Last week I had to make a run from Mesa, AZ to Winslow. On the way back we decided to stop at one of the lakes on the 87. Got out of the truck and heard a hissing sound. All the tires where fine then I finally saw it was steam coming out of the cap of the catch can. Got back on the road and watched the heat peg a few times before we go into Payson. Once there we got dinner and let the truck cool for an hour before adding new water to the system. She took a little over a gallon of water. Get back in town and everything is normal, except a few times when we get home its hissing again. Today it was getting hot again while I was out. The temp would almost reach the red and then fall to normal ranges. I am unsure if this is a thermostat problem or if the catch can is supost to never let steam out or what. If it is the cap on the can is there a way to bypass it for now?

Shouldn't steam out of coolant reservoir. I would replace the radiator cap with a new one if you haven't already and try again. Luck!

x2 on radiator cap.

Also, adding just a gallon of water decreases the boiling point. You'll need to balance that out with non-diluted (no 50-50 mix) coolant otherwise you're going to boil over your coolant. Maybe that's your issue in the first place?

Do both heater hoses on the radiator get hot when the needle goes to red or just the top? If the bottom is still cold then the thermostat is not opening correctly and the fluid isn't flowing like it should.

Not sure how a rad cap makes these symptoms but I will go with it and replace it. Just cuz when I took it off in Payson it looked like it was falling apart and I told myself to replace it after I figured out why I was over heating.... that would have been fun lol.

The radiator cap is a pressure valve. Your coolant is designed to run under pressure, the more pressure the higher the boiling temperature. Eventually though it will reach a critical level and the cap will open and allow gas/coolant to escape. If you cap is bad it will do one of two thigns: 1. it will not open at all causing very bad issues like hoses bursting or 2. open too soon and not hold pressure, causing the coolant to boil.

hmm learn something new everyday, did not dawn on me that the cap is a presser valve.

In this situation, if your adding coolant to raise the boiling point, your just putting a bandaid on the problem. Pure water should not even come close to boiling in the cooling system if its holding pressure the way its supposed to. Either bad cap like others said or small leak elsewhere. If the cap doesnt do it, may want to check tstat house next. New gasket on tstat AND PERMATEX on the mating surfaces. The tstat gaskets dont hold very well by themselves on these motors.

Uh, since when?

I've never had an issue with one. And I've replaced a few.....

Ditto.

Iv had issues with them in a 95 taco and my current 96 runner. Possibly just bad luck? A very thin layer of permatex solved it both time. Im also an experienced tech And have never ran into this issue on any other motor.

The boiling point of strait coolant anti-freeze mix of 60-40 percent is well above the t-stat opening point. And example is DexCool boils at 265 without a pressure cap. If your venting steam out the vent valve either your coolant is mostly water with a dead rad cap or you have more problems than a stuck t-stat or bad cap.

The procedure to burp air out of my Saturn is to run engine without the cap on the tank until the fan cycles 3 times.

And there isn't a difference to me between DexCool and the prestone yellow universal. They are both silica free juice.

I have not had this experience.

I once took a trip from Tucson, AZ to Maine. I had a hell of a time getting out of AZ without problems. The car kept overheating. In Flagstaff I tore into it and took out the thermostat. From there I drove all night and made it to a families house in St. Louis. Next morning I went out and noticed the cap was broken and a spring in the coolant reservoir. From that point I realized it was a pressure valve and that one little part was causing the problem. Made it to Maine, Chicago, Denver, and back to Tucson after I bought a new cap.
**This was in an 1996 Olds Aurora.

Moral of the story is it sounds like your radiator cap.