All the suggestions members are making could have validity. But all the items we're mentioning to check for confirmation of a theory may not always be correct.

For example, a couple of friends just rebuilt a Mercury ecobox type vehicles for a "needy" old senior citizen female....as a courtesy. The vehicle was overheating drastically and they couldn't locate the fault. No "chocolate" colored or frothy oil dipstick. No indication in the radiator. Compression was so-so (can't remember what it read...but it is an older car). They installed a water pump, cleaned the radiator, new thermostat, new hoses (one had a pinhole leak). Nothing was working. They finally borrowing a tool like I mentioned in another post and found that indeed it was a head gasket. The gasket had a very small channel from an oil passage to a cooling passage, IIRC. Once they found that, they rechecked the radiator very carefully, and found the telltale "rainbow". It was easier to see now, because they only had water in the cooling system.

My point for the long winded story......as others have stated, you are now at a point where it will take professional assistance....because you have not located the reason for the coolant loss.

 

Had a similar experience with a small block ( cough hack) ford,,rebuilt, 500 mile mark it started overheating, temp would rise normal untill it hit 180 deg then take off..routine troubleshooting, cap, radiator,heads,,,bubbles under cap....finally pulled block down and found #8 cylinder had a small horizontal crack barely visable if you knew what to look for, just below ring ridge..what was happening was the crack wouldnt leak until the temp got high enough for the cylinder to expand. No water in oil, crack went straight to water jacket, compression kept water out of cylinder as i never lost water except when cap was lifted and it pushed out from the pressure....

 

Well, it has been almost a month since my last dealer visit and I can only report... It has not over heated. Of course, every other day or less I have to add coolant to the overflow tank. I only add to the 'cold' mark so that I may gauge consumption. By every 100 - 150 mi, I'm adding coolant. Still no evidence of external leakage and no visual on the oil dipstick.

It's time to go see the 'man' again. Maybe this time I can get them to inspect the plugs and cylinders.

 

excess water coming from tailpipe?

 

If you know how to "read" plugs, pull them yourself. It's probably a 30 minute job!!

 

Good idea..the affected cylinder will have the cleanest looking plug if water is getting to it...water acts like a bead blast...