1988 Other Plymouth Models engine runs shaky

Do you have a 2.2 or 2.5 single over head cam engine?

If so, I may have some bad news for you.

They are prone to developing a crack between the valve seats.
And if that doesn't happen then they warp and the head gasket looses its seal.

Either one of these cases would allow hot combustion chamber gases into the coolant system. (Steam coming from overflow tank)
or low compression, and keeping the cylinder from firing.

They can be tricky to verify sometimes.
Under just the right conditions, the leak doesn't show up at room temperature.
So sometimes a leak down test, or pressurizing the coolant system doesn't show anything.

But with you leaking coolant out the back of the engine, I suspect a blown head gasket.