Unusual Coolant Problem

[Rockin1]

I am the original owner of a 1999 Tahoe Sport 4 wheel drive with the Z-71 package. It has been an excellent vehicle.
There has been a problem with it that started about 10 years. The water level in the radiator runs about 6 inches down from the cap which means the water overflow tank is always empty. If I fill both the tank and the radiator up it just goes back to the radiator going down to 6 inches below the cap after it is driven awhile. I did buy a new radiator cap at one point but it didn't help.

The antifreeze does not drip out on the floor. The oil has zero residue in it and there is no residue on the oil cap. There is no smoke coming out of the exhaust and the vehicle does not over heat even when driving long distance with the outside temps being in the high 90's. I have let it go this long because no mechanic so far has been able to tell me what needs to be replaced. The engine is the original 350.
Is there anyone out there with a viable idea?

 

[PNW NBS Z71]

Sure would help to know the mileage. Have you ran a compression test? Need to do that with all plugs out. My guess is seepage at the head gasket(s). Do a dry and wet compression test. My guess is that is has a very small leak at head gasket(s) and is going out the exhaust. Also, just because you see no drops on the ground/floor does not mean it can't have a leak. You should pressure test the system with a cold engine and a warm engine to see if that pressure shows drips. Last question, when you see the level drop, is it always that 6" and never any more than that?

 

[Rockin1]

Thank you for your reply. My Tahoe has 210,000 miles on it. It runs well. Since the vehicle has a lot of miles on it there is a small oil leak from the pan. It is possible that a small water leak could be covered up by the oil on the engine and cross member. Even though it bugs me to have any oil leak it doesn't make financial sense to go through the process to replace the pan gasket for a $10 a year oil leak.
I will do the tests you mentioned. I special ordered my 1999 2 door Tahoe Z71 Sport new and have taken good care of it since. I enclosed a couple of recent pictures. It has the original interior, engine(350ci), rims, exhaust and spare tire. It has been garage kept and should be one of the nicest original daily drivers left in the country. If GM made a new fullsize 2 door I would buy it. Either I need to sell it one of these days which I am not crazy about or drive it until the engine gets tired and then either keep it original by overhauling the engine or going with a LS engine swap. It is good to have choices.


Again, thanks for your help

 

[PNW NBS Z71]

Very clean rig. I had a 96 2 door Sport and wish I still had it. I would keep that and do the engine in the rig and upgrade a little for performance.