I need help. Started out with what I thought is everyday simple task: change the coolant. I drained the radiator from the plug at the bottom left corner, then tightened it back up. Filled the expansion tank. Ran the car for a few minutes while I cleaned everything up. Checked the tank and it was still full. That's odd, I thought, but maybe there was not too much drained (I only used about a 1/3-1/2 gallon). Well I took the car out for a short run and the cabin inside temp did not go up on the climate control, that told me there was a coolant shortage. Outside temp was 59 deg and I had set the inside at 72 deg. The engine temp pegged at the top and the red light came on. The last 200 yds home the car revs were up (2700) but the speed was low (15mph). I had only gone for about a 3 mile jaunt.
Anyway, after putting the car back in the garage, i check the coolant level in the expansion tank and it's full to the brim! What next? Any suggestions welcome...

 

There's a 'screw' on the top of the fill tank. It's about as big around as a dime or so, plastic, and phillips head. It's a bleed screw that allows air out of the system. Loosen the screw when you're filling and after your last fill up with the cap on.
An air pocket as severe as what you're describing isn't horribly common, but it's also not outside the realm of the possible and a lot better option than a blockage.

 

filling the expansion tank does not fill the radiator on any car I've had. There is usually a fill cap on top of the rad. The expansion tank just allows the rad to suck back any that was expelled when hot.

I haven't looked at my Jag to see.

 

It's more of a header tank on these Jags. Also, the small screw and the cap should be opened when draining to get as much fluid out as possible.

 

Both HhisIII and cjd are right - the described expansion tank screw is for fill bleeding as well, as void venting. The expansion tank is above the radiator, and gravity fills the radiator. The screw is also where the air lock is in Na5h's coolant system.

 

Thanks to all for their replies, particularly HhisIII.
I had seen this particular screw and mine was so well molded into the expansion/header tank that it looked like it was just that: molded in. Anyhow, I did have the main cap loose (bout 1/2 a thread or so) after the engine has been cooled for 15-20 minutes or so and the fluid got sucked right out. So I topped it off then and once more a little later and no more problems. All in all, the rad took in about a gallon and a third-to-half, just what I had been expecting the first go around; before my cry for help.

To PigletJohn, there is no separate rad cap on the car; just what cjd and rathaway commented... Like you, that was also one of the first things I had looked for prior to the initial fill of the expansion tank, but did not find a cap.

 

Glad you got it all sorted!