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320i Cooling System

Hi, I'm a 320i junkie. I have been working on them for a few years and I 
have had a recurring problem with the cooling systems periodically.

I had a chance this weekend to work on a 1979 BMW 320i. The car's 
motor was really tired - low compression and poor leak down 
measurements. The car had a similar history of overheating over a six 
year period as I have occassionally experienced; unable to idle in 
traffic on hot days. All other driving seemed to be ok.

I first pressurized the cooling system with the Stant system (pump with 
a gauge and adapters for the radiator)and found when pumped up to 14 
psi, it leaked down to about 8 psi in a few minutes. I found a leak in 
the water pump seal indicated by fluid dripping from the vent hole on 
the pump. I pulled the radiator and sent it to Radiator land for 
cleaning. These plastic/aluminum radiators can be flushed but not 
dipped. The result is they can clean the inside ok but they are not 
effective at getting the bugs out of the fins as they only wash down the 
outside with mild pressure as not to deform the fins. I had to do most 
of the exterior cleaning by hand.

I installed a new water pump, thermostat and the cleaned radiator. I 
drove the car for awhile and the standard position for the temperature 
gauge was 12:00. The gauge now ran cooler at 10:00 and was consistent 
so, I am convinced the work on the cooling system lowered the 
temperature. Everything was fine until I went to the car wash and waited 
in line on a hot day - the temp gauge went into the red. I shut of the 
motor and cooled it off. An inspection of the coolant indicated the 
level was lower than when I topped it off.

I took it home and pressurized the cooling system again and it still 
leaked down from 12 PSI to 8 PSI in a few minutes. No obvious leaks.

I pulled the head off and took it to my machine shop. After close 
inspection we found a crack between the combustion chamber and the 
cooling jacket. The machinist indicated all aluminum head/cast iron 
block engines are very sensitive to any overheating - in fact one event 
can crack the head. It depends on many factors including the current 
status of the head bolt torque, the maximum temperature and duration 
incurred, and how the overheated engine was cooled down; let it sit and 
cool it slowly - never put cold water in the radiator when it is in the 
overheated condition as the cast iron will cool at a different rate as 
the aluminum. The machinist indicated the crack may not pass hot gasses 
to the cooling system until it reaches a certain hot temperature. Once 
the crack expands, it then feeds hot gas to the cooling system and 
overwhelms it. This condition may have very well been existant for six 
years and the car still had plenty of power and no dramatic loss of 
coolant.

I have heard of an exhaust gas detector that is inserted into the 
radiator and indicates the presence of exhaust in the cooling fluid. I 
am researching this now - if it isn't too expensive I'll buy one. Is 
anyone familiar with this tool???


The heads are not repairable - if they are cracked, they are scrap 
aluminum and fetch less bounty than aluminum recycleable cans.

I have procured a new head and wish to increase the capacity of the 
radiator. Is anyone aware of an oversized radiator that will fit in 
place of the stock??? I'm tired of running on the edge with the original 
design. Thanks for any help!