I cannot find source for coolant loss--no visible leaks, no coolant smell, no white exhaust smoke, no trace of coolant in exhaust, connection to head is dry. Recent work included a mono valve replacement. After approx. 60 mile drive dash indicator comes on and coolant
replacement of up to a pint is required. Recent issue is #2 glow plug needs replaced. There are no drippings on the garage floor overnight or even parked after driving other than A/C condensation which is clear water. I'm baffled.

 

This is not a once and done episode. I have had to add coolant several times in the passed week. I do think I found the problem though. I took out the ABS Controller and with a flashlight saw that the lower portion of the mono valve was wet. All other hoses/connections looked OK with the mono valve/auxiliary pump. Still don't know where the coolant is going--there appears to be stains on the exhaust pipe so maybe it's dripping onto it and vaporizing. I'll know more once I remove the mono valve assembly.

 

Coolant loss solved--changed out the heater control valve which was not working and while at it the recirculating pump and all associated hoses. Apparently the leak was slow enough that it was evaporating on the exhaust pipe (evidence of stains) with no leaking on the ground when stopped.

 

Opinion.
Even though you found your problem I wanted to add this.

If you cannot find the leak visually as you did if it had been taken to a Shop they would have pressurized the Coolant System to see if they could see a leak.

And, that is better done with the Engine at operating Temp as as some leaks don't show up till things are hot.

The typical Do-it-yourself Kit for pressurizing the Coolant System an assortment of Caps and and a Hand Pump to create the Pressure.

 

Have you check for a fine small crack in your radiator. It happens to me a couple of month ago. The car needed coolant refill every week and I could not find any leaks. I finally found a really fine crack, so small it was almost impossible to see. Replaced the radiator now does not leak anymore.

Rodolphe
www,myrecycledparts.com