This morning I took the rad cap off to take some coolant out to make room for a bottle of royal purple . I have not used the truck for at least a week or more. The truck has 20,000 miles on it. As I took the cap off there was a slight hiss as if the system was still under somekind of pressure. Is this norm? If not what could cause? I have had no leaks /no signs of engines digesting any coolant. tank was right at the mark and no signs of any oil etc in coolant.

did the inside of the tank smell like combustion gasses? might be a slight headgasket leak that is pressurizing the system.

I agree with nosliw:

Things that will tell you you have a blown head gasket:

Water in Oil (milky looking),
Oil in water,
LOTS of pressure in the cooling system. Blown head gasket will allow exaust in the water/cooling system. Once the engine has cooled down, remove the resovoiur cap, If there is lots of pressure that blows the water out, chances are is a head gasket.
Check to see if you have smut or black exaust around the top of the resovoiur neck.
If your truck sits over night & still has pressure in the cooling system (again, remove the resovoiur cap s-l-o-w-l-y & see if pressure blows water &/or air out)
If your top radiator hose holds pressure (is hard) for more than 2 or 3 minutes after you kill the engine.

All signs of blown head gasket.

It was more likely a slight vacuum, which would be normal.

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It was more likely a slight vacuum, which would be normal.

. That's right, sort of! Most probably a faulty cap! It's important that the system be able to "breath." Many factory units are bad! Be safe and save yourself some problems down the road, get a new cap!:grd:

To be on safe side Ill get a new cap since all things described in dixie1 post I do not have. Question is should I replace with orig from stealer or replacement from auto parts?

Most went to a "auto zone" (part # 7046)(about $8) cap because it was from a different manufacturer, and it worked as advertised! Don't be surprised if after a few rides your low on coolant, just means the system is working properly now and has bled the air out . :exactly:

This was a pretty heated argument last year. I tried several new caps and they did nothing for my trucks, one way or another. Some people thought that the system was designed to hold a vacuum to slow the bacteria growth in the Dexcool.

my first vote rad cap.second still in debate

I notice that my truck always has some pressure in the morning. I have been monitoring my coolant level every morning. Sometimes it's down a inch then the next morning it's fine. Mst times when I remove the cap and there is some pressure the level comes up to the full mark.

The dealer sent off a sample of oil to check for coolant and it came back negative. The dealer pressure checked the engine over night and found no leaks or moisture.