[henchman]

I let my Xterra warm up this before leaving for work, and noticed that after a while it began revving high and then low. When I left, I noticed that it's not blowing warm air and overheated until it started steaming.
I refilled it with coolant and checked rad cap which looks ok. When I started it again, it idled fine until it started warming up, then the idle was up and down again and guage started reading high.
I'm guessing it's probably the thermostat, but not sure if that would cause what I'm seeing with the idle. Any ideas what it may be?

 

[dejablue]

any bubbles under the radiator cap ??

 

[henchman]

Not that I saw. If there is, what would that be from?
The first time I started it, I did notice a small amount of coolant under the area of rad cap or coolant overflow bottle. Too dark to tell exactly where it came from.

 

[dejablue]

Bad head gasket . With engine idling radiator cap off do you see any air bubbles ??
If you want to be sure rent a radiator hand pump from AutoZone to test it come with hand pump hose and special radiator cap you pump it up to 15 lb and check it 30 min later to see if you still have 15 lb

 

[henchman]

It was fine yesterday; would the head gasket go just like that? When I filled the coolant and let it run without cap on, it just shot coolant out after warming up a bit.
I'll check around heads and see if there is any coolant. When I drove it this morning, the front of the engine looked wet like it was coming from the rad area.

 

[henchman]

Started it again and can't see any leaks. It idled high and low again but not as long and it's not overheating. Also had heat and Guage is reading a little on the cooler side. It is low on coolant.

 

[dejablue]

Maybe the thermostat just got stuck.What color is the water ?
I would get a new cap first that is only $5 if it doesn't hold a seal it will over heat.