Coolant depleted without any visible leaks

OP is a Lease op.

 

The Shop probably just pumped some air in it, and checked for external leaks. If the Trucks your responsibility, go to Oreillys or Autozone, borrow a pressure test kit. Do it yourself. Put air to system, 15 lbs and wait, see if it holds pressure. Look for external leaks everywhere. If it doesn’t hold pressure, and no external leaks, it’s an internal leak.After that part of the test. Then put just a few lbs. and crank it up, let it run. See if pressure goes over 15lbs, or whatever the cap rating is. If it’s a head gasket or pinhole in cylinder, it will build pressure real quick and just Keep going up, way up high. You can also check the cap with the kit.

 

Good reason to go to an Independant Shop.

 

Never know. Once overheated, thermostats often act goofy, or after running low, and adding coolant, trapped air causes havoc. Especially on the newer closed systems. My car overheated one summer, must have topped it 3 or 4 times. Before it got back to normal. Thought for sure a head gasket was leaking. It’s such a big problem on new aluminum head and block engines, there’s some specific procedures for filling a radiator to avoid air pockets and hot spots ruining the newer engines.

 

Here is an additional test to try. Take one square of toilet paper. When you check the oil level on a cold engine, put one drop of oil on the middle of the piece of toilet paper. Let toilet paper dry completely. The oil dot should be one solid color. If there is an outer ring around the center dot, that indicates oil contamination.

 

We have had 5 t800's with cummins isx15 do this. As a test after not finding the prob for months(new; tank,caps,had shop check for head and inj prob and found nothing . The coolant level would drop to set off alarms.We would not add coolant and the level never dropped any further so we left it for a month(we had a very tollerant driver).The bell would go on till the truck got up to temp then go off and it stayed that way. no more coolant loss.It seemed that the truck wanted to run there. So I modified the sensor so it would ride lower in the tank. Prob solved coolant level stayed there ever since no bell, no light. so I did this to the rest of the trucks and only have one stubborn one that is very occasional and have to add coolant very rarely. Worked for us. It seems the level that sets the alarms off is extremely close to the level the truck wants to run

 

I drove a 2015 579 with a Paccar motor and I had the same issue. Took a month or two but it would start chiming at me the level was low. I would fill it up and it would be fine for a month or two…. I talked with the shop about it (at Magnum) and they said it was a recurring problem with many of their Paccar Pete’s. I just always kept a couple gallons of coolant handy. I drove that truck 3+ years and it never got worse… or better.

 

Do you have an APU?