Originally Posted by wpoll

I found this theory relating to your problem on another forum - I'm not saying it's your issue but it possble...

(from Vector57 on M5board.com)...
Given the circumstances of the fault, I would suspect the root cause of the leak developing in
the side of the heat exchanger was due to coolant electrolysis. Old, "spent coolant" (depleted
additives) turns into a conductor, the same process you have in a battery with it's electrolyte.
All of the aluminum in the engine/cooling systems now become the source for ionic transfer of
material, literally dissolving the aluminum. The thinner the material, the sooner the leakage/failure.
This is why the radiator, heater core and in your case, the heat exchanger are the first to suffer.
It is imperative the tech working on your car should determine just how great this process has
evolved, before draining the old coolant. It is a very simple process to measure; Using a digital
voltmeter (DVOM), place the negative lead on a solid ground on the engine and place the positive
probe into the coolant (do not allow probe to contact anything but coolant). If more than 0.3 volts
DC is present, you have a seriously compromised system. To help determine if it is from a poor
chassis/engine ground, remove one of the battery cables; if no change in voltage, it is not an errant
ground path. However, second to old, "spent" coolant, the usual culprit is high series resistance in a
ground path. This turns the coolant into a weak conductor, and the electrolytic process starts
escalating. The worst of the offenders is first, the starter followed by alternator and ignition system.
All of this can be remedied by cleaning all of the ground connections and in some cases, the addition
of additional "bonding jumpers" between chassis and components (especially the starter).

At this point I would strongly urge you to replace the radiator as well, after the engine and heater
core have been thoroughly flushed. Trust me on this one, your radiator is right on the verge of
developing it's own series of pin-hole leaks. Hopefully, the heater core will be OK with fresh coolant.

Might be worth doing the voltmenter test just to be sure... 'casue something sure is tearing your heat exchnagers to peices!