2010 Volvo Low Fuel economy and losing coolant

I don't know I am not the mechanic on this one. I just know having been a mechanic, anytime I hear, 'well it doesn't throw any codes' it usually means the mechanic is the problem. It is like a computer can be designed to automatically diagnose accurately every problem on a truck or engine. It cannot, any one that says different dose not know trucks, computers, or engines. Get a new mechanic! You got a leak, that thing needs to be pressure tested and even then it might be a headache to find the actual problem.

35k on what trailer? what traffic? what temperature? what weather? what road? What kind of load? It all matters end so do a dozen other factors. I am just not using fuel economy to diagnose a truck. You probably heard of all the mega-crap drivers getting 10mpg in their new Crapcadia. That is because they don't know #### and the gauge in the truck is off. They believe that gauge like they believe the recruiter that told them they are going to get 5000miles a week. I have gotten every thing from 3 to 6.5 real mpg in a volvo like yours; all good real figures from a good engine, depending on load.

Usually rule of thumb is in a shop is: the last thing touched is the first thing broke. I don't know in your case but just guessing from what the mechanic told you; the last thing he touched is probably the problem.

 

Could a bad head gasket be slowly leaking coolant into one of the cylinders, and it burns/steams off through the exhaust as combustion takes place? I mean like a minute leak that opens and closes as the engine temperature fluctuates. Would explain the phantom coolant loss.