My s320 consistently loses just shy of 1qt of coolant every 3-4wks. I can't seem to find leak. No smoke out tailpipe, no fault lights, temp guage reading never over 100, no rough idle, no contamination of oil, no water on ground but I have to top off 1qt every 3-4wks. Car runs fine with good gas mileage. Just did Etest everything was just fine.
Any ideas where it might be loosing the coolant.

Go to a radiator shop and ask them to do a pressure test. Most radiator shops will do this for no charge.

I would suspect the water pump. Keep an eye on it, but I will wager you'll need to replace it in the near future.

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Glen Tokuhara
Beauty & the Beast and the wagon that could!

Sometimes you can get lucky with it being something minor. Like a bad overflow tank and/or hose connections thereto. That's where mine is going.

Well I found my coolant leak. Took the advice here and had system pressure tested. Found small leak half way up the side of rad side tank where it is crimped to the core body. Now to pull it out and get it repaired. Original rad, still looks very good but will be having a thurough check at time of repair.
Thanks for the help.