and coolant will likely be green, pink or orange.^{[3]
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- A leak in your coolant system can cause it to fail to maintain proper temperature in the engine.
4
Check the coolant levels in your vehicle. If you suspect that your vehicle has a coolant leak, pop the hood when the engine is cool and look at your coolant reservoir. The reservoir will have lines indicating the minimum and maximum coolant capacity for your vehicle. Take note of the level the coolant reaches on the reservoir, then check again in a few days. If it has gone down, the coolant is either leaking or being burned.^{[4]
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- Lack of adequate amount of coolant is solely responsible for the malfunctions in the cooling system of a car. So, the quantity of the coolant used in the cooling system needs to be measured regularly and if necessary the coolant should be replaced.
- Besides, deposition of debris also has negative impacts on the efficiency of the cooling system. So, the cooling system needs to be flushed after certain time interval as recommended by technician. Appearance of abnormal signs which shows that the cooling system of the car has some sort of abnormalities should not be ignored. The reason behind the appearance of such signs needs to be identified and repaired in time to enhance the efficiency of the cooling system.
- Make sure to check each time when the engine is the same temperature.
- If you aren’t sure where your coolant reservoir is, consult your vehicle’s owner’s manual.

Method 2

Method 2 of 3:

Visually Inspecting Your Coolant System

1
Allow the engine to cool. Your engine bay gets extremely hot while the engine is running, so poking around the engine bay before it has a chance to cool can result in serious burns. Allow the engine to cool for a few hours before opening the hood and looking for evidence of a coolant leak.^{[5]
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- If the hood still feels warm, the engine inside is likely very hot.
- If your vehicle has been overheating, it will take even longer for it to cool enough to be safe to touch.
2
Put on the appropriate safety gear. Before starting any automotive project, you should put on the appropriate safety gear. Because you’ll be dealing with coolant that may be leaking, eye protection should be considered mandatory for this project. Gloves are optional, but may protect your hands from scratches or bumps as you work.^{[6]
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- Always wear eye protection when dealing with coolant leaks, as they may drip or spray under pressure.
- Glasses or goggles are both acceptable forms of eye protection for this project.
3
Check the radiator cap for damage. Another common point of failure in coolant systems is the radiator cap. When functioning properly, the radiator cap can release excess pressure that builds up in your coolant system, but over time it can become worn out or stuck. If the radiator cap looks rusty, is corroded or caked in grime, it may be the cause of your coolant issues. Replace your radiator cap simply by unscrewing it and screwing in a replacement.^{[7]
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- Radiator caps are inexpensive and can usually be purchased at your local auto parts store.
- Never remove the radiator cap from an engine that is still warm. It could spray hot coolant and burn you.
4
Visually inspect the water pump if you can. The water pump in your vehicle pumps the water and coolant mixture through the engine and into the radiator where air flow helps to dissipate the heat. Look for signs that your water pump is leaking or has failed if it is visible in your engine bay. The water pump is belt driven, so inspect the belt for damage that indicates that the water pump pulley has stopped moving and has been rubbing against the belt.^{[8]
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- Without a properly functioning water pump, your engine will not be able to dissipate the heat it produces as it runs, causing the vehicle to overheat.
- If the belt powering the water pump is damaged, it will have to be replaced after you put in a new water pump.
- If you are unsure of where to locate the water pump in your vehicle, refer to its service manual or the website for the auto maker.
5
Assess damage to your coolant hoses. Look at the hoses traveling from the radiator and to the engine, then follow them along for as far as you can. If any of the hoses are kinked, that will cause the coolant system to fail to function properly. Cracks may also indicate a leak, but even if it isn’t leaking yet, a cracked coolant hose should be replaced before it begins to leak. Keep an eye out for damage to the hoses or signs of rubbing from the serpentine or accessory belts.^{[9]
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- If one of the belts has been rubbing on a coolant line, they will both likely need to be replaced. Make sure to install the new ones with enough clearance to avoid rubbing the belt on the hose.
- Leaks in your coolant can result in puddles beneath your vehicle and overheating.
- Replace radiator hoses that are leaking or are otherwise damaged.

