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BMW and MINI – Losing Coolant But No Leaks – Low Coolant

August 26, 2013

This answer is applicable for many BMWs in addition to the one listed below.

car year: 1997
car model: 328i Convertible
I am losing coolant.  The car does not overheat and no check engine lights are on.  The car seems to run fine. Where would it be leaking coolant?  I don’t have a pressure tester to pressurize and test the system. What do you suggest ?

A loss of coolant does mean that there is a leak in the system …. someplace.  Common leakage issues could be due to an external leak (hoses, gaskets, radiator, etc.), leakage to the vehicle interior,  an internal leak that is leaking coolant into the engine oil, an internal leak that is leaking coolant into a cylinder, combustion chamber pressure leaking into the coolant, as well as a weak expansion tank cap that is venting at a lower pressure than it should.

*  External leakage - Typically, an external leak is easy to spot as it will produce a trail and puddles on the ground.  However, a small leak that only occurs when the system is hot and pressurized, can leak while driving yet not produce any puddles under the car.  In this case, a close inspection when the engine is fully hot and has been running for some time, may produce evidence of the leakage.  If actual wet coolant cannot be found, look for the tell-tale dry white stains from where the coolant may have leaked and dried up.

*  Internal leakage – into the oil – If coolant is leaking into the engine oil, the oil will have a milky tan or yellow color to it.  In this case stop running the engine immediately.  Perform a complete oil and filter change and run the engine.  If the milky oil returns, there is a coolant leak into the oil.  Do not run the engine any longer until the fault is cured.  This leakage could be a failed head gasket, failed timing cover gasket (some models), cracked cylinder head or engine block or a cracked engine oil cooler (on some models).

*  Internal leakage – into a cylinder (combustion chamber) - Coolant can enter the cylinder via a failed head gasket, cracked cylinder head or cracked engine block.  In these cases, it is typical that the exhaust emits a white vapor cloud and the engine may miss or run poorly. The most common would be a failed head gasket that is allowing coolant to enter the combustion chamber.  However, a crack in the cylinder head may be suspected if the engine has been heavily overheated.  In these cases, removal of the cylinder head is in order. If the head gasket does not show evidence of a coolant passage leaking into a cylinder, the cylinder head should be closely examined in the areas of each combustion chamber.  If no obvious cracks are found, the head should be pressure tested.

*  Leakage to the vehicle interior - The vehicle interior heating system uses the hot engine coolant to make the heat.  The coolant runs through a small radiator (called a heater core) that is typically in the forward area under the center of the dashboard.  A leak in the heater core or the piping connections to the core, will allow coolant to leak to the interior of the vehicle.  this may not be initially noticed as it will soak into the carpet and the underlying padding.  A typical clue to this would be a smell of hot coolant (like maple syrup) when the heat is turned on.  In this case, the heater core and the internal piping must be accessed and a determination must be made as to what is actually leaking and needs to be replaced.

*  Combustion chamber pressure entering the cooling system - This can happen when a head gasket starts to fail between a coolant passage and a cylinder.  Initially, the fault is so small that the cooling system pressure is not great enough to force coolant into the cylinder (so no white exhaust smoke and the engine appears to run fine … except for recurring overheating).  However, the great pressure in the combustion chamber when the piston is compressing the intake charge, can overcome the failed gasket and force combustion pressure and gasses into the cooling system.  This not only adds air to the cooling system, but also increases the cooling system pressure which pushes coolant out the expansion tank overflow (via the pressure relief valve in the cap).  This eventually causes a low coolant condition as well as air in the system that needs to be bled out.  A  good symptom to look for here is that the system needs constant bleeding as well as filling and  the expansion tank is still under pressure if the cap is removed after the system has fully cooled down (a few hours).  The cure is to replace the head gasket.

*  Weak expansion tank cap - If the pressure relief valve in the cap is venting at a lower pressure than it should, this can allow coolant to be expelled from the expansion tank overflow as the system heats up and the coolant expands.  This will then create a low coolant warning.  The cure in this situation is to replace the cap.

The Bentley repair manual for your given BMW or MINI will assist in diagnosing a cooling system leak as well as guiding the repairs.  Bavarian Autosport will have the Bentley manuals as well as any repair parts that may be needed, to include; radiator, hoses, gaskets, head gasket sets, coolant, expansion tanks and caps, etc.

Click HERE for BavAuto online store (radiators, hoses, gaskets, heater cores, etc.)

Click below for BMW & MINI coolant:

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