I've got a 5.7 in my 71 Challenger. I noticed that the cooling system would gurgle every now and then, even though the car hadn't been run in days/months. I ignored it for a while, but as I got to thinking about it, I am starting to worry it is electrolysis. It will do it, however, even with the battery disconnected. Had no battery in it for 3 months while doing the relocation to the trunk and STILL does it. It builds enough pressure in a day or 2 to push past the 15psi cap. I have a lever lock cap, so I lit an aim-flame over the reservoir. and cracked the lever. DEFINITELY flammable, and I would assume hydrogen not just oxygen since it will burn gently at the opening like a Sterno can after the initial POP. It displaces A LOT of coolant until there is an air path from the engine to the cap. If there is no battery in the loop, is it simply my green coolant not playing nice with something? Is this why Chrysler used purple coolant? What is correct for the Gen 3's?

Never heard of that. Try draining & flush the system. Add 2 bottles of water wetter than fill with distilled water & see what happens. Add some water pump lube too.

using a DVM on the DC scale, touch the negative probe to the radiator tank while inserting the positive probe into the coolant without touching the tank neck sides or core, and tell us what the reading is.
it should be on the very low side of the mili-volt scale, somewhere around .3mv or less [i think].
if i have my value wrong, could someone please correct me ?

Try a sacrificial zinc anode as a temp fix when stored and see it changes anything. It would need to be electrically grounded somehow to whatever is the source of the electrolysis is.(block/heads which are likely already bonded).

i had forgotten about those anodes. thanks for the reminder.
cheap insurance against corrosion caused by different materials contacting each other through the water's/coolant's surface.

I do vaguely remember from my Ford diesels the coolant additive to prevent electrolysis. I'll get a voltage reading tonight and post it up. I've also read that distilled water may be "electron-hungry", and soft tap water may be preferred. Either way, I am converting to water and water-wetter for drag-n-drove stuff anyway so that will take the coolant out of the equation.

Would an anode in the block drain be effective? Not many other places available for it. on a Gen 3. Maybe at the top coolant bleed pipe plug. No extra ports in the radiator.

i think anodes are available that attach by a clip, inside the upper radiator hose.
the block drain, i believe would work also, but i prefer using one in the upper hose, as it's somewhat easier to install, and is easy to inspect at any time one would want to.

Are you 100% the pressure isn't from combustion? I had an engine act like that with a very small crack in the cylinder wall.

I've bought and used anti corrosion anodes that replace the petcocks in my aluminum radiators,, both of them broke off after installing them
I'm not sure if they needed to be grounded to work or not, I left them in the bottom tanks