Cooling problems on my GMC 250- have a ?

[68gmsee]

Today, the first thing I did was flush it. Then I went to the auto parts store and got some antifreeze. While I was there I got a new cap. The guy at the counter said the recommended pressure was 13 lbs and the one on the truck is 16 lbs.

Not sure why I have a 16 lb one. I replaced it years ago when I bought the 3 core radiator and maybe the rad shop gave me the wrong one.

Anyway, I then took the 180 degree t-stat and put it in a small pot. I used a candy thermometer and made sure it opened at the correct temp and stayed fully open until it cooled back down.

Then I filled the rad up with about a 60 water/40 antifreeze mixture. I drove it around the block for a while and it stayed steady just slightly above the first mark (that's my comfort mark). I'll take it out on the main roads and give it a more thorough test tomorrow.