Radiator cap and drain leaking...

A couple days ago I installed one of those Prestone radiator flushers, and then flushed my radiator. This involved opening up the drain on the radiator a few times, and now it's leaking.

Here's where it gets weird...

When you first fill up the radiator, it leaks out the drain. Drive for about 2 miles, and it stops leaking. Stays stopped even when it's cold. Add more water (about a quart) and it leaks again. Now, I could probably get a new gasket for the drain at a parts store, but this is an aftermarket radiator, and the drain is a bit different than stock - there isn't really a place for a gasket.

Now, tonight I just drove about 10 miles. It leaked in the beginning and then stopped as usuall. After the engine had been off for about 30 seconds, it started puking water out of the radiator cap. WHY?!?!!!

I'm supposed to drive it 300 miles tomarrow...

You mentioned a radiator cap, so I assume you either have a '91 or newer XJ or that you have changed the radiator to an "open" system. Are you using an OEM radiator cap? Do you have a tube connected to the coolant recovery bottle?

The system operates at a temperature that's at or slightly above the boiling temperature of water. The ehtylene glycol in the mix raises the boiling temp, but not that far. As long as coolant is circulating, things are in a sort of equilibrium. When you shut off the engine, the coolant ceases to flow. Where the hottest parts of the engine are located, there can be localized boiling of the coolant, which raises the pressure enough to exceed the rating on the radiator cap.

When that happens, the coolant is forced out the neck. It is supposed to flow into the coolant recovery bottle. Then, as the system cools down, it creates a vacuum and draws the coolant back into the radiator. If your radiator cap is defective or the wrong type, this won't happen. Likewise if you don't have a tube running to the recovery bottle.

I don't believe it is a stock radiator cap...says 13lbs, what is the stock pressure? The overflow bottle has been working fine up until today, the level going up and down every now and then. I did take off the cap several times, so maybe I f'ed it up...