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by Damien Civiello
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So you’re done working on the coolant system in your reliable EEK, you top off the radiator, fill the overflow tank to the minimum mark and take her down the road to test your repairs/maintenance. But you’re not done, if you drive away now you’ll have a wildly swinging temp gauge at best, at worst you’ll warp the head.
The venerable Chrysler 2.2/2.5 has the thermostat located at the front of the engine block along the top of the head where the upper radiator hose connects to the engine. The thermostat is located in the “water box” behind this connection point. The problem is that that due to the slope of the hood, the water box is higher than the radiator, so any air in the system will collect at this point. This air prevents the thermostat from opening properly and disrupts proper water flow through the motor. Therefore any time you loose or drain coolant from the system you need to bleed the system of all air when you fill it. There are two ways to do this:
If you had a copy of the FSM (Factory Service Manual) or equivalent (Haynes, Chiltons, etc.), you wouldn’t need this web page but it would give you the following procedure:
NOTE: Before you fill the system completely it may be a good idea to remove this plug and apply some anti-seize or thread sealant to the threads. Over time the steel plug tends to seize to the aluminum head and so step two above may prove just shy of impossible without explosives. If you do manage to remove it, some anti-seize or thread sealant should make it easier to remove next time.
Some people have replaced the plug with a brass one that is less likely to seize to the head, others have adapted a small valve or petcock into the threaded hole, allowing them to vent the system by simply opening the valve. A good hardware or plumbing supply store should have the parts needed for either of these modifications. Be sure that anything you adapt can withstand the temperatures and pressures under the hood.
If removing the vent plug proved to be impossible then there is an alternative method. Air rises to the top of the cooling system and gets trapped, causing the cooling system to vapor lock. However if the radiator is made the higher point in the system then the air will escape into the radiator where it will be vented out through the radiator cap and the overflow system.
NOTE: If you’re replacing the thermostat you can save yourself the wait and drill a very small (1/8”) hole in the metal body (not the brass valve) of the thermostat before you install it. This will allow the air to escape into the radiator while the thermostat is closed, yet the hole is too small to affect system operation.
NOTE: If you are not comfortable leaving the radiator cap off during warm-up then you can accomplish the same thing by leaving the radiator cap on and allowing the car to cool down after step four and then repeat steps one through four again, making sure the overflow bottle is maintained full. The heating and cooling cycle will push the air out through the overflow bottle and then suck coolant in to replace the air when the engine cools. The car must remain inclined for the whole procedure.
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