I bought an 07 M35 x with 146k miles a couple of weeks ago. The car seemed to be in really good shape overall and it drove great. it only had one owner and I could tell that they took care of it both by how the car looks and feels and it's service records.

Yesterday however it gave me a big problem. While driving, it suddenly felt as if I hit a wall and then both the check engine light came on and the temperature gage was at max. It also started revving oddly and couldn't get up in higher gears from a stop. Just felt like it lost any engine power.

I drove it for about a mile to a safe spot and had it towed to a Nissan dealership. I could see that coolant had leaked and the coolant reservoir was empty. But the dealership couldn't find the leak and the check engine light had turned off, with no code, and even after one of their techs driving it for a couple of hours, they found nothing but the coolant reservoir being empty. All they did is fill it with antifreeze.

It seems to drive okay again today but I am really surprised that all the coolant would leak and engine act so terribly without a visible sign or error code.

Wondering if anyone has any tips.

Try not to keep that "2 week NEW car " - bust their Azzz to take it back . You and I know , that they KNEW what they "had in that car" , and didn't disclose you . For your own observation , see if water in oil -cap shows milky.
See if oily / exhaust smelly coolant (on a COOLED DOWN engine , Rem. cap).If it's a VQ , the w/pump has a weep hole , pump is INSIDE ENGINE CASE , and coolant could backflow into motor especially if some Clown tried to EPOXY the weep hole And HIDE a $2000 ++ W/pump/timing repair - until they found you
Gonna need a lawyer, Lennar and Row - one call that's all

Thanks macgiver. I may need a lawyer after all.

No mayonnaise under the oil cap. That was the first thing i checked. And the exhaust seems to be clean. So hopefully no head gasket.

The dealer did pressure test the coolant system and couldn't find a leak but that's not to say that the water pump or radiator don't have a leak. I am wondering if there is a way to test this myself.

I had the car parked in my garage overnight and didn't see a puddle or any signs of major leaks. I did see a small, very faint mark on the concrete floor, but it was faint enough that I couldn't tell if it had been there before.