When it comes to coolant, you never can be too careful or too observant when it comes to checking your reservoir. Coolant leaks are common and the last thing you want to happen is to end up in the middle of nowhere, on the side of the road, completely unprepared.
If your engine is overheating, cut the engine off and wait at least 30 minutes before opening up the radiator cap and topping it off with coolant. In terms of maintenance, you should top off—if necessary—every two weeks.
Checking your radiator fluid two to three times a month is a good habit to get into. You never want to be that person with the hazard lights on and no coolant or emergency water to get you back to the house.
Why Wait 30 Minutes?
The radiator cap keeps your coolant from blowing out of the top of your radiator when the system is operating. For one, it’s very hot, which means that the molecules are racing around and expanding. For two, the system is under pressure when it is operating.
Unfortunately, due to the heat of the coolant and the pressure in the system, you have to wait until the engine cools down before removing the radiator cap, a process that will take about thirty minutes.
Of course, you may have seen an old-school mechanic use a towel or a heavy glove to remove the radiator cap when the system is still hot and under pressure. You probably noticed the explosive nature of what follows.
While you shouldn’t remove the radiator cap when the engine is still hot, an experienced person can do it, so long as you know what to expect and properly prepare yourself with some sort of shielding.
The coolant will shoot out of the radiator cap hot enough to boil your skin and cause severe burns and it’s definitely not recommended that you remove it. Once thirty or so minutes have passed since you cut the engine off, it’s generally safe to remove the radiator cap, even though it may hiss a little.
Can You Top Off the Radiator When the Engine is Hot?
You can but it’s certainly not advisable. After thirty minutes have passed, it’s generally okay to remove the radiator cap and top it off. You also want to pay attention to your reserve as well and fill it to the fill line.
When your vehicle is leaking coolant, the first thing that it does is draw extra coolant from the reserve. Once it can no longer do that, it will continue to lose coolant until the engine overheats.
So you should always fill it up as well as top off the radiator. The same goes for water. If it’s an emergency situation and all you have is water, be sure to top off the radiator and the reserve container as well.
How to Know if It’s a Radiator Leak?
You may pop the hood and realize that the reserve container is still at the fill line with coolant. If that’s the case, the system never lost enough coolant to draw from it, which is a good indication that it’s not a leak.
In this case, it’s going to be one of three things:
- Thermostat
- Clog
- Fans
A common issue with a coolant system is the thermostat no longer opens when the temperature gets high enough that it triggers an inflow of coolant. If the thermostat is no longer operating as it should, you’re getting absolutely zero coolants in your overheated system.
Clogs can stop the flow of coolant as well, or a pinched tube within the system. If there is no visible leak, you definitely want to check the tubes that are running both to and from the radiator.
If the fan is no longer operating, it has nothing to do with the coolant. While the coolant does a great job in keeping your system cool, it isn’t the only thing that performs that function. The fan also contributes to the cooling process and if it’s no longer working, that’s a problem.
Clutch fans usually operate the water pump as well, so a failed clutch fan also means a failed water pump, which means that the coolant is no longer flowing through like it’s supposed to.
Electric fans are separate from the water pump, but a failure in one of these fans will also cause the engine to overheat, especially in the heat of the summer. So if you have a full reservoir, it’s no longer a matter of waiting until you can add coolant, as the coolant isn’t the problem.
Burping the Engine
This is mechanic slang for purging the air out of the coolant once you’ve topped it off. It may not be possible to do if you’re stuck on the side of the road somewhere, but if it’s possible, you should try, especially if you’ve just done a full flush and replacement of the system.
All it involves is parking your vehicle on a slight incline—ramps are great for this—since air rises and you want all of the air in the coolant to flow in the inclined direction. Make sure that the engine is cool and the radiator cap is off.
Crank up the engine and let it run. Observe the coolant level just inside the radiator cap. You’ll notice that it rises and falls. Sometimes it will rise high enough to flow over but that’s okay. As the engine warms, you’ll also notice bubbles flowing to the top and as each one pops, the coolant level drops.
When it drops far enough, top it off a little. Let it run until the engine warms up enough to push radiator fluid out at a constant rate and then cut it off, replacing the cap when you’re done.
All Things Considered
Removing the radiator cap when the engine is hot is dangerous and should only be done by a professional mechanic or someone that is very familiar with the process. You should always try and give the engine thirty minutes to cool down before you attempt to top it off.