Method 3

Method 3 of 3:

Testing for Common Coolant System Failures

1
Determine how long it’s been since the coolant has been flushed. If there is no sign of a coolant leak and your vehicle is failing to properly manage temperature, it may be a result of your coolant going bad. Most auto makers recommend flushing your coolant every thirty to sixty thousand miles, so if it’s been longer than that for your vehicle, it may be the culprit. Drain and flush your coolant by opening the petcock on the bottom of the radiator and draining it into a container, then filling the coolant system with water and running the engine for a few minutes again. Repeat this process a few times to clean all of the old coolant out, then refill it with a 50/50 mixture of water and coolant.^{[10]
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- Most coolant comes pre-mixed, but you can purchase only coolant and mix it with water yourself.
- Coolant can be purchased at auto parts, gas stations and large retail stores everywhere.
2
Look for signs of a bad head gasket. If you notice coolant leaking from the engine beneath the exhaust manifold and white smoke coming from your exhaust, your vehicle’s head gasket is probably blown. A blown head gasket will result in coolant leaks, engine overheating, a severe loss of power and the previously mentioned exhaust coloring.^{[11]
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- A blown head gasket requires taking the cylinder head off of the engine to repair, so it may be best left to professionals.
- If you believe the head gasket in your vehicle has blown, stop driving it immediately.
3
Determine if your thermostat is functioning properly. Your vehicle’s thermostat determines the functioning temperature of your engine. If the thermostat fails in the open position, the coolant will flow continuously through the radiator and the engine will run cold. If it breaks closed, the coolant will not be able to reach the radiator, resulting in overheating. Look for signs of leaking or oxidization around your thermostat to determine if it is causing the issue.^{[12]
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- If the thermostat is rusted, it will probably not be able to function properly.
- A leak coming from the area around the thermostat would cause it to fail to function.
4
Use an OBDII scanner to assess engine error codes. If the check engine light has come on in your car, plugging an OBDII scanner can help you identify exactly what’s gone wrong. Plug the OBDII scanner wire in the port beneath the dashboard on the driver’s side, turn the key to “accessories” and turn on the scanner. Depending on the scanner, it will provide you with an error code to look up or the English description of the error that occurred.^{[13]
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- Many auto parts stores will use their OBDII scanners to check the codes in your car for free.
- If your scanner only provides the error codes, check the auto maker’s website to find out the description of the error.
5
Look for coolant leaks. If your coolant system is leaking, it will not be able to properly cool the engine may even result in overheating. When visually inspecting the coolant system, be sure to look for signs of spraying or leaking coolant coming from the radiator as well as the hoses and pump. Follow any coolant you see in your engine bay up to its highest point to identify the source of the leak.^{[14]
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- You may want to spray the engine with a hose to remove all the old coolant, then start the engine again to identify where new coolant leaks out.
- If the leak is in your radiator, it will either need to be repaired or replaced by a professional.
6
Use a pressure tester to locate hard to find leaks. Another tool you may opt to purchase to identify issues with your coolant system is a pressure tester. Remove the radiator cap from your engine and attach the pressure tester in its place. Turn on the heater in your vehicle without starting the engine to allow the coolant system to develop pressure. Watch the gauge on the pressure tester for sudden drops in pressure. A drop in pressure indicates a leak. Then listen in your coolant system for the sound of air escaping to locate the leak.^{[15]
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- Pressure testers can be purchased at your local auto parts store.
- Your coolant system will need to be drained to properly utilize a pressure tester.

Community Q&A

Question

I replaced my radiator, my thermostat and the top radiator hose and my car is still overheating. What's the problem?

Community Answer

Try doing a pressure test to ensure there is no leak in your coolant system. Assuming there isn't, your water pump has likely gone bad.
Question

What makes my engine overheat?

Logic Johnson Lafontaine

Top Answerer

There's either not enough coolant going around to handle all the heat that your engine is producing, or you may have a leak under the hood.
Question

What do I do if the thermostat gauge won't move?

Community Answer

If your thermostat gauge fails to work, it may be the result of a faulty coolant temperature sensor. Refer to your specific vehicle's service manual to locate and replace the sensor.