My nephew asked me to search for what might be the cause to his Chevy pickup losing coolant without a visible leak.
After a little searching, and frankly not finding much, this seems to be the only page that actually points to a few ideas: http://www.fullsizechevy.com/archive/index.php/t-432699.html
Here are a few of the answers: (I didn't type these, so don't blame me for the English)
1. not completely sure but i think ive seen on here that the water pump gaskets are pretty bad on a 5.3 might check into that otherwise id say its safe to assume head gasket, if you dont have money to fix it that sealer **** will get u a little more time otherwise i personally wouldnt use it.
2. might be a cracked head. pressure test time.
3. I chased an elusive coolant leak on my 1999 / 5.3. Two water pumps, a few new hoses, intake gaskets and alot of overpriced coolant later, I found that there was a problem with the aluminum heads mated to a cast iron block on the 5.3.
The local dealer was ready and willing to sell me new ones at around $1100.00. I had a local Hot Rod/machine shop deck the heads for much less. That was about 40k ago and running fine.
I also had moisture under the oil filler cap but no milkshake. It happened to me around 120k.
4. Is the oem rad cap still on it? If so replace it first, fixed my coolant loss problem with no signs of coolant any where.
5. sounds like the same problem i had. No sign of leak..
I assume it was a cracked head in my case, the oil was showing signs of coolant sludge and low coolant, like had to fill res. once a week.
I put two bottles of bar stop and vwalla.. no more leak.
that was 70000 miles ago on my 2003. 5.3 liter I also switched to royal purple 5-30 synthetic.
I had the same cold start up knock that would go away after 1-2min.
Once i replaced the oil, added the barstop , it was good as new and runs great now... knock is gone and oil pressure is always 40-60
I dont believe that statement of not using rad-fix. Ive talked to several techs that swear by it in the newer trucks with this problem.
This was after replacing the waterpump, thermostat,radiator cap.. these didnt make a difference
Heres the write up on the head problems
scroll down to the middle of the page
6. I had the same problem a few months ago. My "Low Coolant" light would come on, I'd add coolant, drive for about a week, then the light would come back on, add more, and so on... Turns out it was leaking right above my skid plate that's under the transmission. The skid plate was preventing it from leaving a spot on my driveway or any other evidence of a leak. Just some FYI for ya.