mystery coolant leak

[K2500]

My 94 K2500 Sub has a small coolant leak. I am losing about a quart every 250-500 miles. There is no visible coolant anywhere. No freeze plug leaks, no coolant in the oil, no coolant on the ground. I have had this thing in two shops with two competent mechanics, and no one can find any leaks. Any help would be great.

--Kevin

1994 K2500 Suburban, K&N, Kennedy 3.5" exhaust, straight thru, no cat, JL Audiosound. Otherwise stock.

[charliepeterson]

Try running the truck for a few minutes and before the thermostat opens squeeze the top radiator hose. If it's hard then the coolant is most likely going out the tail pipe.
Feel the carpet under the heater box inside the cab. Another fantom place can be the coolant pipe feeding the heater box at the thermostat housing. If the pipe is stock look for any white powder around the fitting. This is residue from the coolant evaporating. If you bang it, it will break off. This is not one of GM's better idea's.

[DogDiesel]

Critical for these type problems is a radiator pressure tester.

With engine off, Install it and pump (air) pressure to radiator cap pressure or a few pounds more. If needle on gage droops, drops, listen for air seepage and tighten clamps. Test cap, replace if it won't hold breaking pressure (usually 14-16PSI).

Look for the simple stuff before worrying about the inside engine problems like headgaskets or block damage.

These are available from JC Whitney or other tool vendors for 30-50 bucks, a wise investment IMHO.

Good luck with your problem.

[DogDiesel]

More cooling system theory...

For every PSI of pressure that the radiator cap successfully holds inside the radiator ( a closed system), three degrees Fahrenheit is added to the (212) boiling point. A one PSI cap would give a 215 boiling point, a 16 PSI cap makes for 260 degree boiling point. 15=257, 14=254,etc

Factor also that a good coolant mixture reduces boiling point, not to mention pump lubrication. While pure coolant is not best, and to, none is not best, a mixture that gives a -10 to -20 reading is still another tool in keeping temps down.

Wayne

[DogDiesel]

My wife said her Mercedes diesel was using water. Had to put a large bottle of Avion in to get home. Popped the tester on the radiator and it held steady like a champ.

Tested the cap, which consistently broke pressure at 8 PSI (236 deg) vs 15 (257). Bought the cap, replaced, and her mystery leak resolved.

Same scenario this spring on my old VW pickup, this time under pressure, wetness would weep up on the radiator return hose, which was quickly evaporating without leaving an external trace. The tester needle dropped and wetness formed.

Good luck with that mystery leak, and pray it be a simple leak like I describe or a bad cap, vs big money inside the block or heater or stuff like that.

Wayne

[Dimsdale]

Seems like you have the wrong (right?) year for a block crack, but don't discount the possibility. Check your oil for muck, and the filler cap for condensed steam.