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Bleeding the Chrysler 2.2/2.5 liter [and similar] cooling system

by Damien Civiello

Click here for overheating issues and solutions

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:

Method 1: The "correct" way

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:

  1. Fill the system as normal with your antifreeze mixture. Always use a mixture of antifreeze and water (distilled works best) of at least 50%, though you can go as high as 70% antifreeze. An in-expensive tester available in most auto parts stores or the auto parts section of larger stores (Wal-Mart, etc.) that will tell you the concentration of antifreeze in your system.
  2. At the top of the water box where the upper radiator hose enters the engine there is a small threaded plug with a hex fitting. Using the correct Allen wrench loosen the fitting to the point where air starts to escape. Ensure the heater is left on to allow water into the heater core.

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.

  1. Continue to add antifreeze to replace the air being expelled through the plug.
  2. When fluid starts to seep out of the plug tighten it up, top off the overflow tank, and you're done. If you get erratic temp readings, or lack of heat from the vents in the car, re-open the screw and allow any residual air to escape.

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.

Method 2: The "other" way

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.

  1. To vent a system in this method raise the car such that the radiator is higher than the water box. You can do this through jacks (and jack-stands, never support a vehicle by a jack alone), ramps, or Mother Nature, by finding a nice steep hill and parking with the nose of the car pointing up the hill.
  2. You'll need to let the car warm up so that the thermostat opens for the system to vent in this method. Never open the cooling system when hot or you could end up with a very hot shower of antifreeze. Therefore let the car warm up from cold with the radiator cap removed. You may get some spillage while the coolant expands and the air bubbles out. Be careful as this coolant will be hot.

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.

  1. Allow the engine to run, with the heater on (fan can be off or on low) until the thermostat opens and all the air is allowed to purge. Once the thermostat opens you will see the coolant level inside the radiator bubble and drop. Continue to add antifreeze to maintain fluid level. Again be careful as the coolant and any steam released by the system will be hot. Once the upper radiator hose becomes hot to the touch and no further air issues from the system carefully replace the radiator cap and ensure the overflow bottle is filled to the Max line.
  2. Continue to allow the car to run to allow the temperature to stabilize. If you don't have a temperature gauge allow the car to run until the fan cycles on and off at least once. During this time ensure there are no leaks from the system and that the upper radiator hose gets hot to the touch (especially close to the radiator). If not then allow the system to cool, and repeat the steps above to purge any remaining air.

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.

For more information and tips, click here.

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