Fill ‘Er Up! How To Properly Fill A Radiator

By Jefferson Bryant September 1, 2024

in Engine & Transmission, How To, Sponsored Content, Sponsored Content by U.S. Radiator

Not all cooling system issues are caused by faulty parts, especially after a new installation or fluid change. The biggest mistake most DIYers make when adding coolant to the radiator is not properly burping the system. There are multiple ways to do this, depending on the type of system (open vs. closed), position of the radiator in relation to the engine, and the engine itself. 

Feeling Bloated?

The issue with filling a radiator is that air is easily trapped inside the engine, hose, and heater core, leaving you with multiple issues. The first one is that your cooling system is not properly filled. An air pocket not only takes up room where coolant needs to be, but also can block coolant from entering other areas, so you think the system is full, but isn’t. If you only poured a half of a gallon, but the tank is full, you likely have an air lock. 

Symptoms of an air pocket in the cooling system include erratic temperature swings, overheating, no heat from the HVAC (when set to heat), or low temperature readings. Notice that we mentioned temperature twice. This is because there are two different symptoms here. An air pocket can cause erratic temperature readings, such as going from cold to normal operating temp in seconds, and then getting very hot, and then suddenly swinging down to the normal operating range again. This can be caused by an air pocket interrupting the flow of water through the thermostat. The other issue, which is more serious, is air becoming trapped around the temperature sensor, so the gauge is not seeing the coolant at all, so it can’t read the actual temperature. 

While any engine can get air pockets, some engines are inherently prone to trapping air. LS engines for instance have a habit of getting air locks. The steam lines in the heads help reduce this, but it takes time for them to work out naturally. If the engine or heater core are mounted above the radiator cap, air is easily trapped in the system. 

Under Pressure

While the average DIYer doesn’t have to fill their cooling system often, the reality is that for less than $100, you can own a cooling system filling kit that uses vacuum (negative pressure) to draw all of the air out of the entire cooling system, and then with the flip of a valve, that same vacuum draws fresh coolant into the system. This is the fastest way to guarantee that the system is not only full, but the vacuum process also shows you if there are any leaks. A slightly loose hose clamp will ruin your day if the hose pops off on the highway, finding it before that happens is worth the cost of the kit alone. Some filling kits also have pressure pumps, which are very useful for finding leaks in a system.

The basic process with a vacuum filler is as follows- attach the cap valve to the radiator, close any expansion tank cap. Connect an air compressor to the venturi valve, open the valve to draw vacuum on the cooling system. The radiator hoses may collapse, this is normal. Once the gauge reaches about 25 inches of vacuum, close the valve and disconnect the air compressor. The vacuum should not drop, it should hold at the same level for at least 30 seconds. You don’t need to let the system rest under vacuum for a long period of time. If there is a leak, the vacuum will not hold for long. 

Insert the filler tube, which is attached to the main valve, into a fresh jug of coolant and open the filler valve. This will draw coolant into the engine. Once the vacuum pressure is gone, the process is repeated until the system is full. This usually takes 2-3 vacuum draws for most vehicles. 

Manual Fill

Most DIYers manually fill their cooling systems, which is just fine, it just takes a little more time and effort. Start with a cool engine, (Never open a radiator on a hot engine) fill the radiator with the correct type of coolant until it is at the top of the radiator. Wait a few minutes for it to lower, as the air takes some time to work its way out. Once the radiator (or expansion tank) is full and no longer dropping, start the engine and allow it to come to operating temperature. Turn the heater to max heat, this opens the heater core coolant valve, allowing coolant to flow through the heater core as well, where air can become trapped. During the warmup period, the coolant level will drop. Add more coolant as it drops below the bottom of the water neck or “full” mark on the expansion tank. Leave about a half inch of air below the radiator cap mount, this provides room for the coolant to expand when hot. 

Once the engine is at operating temperature and the tank is full, replace the cap and go for a drive. Bumpy roads are great for breaking up trapped air. Return home, allow the engine to cool, and then open the cap, top off as necessary. It may take a couple of days to get all the air worked out of the system. 

Got A Headache?

An old timer once taught us a trick years ago for dealing with stubborn air pockets. His trick was to remove the thermostat, pry it open with a screwdriver, and slip a couple of headache pills between the body and spring of the thermostat, and then reinstall the T-stat. The pills held the thermostat open for about 15-20 minutes once the engine was started, allowing the coolant to flow freely, purging any trapped air in the engine within minutes. Within 30 minutes, the pills completely melt, and the thermostat operates normally. 

If you find yourself fighting a tricky air pocket, you could have a small air leak that is big enough for air to pull in, but not quite large enough for coolant to weep through. The vacuum kits are really good at finding these leaks. If you pull vacuum on the system with coolant in it, you can often hear the bubbles in the tank. 

Ensuring that your vehicle’s cooling system is properly filled is paramount to its operation. Leaks and air pockets generate a lot of frustration. Take your time to do the job right and your engine will run at the correct temp without issue season after season. For more information on setting up your vehicle’s cooing system needs, give the folks at U.S. Radiator a call at 800-421-5975.