Pressure testing might not reveal anything if they only leak when warm and the metal has expanded. Here is a TSB on what to look for when you pull the valve cover. It also says pressurizing the cooling system could help revel the leak. I would do this when both warm and cold.

http://ww2.justanswer.com/uploads/Bl...t_loss_5.3.pdf

 

Hey just a quick FYI for a cheap/easy fix for this issue. My buddy had a 04 that had the same issues. Coolant loss, no visible leaks, nothing in oil, etc. He had to add coolant about every 2 weeks with regular driving.

We ended up getting some Bars Leaks tablets and putting all 6 in the top radiator hose after dissolving them in hot water.



The light hasn't come on since. God knows where it was leaking from but its not doing it any more.

 

Well there are no bubbles in the reservoir and I had it pressure tested and they found nothing. They tested it hot. They put pressure on it and let it completely cool overnight, nothing. Put pressure on it and warmed it up and nothing. They pulled the plugs and found the number 8 cylinder plug was very clean compared to the others. So they pulled the plug wire and ran it and nothing. They would hope it pooled or puddled but nothing. Now, I am driving between 1200-1400 miles in between refilling with usually about a gallon so that is minimum amount and apparently not enough to diagnose an issue. I am going to check the head casting when I get a chance and see if it is the casting that had the flaws. hoping that is what it is but who freaking knows at this point......... Thanks for all the suggestions guys. Will update as I get a chance to work on it.

 

For me, it was obvious it was my heads. I pulled the valve covers and there was sludge all over the top of the head and inside the valve cover.

 

Sounds like an all too familiar problem. I had the same symptoms. Ended up being the drivers side head or head gasket. Not sure which as I did not tear it down, but that was the side with the milkshake. It took a long time for the the oil to sludge(gel) up. When it did, it caused the Oil Level light to come on. I knew then it was time to swap up to the 6.0

 

Well I still have no answers as I have not pulled it apart and will not anytime soon. I have torn my patellar tendon and going to have surgery soon and am pretty much limited in what I do as not to make it any worse. Actually had a conversation with someone at GM who said due to the mileage they would do nothing towards the expense. Guess I am going to drive it until it blows then look at alternatives. 6.0l maybe...

 

I had the same problem on an 04. I chased it for 2 years, with head gasket, pressure test, die test, smoke test. I finally changed the heads out and it fixed it. there was a small hair line crack in one of the heads. it would burn the coolant up slowly but only enough to notice after about 2000 miles. it would also pressure up the cooling system and pop my heater cores (3 of them). truck only had 30,000 miles at the time. I bought it used. ran perfect just used water. buy a set of heads and gaskets and be done.

 

I have had the same problem with 2 of my vehicles, the first one being my wife's 2000 tahoe with 148 on the clock, nearly drove me nuts for a year looking for the leak until it showed itself when I was changing the oil one day. I had a newer low mile set of heads and valve train sitting around so I re did the top end, since then approx 1,000 miles there has been zero coolant loss. The second was my 2000 z71 with unknown miles, I wasn't as lucky with it, ended up locking the motor down before I found the problem and had to swap it out with a junkyard motor. These heads can be found for cheap and the parts are not costly and it's not a very hard job. IMO well worth the effort to swap it all out.