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Mystery leak

I have a 99 chevy 6.5 liter turbo diesel and it has been eating 2 bottles of coolant a week. The tempature is normal, there are no signs of leaks, no water in the oil, no steam out the tail pipe. On that model is it possible the head or the head gasket can leak directly into the exhaust? Other possibilities?

Before you do anything complex, try replacing the radiator cap just to make sure it’s not weak and letting the coolant out the overflow. It’s cheap and easy, you may get lucky.

The cap is on the reservoir bottle back near the firewall - does that make a difference? I’ll try it though the cap seems new.

No, not the reservoir cap. There ought to be a cap directly on the radiator itself although on some cars it is on the upper radiator hose a few inches away. That’s the one that might have worn out.

I was referring to the pressure cap, on some cars it’s on (or near) the radiator, on other’s there is a pressurized expansion tank (not a vented reservoir bottle) that is directly connected to the cooling system. Either way, trying a new cap is a reasonable first step.

Lets say I replace it ( the resivoir is pressurized by the way ) and it still happens - what do you think about radiator stop leak? Not the gunky kind but the powdered aluminum? Or just bite the bullet? G

Also, I went to a radiator shop - we did a pressure test - it lost about 3 - 4 lbs in 10+ minuites. I’m not certain if that is bad or good.

Personally, I’m not a fan of stop leak, especially if you don’t know where the leak is. What did the radiator shop say about their results? I think you need to determine where the coolant is going before you can fix it.

Both the diesel shop and the radiator shops had no real idea where it was going. The diesel mechanic thought perhaps there was a small crack, and that over time it would get big enough to do something about and that I should just keep driving it and do nothing except put fluid - but the truck takes dexcool which costs $14 a bottle. At 2 bottles a week I thought “That sucks”. I asked about block sealant and he suggested the aluminum powder kind. So I dunno what to say at this point. I bought the truck used over e-bay - the former owner thought it had a blown head gasket but it must be a very small crack or whatever because it has shown none of the usual signs of it other than the coolant loss. I would like to nail it down more before I tell them to tear it apart. The “know before you go” approach rather than throw parts at it till the problem goes away. One other thing, the truck smells of coolant after you drive it somewhere. Could it be oozing out somewhere and evaporating and thus no sign of a leak? If so, how would I find it? Any thoughts?

This is two years after the last post. So what every happened to the mystery leak? I’ve got a 97 GMC 6.5 turbo diesel that I’ve had to add a bit of coolant to. When I was under the truck I thought I could see some drops of coolant on the oil pan. After a couple weeks of long road trips the leak seems to have been greatly reduced.