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Thread: 08 Dodge Avenger 3.5L cooling system

  1. #1
    Join Date
    Aug 2015

    Post 08 Dodge Avenger 3.5L cooling system

    Besides the obvious, I am having trouble understanding the difference between the pressured cooling system and the non pressured system, and how they differ in function. I have the reservoir that looks like the pressured system. Why is it embossed with the word gas? Why does it appear to have immediate flow of coolant when the vehicle is started? I have low compression on number 2 cylinder and by all indications it appears to be a valve problem, but I need to eliminate the idea that it could be a breech in the lower intake gasket. With coolant flowing immediately after starting is this compression gas or is it normal operation? I know there is a hydrocarbon test I could do, but it would simplify my thought process knowing how the system functions.

  2. #2
    Join Date
    Jan 2007


    On the non-pressure system, the coolant reservoir only acts as a container to catch coolant that escapes past the pressure cap if the system pressure exceeds the cap's pressure rating. On the pressurized system, which your car has, the tank is an integral part of the cooling system; it's normal to see coolant flow in there as there is a supply hose and a return hose. It might be stamped "Gas" because it is pressurized when the engine gets warm and if you were to open the cap, steam would come out. It also serves to bleed air (gas) pockets from the cooling system (Ford actually calls their expansion tanks "de-gas" bottles). However, if you see bubbles in there it is possible that they are being caused by a compression leak (such as a bad head gasket). A combustion leak check would let you know if you have a bad head gasket, which could certainly cause low compression.
    A leak in the intake tract will not cause low compression.

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