I just replaced the Radiator in my 89 4runner "22RE" however I still have an over heating issue. Within 10 miles or so on the highway at 70mph the temp gauge will start creaping up and not go back down till I stop and let it cool off. The system will exspell the exsess coolant to the overflow when it gets hot like it should but doesn't suck it back in after it cools. When I take the Radiator cap off it makes a suck sound like its holding a vacuum, because shouldn't be pressure that wouldn't make any sense.
In the process of a cooling system pressure test as we speak and its holding at just over 12 lbs from 15 lbs 5 hours ago. FSM says 10.4 to 14.9 lbs so I'm on the low side but still within specs so far.
I'm not sure WTF is going on? I'm wondering could it be that both Radiator caps I have are defective? I'm thinking get another new one and see what happens BUT?
Any Ideas would be gratefully appreciated!

 

On a related note:
Either somebody omitted part of my FSM or I don't have a heater core? Since it gets hotter inside when I slide the little lever on the dash I'm inclined to believe its the former not the later. However nowhere in my FSM does it even say one word about any heater core let alone what all has to come out to get to it? I know, I know, start with everything in, on or around the dash and go from there. I've done a core or two in my day but never on this ride and it would be nice to know any tricks or tips or shortcuts if anybody has any?

 

the radiator overflow will tend to fill up, if it was overfilled to begin with.

i filled my radiator within 1" or so, and it still overflowed for a bit... same thing as yours, it had vacuum under the radiator cap, after filling the overflow.

i'm not sure that a heater core can make the engine overheat?

 

No I didn't mean heater core can make it overheat however it does a pretty good job of cooling it down in a pinch!
I just found it strange as detailed as the FSM is no mention at all about the heater core or how it's tied in with the cooling system.