heater core failure; reading tea leaves post mortem

[Tom Jennings <tomj@xxxxxxx>]

(STUPID G4 TRACKPAD)

So I was stupid for putting an old, used heater core in so difficult  
a place. The lowest corner of the core has some crusty whitish  
corrosion; the rest looked good.

I had vacuum-pump tested it; I even re-tested it before I extracted  
it. It held vacuum (15" or so) for >> 30 seconds, and still leaked.  
Oh well no test os foolproof.

It never occurred to me to check for electrochemical corrosion  
before. So I did. All of my cars have been cast iron blocks with  
copper/brass radiators; you keep the juices fresh and electro  
corrosion is basically not an issue.

But now I have an aluminum intake manifold, with hot coolant running  
through it. I assume that if everything is grounded the corrosion  
will be close to zero, but when I got the (used, junkyard) fold, the  
iron hose nipples had corroded into the manifold (since repaired of  
course). Millions of cars were made like this -- do they simply eat  
more brass than older American cars without aluminum in the system?

So anyways. I few weeks back I flushed the system (mild cleanser),  
new 50/50 coolant, anti-rust/anti-corrosion additive, and a wetting  
additive. No cooling system problems.

Tonight I got the car hot with the radiator cap off. Put a DVM from  
battery - to the juice in the radiator. Right after the thermostat  
opened, the voltage on the probe went up to 0.200 volts; after that  
the voltage never went higher than 0.100 - 0.140.

I tried grounding the heater core, which floats since it sits in a  
plastic box. No effect.

Should I be concerned?