Is your car overheating when idling? A car that runs hot just idling in traffic can be frustrating and will cause engine damage if left unchecked. In this blog post, I’ll cover the common reasons cars overheat, especially at idle. Then I’ll show you how to diagnose the issue so you can beat the heat!
There are several things that can cause a vehicle to run hot when idling, from low coolant levels to a malfunctioning radiator fan, even a faulty thermostat. This article will run you through some of the key things you need to know about your cooling system, and where to go next.
Let’s get started…
There are many causes of engine overheating, however, if the issue only happens at idle there is a big chance the cause is either poor airflow across the radiator, or a restriction in the cooling circuit.
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The first thing to consider when a car ONLY RUNS HOT AT IDLE is the fact a cooling fan, either electric or mechanical needs to provide enough airflow over the radiator to cool down the engine.
The second thing is there needs to be enough coolant circulating properly through the engine, at idle the water pump impeller is moving at its slowest RPM. There is a good chance that when you are at 2,000 rpm driving down the highway the car will run cool as a cucumber.
At idle the game changes however natural the airflow stops, and the coolant moves much slower through the cooling passages.
If the car runs cool driving down the road, this proves with enough air flow and or higher water pump impeller speed, the cooling system is capable of cooling the engine. This helps us narrow down some key things to focus on first…
- Low coolant
- Inoperative cooling fan
- Bad thermostat
The things listed above are the most common causes of a car overheating when idling so we will break these down first.
Let’s get started…
Low Coolant Level
A low coolant level can cause overheating anytime the engine is running, at idle however the problem is amplified due to the slower water pump rpm and coolant flow. When the coolant level is too low obviously there will be high running temperatures.
Whatever coolant is left in the engine can’t displace the heat fast enough as it travels through the coolant passages and radiator, the result is high engine temperature, especially at idle.
The first step to diagnose high running temperature at idle, (or anytime for that matter) is to check the coolant level. If the level is low the system should be topped up and pressure tested to check for leaks.
Malfunctioning Radiator Fan
A malfunctioning radiator fan will cause the engine to overheat at idle especially as it fails to provide airflow through the radiator. The fan needs to take the place of natural airflow through the radiator when driving down the road.
This is the most common cause of an engine overheating at idle. Be sure to pay attention to the fan when the car is running hot, if the fan is not running it should be. If you have access to a decent scan tool, hook it up to the car so the live engine temperature can be monitored. If the engine reaches over 90-104 Celceus the cooling fan should be running.
There may even be a trouble code for fan activation. If the engine is hitting the cooling fan setpoint and the fan is not running, you need to find out why.
Faulty Thermostat Valve
A malfunctioning thermostat can cause problems at idle and while driving down the road. A thermostat is designed to open and close to keep the coolant circulating in the engine under control, this allows the engine to stay at the optimal temperature while driving. Many times thermostats can jam, either in the opened or closed position.
If the car runs too cool driving down the road chances are the thermostat is jammed open. If however the thermostat jams closed, the problem becomes much more of a bigger issue. When the thermostat jams closed no coolant can flow and major overheating will occur.
A clogged radiator can lead to engine overheating. Inside a radiator, there are very small channels that the coolant flows through as the outside air travels through the fins, stripping the coolant of heat.
Over time in neglected cooling systems, these channels can get clogged and coolant flow is diminished, the radiator can no longer flow enough coolant to do its job and the high steps start. A restricted radiator can be a bigger problem at idle because the water pump is turning slower and not forcing coolant through the restriction.
Bad radiator cap
Radiator caps are designed to hold a set pressure, the pressure within the cooling system is what changes the boiling point of the coolant. If the cap does not hold pressure or doesn’t vent pressure after the pressure exceeds the predetermined value, there will be issues.
I have fixed a Honda that was running hot with a simple radiator cap, The cap was not holding pressure, and the dealer’s diagnoses?
A new engine… guess who that customer comes to for work now? You guessed it, Scoot’s!
Bad Water Pump
The water pump is the heart of the cooling system, without it no coolant can move through the system and the engine will overheat very fast. Most of the time water pump bearings will fail, or they will leak, however, there are times when impellers rot or break, or some other weird issue happens and these fail.
New vehicles, especially BMW use electric water pumps. These often fail and set trouble codes, often while the engine is red hot. A good way to check an electric BMW water pump is to run the vehicle and set the defrost to full heat and remove the coolant cap, you should see coolant “pissing” back into the reservoir.
How to pinpoint a car overheating when Idling
Now that we covered the potential reasons for a high reading on the temperature gauge, let’s take a look at what to do next to rectify the problem. First, confirm the coolant level and make sure there are no leaks in the cooling system. I also recommend checking the radiator cap to make sure it can hold the rated pressure of the system.
Scan the vehicle for trouble codes and record the Data
If you have access to a good scan tool hook it up and see if there are any codes in the engine computer, don’t forget to scan all of the other modules as well to check for any other underlying issues that may provide clues.
If there are any stored codes check the freeze frame data which will give you a snapshot of what the car was doing when the code was set, rpm, engine temp, etc, this can be invaluable in diagnosing an intermittent issue.
Check for proper operation of all cooling system components
Once the codes are pulled and the radiator cap and cooling system have been pressure tested, you may have the answer, if not the next step is to shift our focus toward ensuring that all other components are functioning correctly.
If there are no codes…
Using the scanner, monitor the engine temperature, if the engine reaches over 90 to 104 Celsius, the cooling fan should be on, if not keep running the vehicle a little longer and see if the fan kicks on after getting a little hotter. Turn the ac on, when the air conditioning is on, the electric cooling fan should run to keep the high side pressure at bay. If the fan stays off you have an issue with the power supply, a pressure or temperature switch, the fan controller, or the fan itself.
If the car has a mechanical fan, be sure the clutch is locking up at this point. If you rev the car a bit it should sound like a small plane under the hood, this confirms the fan clutch is engaged.
If the cooling fan runs… or locks Up…
If the fan is moving air through the radiator you should feel hot air blowing into the engine compartment. If excess heat is not felt blowing through the radiator, just cool air, you most likely have an issue with the thermostat. If you can reach the lower radiator hose CAREFULLY feel the hose and see if it’s cool. If the hose is cool and the upper radiator hose is red hot, this indicates a definite circulation issue.
Replace the thermostat, bleed the cooling system, and re-check for overheating. Either way, a thermostat should be replaced after a vehicle overheats a few times, either while driving, or at idle.
If the cooling fans are running and the airflow is still cool after replacing the thermostat, be sure you bled the cooling system properly. If you get to this point and there is still an issue the problem may be internal restriction or something more complex to fix.
This more in-depth diag needs to be done by a pro. There may be deeper issues such as a blown head gasket, a cracked cylinder head, or even an electrical issue with the gauge itself or a temperature switch.
A car that keeps overheating when idling can be a frustrating and potentially serious issue. It’s important to understand the common causes of this problem and take action before a problem turns into a HUGE problem.
Low coolant levels, malfunctioning radiator fans, faulty thermostats, clogged radiators, and bad water pumps are some of the main culprits behind overheating when idling.