Have 1998 3.5 RL V6, 166K miles with a mystery coolant leak. I need to fill the reservoir every week with about 1.5 liters of coolant/water. There are no visible leaks. Repair garage pressure tested coolant system for days and could not find what problem was. Replaced radiator hose and cap and gasket, replaced all hoses. As long as the reservoir is filled the car does not overheat. Tends to use more from the reservoir if I use the a/c. The heater is not a problem, there is no leak under the dash from the heater core. There are no smells, no smoke and car runs like a top. Will a bottle of leak sealant in the radiator fix the problem or do I have a larger problem with the water pump or head gasket. Appreciate the help...

wnflygirl wrote:

Have 1998 3.5 RL V6, 166K miles with a mystery coolant leak. I need to fill the reservoir every week with about 1.5 liters of coolant/water. There are no visible leaks. Repair garage pressure tested coolant system for days and could not find what problem was. Replaced radiator hose and cap and gasket, replaced all hoses. As long as the reservoir is filled the car does not overheat. Tends to use more from the reservoir if I use the a/c. The heater is not a problem, there is no leak under the dash from the heater core. There are no smells, no smoke and car runs like a top. Will a bottle of leak sealant in the radiator fix the problem or do I have a larger problem with the water pump or head gasket. Appreciate the help...

did they check the over flow tank and hose
I'm not a big fan of rad seal
you can have the headgasket checked
good luck

I doubt the water pump would be at fault. That would show signs of leaking and get steadily worse fast. I would put my money on a head gasket problem. You could have a leak down test down to find the source of water/coolant disappearing act. The car does not need to be fogging smoke out the back to be using it or attempting to burn the coolant. All fingers are pointing to the head gasket. One word I might offer is to NEVER use radiator stop leak. The junk compounds the problem with blockage in other locations, etc. Bad choice to use that.