please house I have a 2007 s40.
it just started overheating about a week ago.
then the water can always goes dry whenever I check.
Within the same week it developed hard start problem. It only starts well when I accelerate while starting.

we have checked everywhere for leakage. But it isn’t leaking anywhere.

please what could be the problem?

 

You have probably damaged the motor by driving it with no coolant. Once you get the motor hot enough, you will blow a head gasket and/or warp the head, and the coolant goes into the pistons.

Sorry, but I'm betting you let a small problem turn into a huge problem. Never, ever, ever, ever, ever drive a car when it's overheating or very low on coolant.

 

I thought so too. But then it Doesn’t overheat everytime. It has done so twice in one week. And the rest of the week, it moves fine.

 

coolant is going somewhere. There's a few tests for a head gasket you can do - you can buy an HC test kit for about $40 that measures exhaust gas that made its way into the coolant, you can do a compression test to confirm the seal etc, check for coolant in the oil (dipstick or oil filler cap foam?). When you say water goes dry, how much do you need to add to get it back up to the level in the overflow reservoir? does it run ok once you refill? Best case is you have a sticky thermostat and it is pushing coolant out through a hose (including checking for odd cabin smells and wetness in the footwells in case you have a heater core issue. But if you have searched and can't find any signs of leaking (radiator seams/upper lower hoses, heater core, overflow tank (common fail point) or overflow cap then you'd want to check for any white exhaust/condensation and possibly a sweet smell in the exhaust, particularly after its been sitting. You can also pull the plugs to see if there's a color variation.