exhaust pressurizing cooling system?

[Calvin Krug]

At 1:50 AM -0500 11/10/00, quattro-request at audifans.com wrote:
>
>Someone just posted a link to a place that sells Modine radiators and I
>checked it out since my 4ktq's radiator is leaking at the seam near the top
>(a fresh puddle filling up that little crack on top whenever I go for a
>drive).  One of the pages on that site talks about types of radiator
>failures and when I came upon this one
>(http://www.radiatorinfo.com/blown.html ), it describes my symptoms but
>seems very far fetched.  I just don't see how the exhaust gasses could
>penetrate the coolant jackets?  Besides the fact that if the pressure got
>that high anyway the expansion tank cap would just let off the excess
>pressure right?  So am I nervous over nothing other than a radiator that is
>past its service life?
>
>
>Thanks!
>
>-Marc-

This type of leak is generally caused by a bad head gasket, or worse, 
a cracked head or engine block.  Exhaust gasses in the top of the 
cylinder are forced through the crack (or bad spot in the gasket) and 
into the nearest cavity they can find, the water jacket.  Don't get 
too worried yet, since I haven't heard anything in your symptoms that 
would point to this.  Odds are that your radiator  is just a little 
weak at that seam, and as the coolant gets hot, the pressure causes 
it to leak.  The best way to check for this other, exhaust gas, 
problem would be to do a compression test on all the cylinders.  If 
one is significantly lower than the others, that would indicate that 
there is some kind of pressure leak in that one, and you should 
probably investigate.

Calvin Krug