Check coolant level

My 2001 Deville looses about a cup of coolant every 200 miles. I've had this problem for several months. I replaced the water pump and pump seal to try to fix the problem. I also added 3 GM stop leak pellets to the coolant system when I replaced the coolant. Three weeks later I added 5 stop leak pellets. But the coolant is still leaking. Apparently the leak is too big to seal with the tablets. There are no visible signs of a leak and nothing drips. Any suggestions?

I'm glad there is a coolant level sensor is the expansion tank or my engine would have burned up by now.

I borrowed a coolant leak test kit from my local auto part store. With the coolant at ambient temperature, I pressured the coolant system up to 18 psi. I noticed a small drip at a hose and another small drip on the back side of the water pump. Both were very small leaks, about one drop of coolant was leaking every minute. Even with the small leaks, the system held 18 psi for 10 minutes. After 10 minutes I depressurized the system. Next I checked for compression and combustion leakage into the cooling system with the pressure test kit while the engine was on. The system held pressure with the engine on and the gauge indicator did not oscillate. I tested the expansion tank cap and it leaked air rapidly. So I'll get a new cap, torque the water pump so to better seal the o-ring, and replace the leaking hose.
I am surprised the stop leak tablets did not seal the small leaks I have in the cooling system. I have plenty of stop leak in the system. When I drain the coolant, I can see the small stop leak particles in the coolant.

If I were you I would flush that crap from my coolent system. GM stuff I am told works well but those tablets are suppost to be hammered to a powder then put in system. I just dont like putting something in my eng that could screw it up.Glad you found you leak

My 2001 Deville looses about a cup of coolant every 200 miles. I've had this problem for several months. I replaced the water pump and pump seal to try to fix the problem. I also added 3 GM stop leak pellets to the coolant system when I replaced the coolant. Three weeks later I added 5 stop leak pellets. But the coolant is still leaking. Apparently the leak is too big to seal with the tablets. There are no visible signs of a leak and nothing drips. Any suggestions?

I'm glad there is a coolant level sensor is the expansion tank or my engine would have burned up by now.
Did you add the tablets to the expansion tank (where the radiator cap is)?

Coolant simply does not circulate through the expansion tank, so if the tablets were added there, they will simply sit there and do nothing.

All tablets must be placed inside the lower radiator hose, when the cooling system is empty. Crushing them helps, but is not necessary.

The specs call for 2 or 3 GM tablets, but on the advice of several Caddy mechanics, I added 6 to my car, with good results.

I was just passing on info from caddy service. They told me those tablets needed to be hammered to powder. I did not use at the time cause I too found my problem. And I was determined to do so cause I dont like making my coolant system less effective even if its minimal.