Sudden Coolant loss—I don't think it's the head gasket

Hotter plug doesn’t make engine run hotter, just makes plug less likely to get a build up.

 

No, EGR plugging does not cause the engine to run hotter, but may be a factor in killing the head gasket, and consequent coolant leakage.

 

Correction, hotter plug = hotter tip, can in extreme cases cause predetonation. EGR recirculates inert exhaust gas which effectively enriches the mixture. Am I still off somehow? We’re talking chamber temp now…

 

You're getting closer. EGR dilutes the fuel-air mixture, which, among other effects, effectively reduces the (very brief) peak temperature of gases in the combustion chamber.

 

^ This.

Adding fuel would enrich the mixture; adding oxygen would lean it. EGR adds neither fuel nor oxygen, which is what's cool about it. It neither leans nor enriches the mixture. The mixture stays where the ECM has targeted it, the reacting molecules are just farther apart. so the energy release from combustion spreads out over a longer portion of the piston stroke, with a lower, broader temperature curve that never gets into the temperatures where oxides of nitrogen are formed.



(The left curve shouldn't be read directly as combustion temperature. It's given as Rate Of Heat Release, in joules per degree of crank angle.)

 

Low tension rings respond to nothing but new rings and ideally pistons. Assuming the cylinder walls are not scored.

 

The revised rings won’t fit in the old pistons; they’re dimensionally different.

 

That left graph should make clear why blocking EGR without retarding ignition might be a bad idea.