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Audi A4 - losing coolant rapidly but no leak?

Hi all, I have a problem that my mechanic hasn't been able to figure out. I'm not particularly savvy about cars, so any help is much appreciated!



I have a 1998 A4 Audi Quattro 2.8. In the last 2-3 months, the day after I drive more than hour or two, I find that my low coolant light comes on and my reservoir is empty. I have noticed that after some of these drives, I'll see steam coming up from under my hood. Opening the hood, I'll find that coolant is coming out of the bottom of my reservoir through what I think is an overflow valve.



The strange thing is that my mechanic has flushed the coolant system, pressurized it, and says that there is no leak that they can find. Certainly no leak that could result in a loss of all the coolant so quickly. They also say that the water pump looks fine. My engine isn't overheating, at least according to the gauge.



However, the problem is really disconcerting since I'm filling up on coolant every few days.



Any suggestions on what might be going on?
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Comments

  • edited March 2011
    First you should change out the thermostat. It might be as simple as that. Try that and get back.
  • edited March 2011
    Are you talking about the thermostat measuring the engine temperature? My mechanic said he checked that and it was fine. Why would the thermostat lead to the water overflowing out of the reservoir? Are they linked in some way?

    Thanks!
  • edited March 2011
    If all the things you say were done properly, you might have a head gasket leak. Have a compression check done to verify that. If so, budget about $1000-$1500 for a proper repair, since this is an Audi.
  • edited March 2011
    Interesting. I actually had the head gasket replaced because of a rapidly worsening oil leak at the same time they were trying to figure out the coolant issue. They had noticed a few months back and thought it was fine but I was losing oil pretty quickly so they went ahead and changed it.

    Someone on another forum suggested that there might be air in the system and to start by replacing the reservoir and cap.

    Hehe, if I had known the costs of repair on Audis, I might have thought twice. I've sunk quite a bit of money into the car, but now am sort of stuck. Still working out to be cheaper to fix the car than get a new one, but if this keeps up...
  • edited March 2011
    Not the temperaure sensor, the termostat, which is a valve that opens when the engine warms up to allow coolant to circulate through the radiator. If the thermostat is defective it will prevent coolant circulation and thereby cause the engine to overheat.
  • edited March 2011
    Ahh. Makes sense. Will check on that too. Thanks!
  • edited March 2011
    I'm seeing a couple of possibilities here in addition to the T-stat mentioned by Texases, one simple and one serious.

    The radiator cap should hold the hot coolant in until it reaches a specific pressure, typically about 15-16psi, and only then allow it to flow into the reservoir. It does this because pressurized fluid boils at a higher temperature than unpressurized fluid. If the radiator cap is defective and allowing the fluid to freely run into the reservoire as it expands, it will boil over.

    Now, the bad news. If you have a headgasket leak, and based on the engine's history you might, then two things will happen.

    The first is that with every combustion in the cylinder where the breech is, hot combustion gasses (up to perhaps 2500 degrees F) will be blown into the water jacket, heating the coolant beyond the ability of the cooling system to dissipate it.

    The second is that every time the piston goes down for the intake stroke it'll pull coolant through the headgasket breech and into the cylinder, where it'l be turned to steam and blown out the exhaust.

    You can see where a headgasket breech will not only cause overheating but also severe loss of coolant.

    Fortunately, there is a definitive test for a headgasket leak. It's called a "pressure leakdown test". The cylinder is pressurized with a setup with a valve and a gage and checked to see if it holds pressure. If the headgasket has a hole, it won't. There are other more simple ways, such as if the cooling system is full and the engine is running and bubbles are coming up out the radiator cap, those will be the combustion gasses migrating up to the system high point and bubbling out.

    Your mechanic should know all this stuff. Perhaps the first thing you should consider replacing is your mechanic.
  • edited March 2011
    Thanks for the clear explanation. I should say that I am pretty much clueless about cars (ask me about something simpler like the human body or my bicycles!), so it's quite possible some of these have been done by my mechanic already. I don't have any reason at the moment to doubt his expertise but will keep it in the back of my mind (given that this is still ongoing).

    Nonetheless, I talked briefly with him this morning and will drop off the car tomorrow with all of your suggestions and see what he has to say. I'll circle back in a few days and let you all know what happened.
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