2002 5.3 loss of coolant

[SP543]

My 2002 yukon 5.3 with 236K miles has begun to loose a little coolant over a few weeks or so, about an inch in the resevoir for every 1.5 to 2 weeks. I have been researching the issue, and it seems like the 706 castech heads have been causing what i have been experiencing. I pulled the oil filler tube and it looks clean with no signs of coolant and from the valves i can see there is only oil, no milky mixing. Are my heads the good heads from this photo, there is an 862 stamped to the bottom right of the filler cap.
Next guess would be head gasket, but would there still be pressure released when opening the resevoir cap after 14 hrs or so from starting if it was a head gasket issue? I dont want to dump much many into this high mileage truck, so im thinking of using one of the stop leak products. Would that Bars Stopleak product, or the oem GM tablets be best? THe waterpump has also been replaced and i have looked all underneath the engine for any leaks, and there are none. There is also no signs of coolant in the oil and no loss of oil after 3K miles.
Thanks

 

[Wake]

Can't help you on the heads, but how about inside the vehicle do you smell any sweet smells of coolant? Heater cores sometimes will do this as well.

I'd be careful of the block sealing junk out there. It's been about 20 years but the first and last time I used it, it really clogged things up. Ended up swapping out the radiator and heater cores because they got so gummed up.

About the only thing I'd try before getting into the engine would be the GM or Barh's stop leak powder. It's ground ginger root that swells up to clog small leaks. GM recommends it be added to a lot of the aluminum block engines like the Northstar to prevent small leaks and coolant loss.

 

[SP543]

every so often i do get a little wiff of burning coolant, but thats maybe once a month if that. My heater works fine, what should i check if it is the heater core? There are no visible leaks anywhere on the engine that i can see. Ill moniter it for another week then i may go the tablet route. Do i just crush the, up and drop them into one of the hoses?

 

[Wake]

every so often i do get a little wiff of burning coolant, but thats maybe once a month if that. My heater works fine, what should i check if it is the heater core? There are no visible leaks anywhere on the engine that i can see. Ill moniter it for another week then i may go the tablet route. Do i just crush the, up and drop them into one of the hoses?

How does the inside of the windshield look? A lot of times the windshield will get an oily film on the inside if the heater core is leaking, although if it's very slight it might not be much of a a problem. I don't know how hard it is to get to the heater core in our vehicles, but if there's a service door available you might be able to peek at it and see if it has any stains on the outside, there's a dead giveaway to a heater core problem.

From what I remember, the tabs are to be crushed and put in the lower radiator hose, not the overflow tank as the powder will just settle there in the bottom.

Instead of the GM tables I ususally use the product from Bars, it's already in powder form and comes in a little cylinder and is much cheaper and available at around $1 a tube.