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Causes (and cures) of Erratic Engine Temperature Swings

Causes (and cures) of Erratic Engine Temperature Swings

Engines do not rise in temperature and immediately drop so it is likely you have air trapped in your cooling system.  Air rises to the top of the cooling system and gets trapped, causing the cooling system to vapor lock. 

When the radiator is positioned at the higher point in the system, the air will escape into the radiator and it will be vented out through the radiator cap and the overflow system.

There are a couple of ways to resolve this and free the air.

    1. Free trapped air via the vent plug
    2. Fill the system as normal with your antifreeze mixture.  Use a 50-50 mixture of antifreeze and distilled water.  Auto parts stores will sell in-expensive to determine the concentration of antifreeze in your system.
    3. Find the small threaded plug with a hex fitting at the top of the water box where the upper radiator hose enters the engine.  Loosen the fitting to the point where air starts to escape and inspect the thread sealant – re-apply as needed.  Be sure to run your heater to allow water into the heater core and free rapped air.
    4. Add antifreeze to replace the air being expelled through the plug.
    5. As fluid starts to seep out of the plug tighten it up, top off the overflow tank, and you’re done.  If you continue to get erratic temperature readings, or fail to get heat from the heating vents, re-open the screw and let any residual air to escape.
    6. 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 a nice steep hill and parking with the nose of the car pointing up the hill.
    7. You’ll need to let the car warm up so that the thermostat opens for the system to vent in this method.  AVOID BURNS AND PROTECT YOUR EYES! - Never open the cooling system when hot.  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.  This coolant will be hot.
    8. 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.
    9. 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.

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