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The Right Way to Change Coolant in Your C4 Corvette

Technically Speaking

James Berry Jul 25, 2018
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Question: Hello to everyone at Vette magazine, I recently bought my father’s Corvette and it seems to be running a little hot. When I checked the coolant it looks very dirty and there is a lot of red, muddy rust around the radiator cap.

I would like to do a coolant flush. Is this just as simple as draining and refilling the coolant?


Answer: Mark, it’s great to see these Corvettes being passed down from generation to generation.

When the cooling system becomes contaminated, the cooling system should be flushed thoroughly to remove the contaminants before the engine is seriously damaged from not cooling correctly.

Some common problems from contaminated coolant can range from the radiator or heater core becoming partly stopped up from the rust, head gasket problems from the system not cooling properly, or the freeze plugs rusting out and causing a leak. These are just a few of the problems that can be caused from dirty coolant.

Draining and Filling the Cooling System

To avoid being burned, do not remove the radiator cap (surge tank cap) while the engine is hot. It’s possible for the cooling system to release scalding fluid and steam if radiator cap is removed while the engine and cooling system are still hot and under pressure. Let the vehicle completely cool down before starting this process.

It’s best to raise the vehicle and make sure that it is level for getting the coolant out. Before raising the vehicle please refer to the lifting and jacking instructions for your vehicle.

Remove the radiator cap.


Place a drain pan under the drain plug and then turn the radiator drain plug. If no coolant comes out of the radiator the radiator drain may be stopped up with debris. You may need to use a small wire and poke it into the hole to try and clear the debris so the radiator will drain. If this does not work you may need to remove the lower radiator hose. Again, this should be done when the vehicle is completely cooled down. Be careful and remove the lower hose slowly so the coolant can be directed in the drain pan.


In order to get most of the coolant out you will need to remove both knock sensors out of the engine block. The engine block holds more than 50 percent of the coolant capacity so this step is necessary to get a suitable flush. These sensors are located in the center on both sides of the engine block just above the oil pan. To remove the knock sensors you will need a 7/8-inch socket.

If the system is contaminated with rust you may need to flush the coolant. I do not like the idea of using a chemical flush. If you are flushing the system I prefer this procedure:

Remove the surge tank and flush the surge tank with clean, drinkable water.


Remove the air bleeder screws located at the water pump and throttle body and make sure the passage is not stopped up.

Remove the thermostat and rinse through the engine with a garden hose, being sure to capture all of the contaminated water so it can be discarded correctly. This would be a good time to replace the thermostat.

If your system was very contaminated you can install distilled water (see the filling procedure below) and run the vehicle for approximately 20 minutes, let the engine completely cool down then follow the draining procedure again.

Filling Procedure

Install the knock sensors, install the radiator hose and install the drain plug and anything else you remove for draining or flushing the cooling system. If the thermostat was removed for replacement or flushing, fill the engine block with coolant before installing the thermostat, this is not necessary but makes it easier to get all of the air pockets out.

Never use tap water in your cooling system you should use distilled water or, better yet, de-ionized water. The reason for this is that tap water has minerals that can form deposits inside the radiator and the cooling system passages of your engine, these deposits can interfere with the flow of the cooling system and lead to over-heating.

The proper coolant concentration is 50 percent coolant, 50 percent distilled water or de-ionized water. You can use AMSOIL cooling system treatment, which provides corrosion and rust protection for modern aluminum and cast-iron cooling systems. This additive can also reduce coolant temperatures by as much as 20 degrees Fahrenheit.

The use of a coolant filling funnel will help with getting air bubbles out and the coolant not spilling when filling or when the thermostat opens up (see the lead photo). Simply refill the system through the surge tank using the filling funnel until the funnel is approximately 1/3 full.

Start the engine and allow the engine to idle for approximately seven minutes with the heater on. Monitor the coolant temperature during this time and make sure not to overheat the vehicle.

Also monitor the filling funnel level closely. The funnel coolant level will begin to decrease once the engine is started or the thermostat opens. Be sure to keep coolant in the filling funnel.

Open the air bleeder screws located at the water pump and throttle body long enough to bleed any air out of the system. When coolant runs out of the bleeders there should not be any air left in these areas.

When the thermostat opens the coolant in the filling funnel will become hot. After approximately 10 minutes and the coolant in the filling funnel is hot shut the engine off, remove the filling funnel and install the radiator cap.

Turn off the engine and let the car cool completely down (approximately 3 to 5 hours). Inspect the cooling system for leaks, and then remove the radiator cap and top off the coolant and the overflow tank as necessary.

Mark, be sure to store used coolant in the proper manner, such as in a used engine coolant holding tank. Some auto parts stores will take your used coolant. Do not pour used coolant down a drain. Ethylene glycol antifreeze is a very toxic chemical. Do not dispose of coolant into the sewer system or ground water.

There’s a label under the hood of your vehicle that will give you the coolant requirements and the filling quantity. This information can also be found in your owner’s manual. Thanks, Mark, and let me know how this works out for you.

Photography by James Berry



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