2000 GMC Jimmy. Yesterday I found about a half a cup of coolant had leaked from the truck on to the ground, looked like it was coming from dead center/front of the vehicle. I couldn't see anything wet in the engine compartment however. I added more coolant (same color/universal) and ended up adding about 1/2 gallon back into the cooling system. Today I didnt see any new puddles before leaving, however while driving to work I noticed my heat stopped blowing hot or even warm. If I turned off my heater and turned it back on it would blow normally for about 2 seconds and then taper off to cool again. I pulled over - worriedly - popped the radiator cap off (yes I was safe about doing this) but I dont recall the cap even feeling all that warm and when I did pop it off I only got a small amount of coolant that spilled out of the hole. It was still topped off from yesterday. I put the cap back on and continue don my merry way. My temp gauge has stayed normal through all of this. However I started to think I had a vapor lock so I just let the truck run without the rad cap on for about 20 minutes, came back to it, the temp gauge was higher than it normally is (I usually stay between 1/4 and 1/2 tick marks and this time it was at the 1/2 mark). The was a small stream of coolant spilling over the side of the hole down the side of the radiator, but it didn't seem like any air had come out. The T-stat and radiator were replaced recently (about 4 months ago).

Thoughts?
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Update - Drove home (25 miles, 40 minutes) no issues the whole way. When I got home I put my hand on the radiator cap - was pretty cold. Water pump?

 

Hmm, if the rad cap was cold it sounds like could be a water pump, but its not very often that the water pump would simply just stop pumping water.

This still does not explain why you had a leak before. From the center of the vehicle it could be a few things, your..umm..i think its head gasket, whatever it is its a bit of a pain to replace, i had mine go last year, and started off with a trickle here and there, and then a gush, with this did not see anything in the engine compartment as its right at the back of the motor...Also have had a heater core go, now when that happens the leaked fluid tends to go out the drain for the ac. Only other thing i could think of for a leak back there would be something with the lines running to the tranny.....

As for the temperature issue....You said with the cap off running it was spitting a bit, which would suggest that coolant is pumping. Sounds to me like a thermostat issue but it was just replaced and your temp read normal on the dash.........What i would do is run it enough so things would warm up, then feel the hoses, see which ones are hot, and which ones if any are cold. This could possibly help narrow it down.

 

If it is a Votec V6, it is possible that your intake manifold gasket is leaking. This is a very common problem on these engines in that year range. Keep an eye on it,because when it goes, it goes, and will blow coolant all over the engine compartment.

 

^ AH thats what i was talking about that i had to replace!