I need some advice.I have a 98 tahoe 5.7 4x4 and am experiencing constant coolant loss.after doing the coolant pressure test I find 2 external manifold leaks,but after pulling spark plugs i find that the 3 and 4 cyl plugs are clean on half of the porcelain by the electrode .Does this always indicate water in the cylinder and Is this condition possible with just a manifold leak or does this definately indicate a head gasket problem? Is it possible that I'm only getting partial combustion due to the manifold leak causing the plug to appear this way? There isn't any coolant in the oil and It never overheats and has plenty of power with no power loss or misfires. Just wondering if I should dig deeper? compression test?Thanks for the help.

A cylinder leakdown test will tell you where your system leak is at if it is due to the head gaskets. Bubbling into the radiator means the head gasket is leaking between the cylinder and cooling passage.

Also do a cooling system pressure test, look for external leaks, then recheck the plugs for wetness.

I would also check your transmission fluid to see if the level has been mysteriously rising. That's a leak from the tranny cooler that is inside the radiator.

Good luck!

thanks for the help.I done the pressure test and found a couple external leaks but didn't chk the plugs.will redo this along with the leakdown test tomorrow.thanks again