1994 GV U-300 Detroit Silver.
After a 700 mile trip and everything cooled down I noticed the anti-freeze overflow tank was empty.
I added anti-freeze bringing it up a couple inches above the COLD level. Did a 300 mile run, let it cool off and the level had not moved.
300 miles back home, let it cool and empty again. My thought is it's burping air from the system.
I filled the overflow to almost the hot level, cranked the RV and let it run long enough hot. After cooling it brought the level down to about the cold level.
I am confused to say the least. No visible leaks anywhere. The radiator is dry, hoses and connections dry, engine bay is dry and no visible evidence of a leak underneath or on the rock guard. Thoughts?
Ours did the same thing. Turned out to be the soldered on tube to the plastic reservoir up in the neck at the radiator cap. It came unsoldered and we lost coolant like you did.
I take it that you did check the oil dipstick for gray foam indicating coolant in the oil? How about the transmission dipstick for failure of the transmission cooler. The Detroit has a well built tranny cooler but check it.
Did you check up front for a leak in the heater hose in the nose when you open the nose access? You would smell it if it were inside the coach.
You might take a electric screwdriver and pull the outside cover off the radiator. Check for radiator rot over the whole radiator but especially the lower right corner. Check bolts where the tanks are attached.
Pierce
The nipple out of the fill neck looks good, no cracking or moisture around it. Ours has a rubber tube clamped to it that runs to the overflow tank, it's not cracked and dry as well. Checked the oil, no apparent contamination. Did not check up front, the previous owner had turned the water off to the front heater in the wet bay, but I'll check. I did look under the coach and no leaks.
I can see through the vents to the radiator with a flashlight and nothing is wet. I went under the coach to the engine side of the radiator and it's dry as well. When I open the metal, antifreeze reservoir when cold it's holding some pressure and is full.
Did not check the trans fluid but will next.
On our 99 U270 with the Cummins 350hp if you have drained and replaced all the coolant to full, it will burp and need an additional gallon of antifreeze to fill it after a five mile drive, and a second time again for a total of two extra gallons to get a true fill of the coolant system.
Thought it was a leak when it happened.
Update: Checked the hot water connection in the nose and it's dry.
Checked the transmission fluid and it's clean.
Gaelicdog,
Have you or someone else done a coolant change recently? If so reply #3 is your answer.
I am leaning towards that, but I do not know. The previous owners were not exactly forthright about ANYTHING to do with this coach. The first day after buying it, when I started finding more issues they stopped communicating. We bought it in July last year, didn't check the overflow tank level before heading home with it, was overwhelmed and my bad. After getting it home and learning it's systems did I start looking more closely at everything, taking pictures, taking notes and comparing the info I compiled.
Last on the list would be checking for products of combustion in the coolant. Lots of places sell the kits including HF, etc. Here is one from Amazon. Make sure to order the diesel fluid. Amazon.com: Lisle 75500 Combustion Leak Detector: Automotive (https://www.amazon.com/Lisle-75500-Combustion-Leak-Detector/dp/B0007ZDRUI)
Pierce
Visually it's clear and bright, just as pretty as the the day I put it in.
Yesterday I had the coach running for over an hour as we are prepping for a trip. Engine up to temp, level in overflow tank came up a couple inches. As it cooled the level returned to the pre-run line.
Visual condition may not indicate products products of combustion.
Unless actually driving, a leaking head gasket may not cause loss of coolant.
Check each morning on your trip.
Pierce
I am going with BURPING or purging air from the system.
After getting the coach up to temp twice last week the levels stabilized and no further loss. Marked the cold and hot level on the tank with tape prior to doing two, 300 mile runs this weekend and the levels were spot on with no loss.