Slow to start. New plugs and fuel filter. Mechanic is at a loss doesn’t happen all of the time.
Do you mean you have to crank it a long time before it starts? If so, do the “key dance” and see if that fixes it. The key dance is turn the key to ON but not start. You should hear the fuel pump hum. Turn the key OFF, back ON, OFF and ON then start. If it starts right up, likely the fuel pump check valve is leaking or possibly an injector is leaking.
A bottle of Techron or other fuel injector cleaner in the gas tank may make it better. If not, it is the fuel pump. You don’t have to replace it right away. Heck, you can go quite a long time with this but if the car has over 100K on it, the pump’s life may soon come to an end anyway.
Your mechanic should know this. Maybe you need a better one?
We put in BK in the fuel not any better. The car has 125k miles
Here’s some simple questions . . .
Does the engine crank over at a normal speed?
Does it crank over a long time, before it does start?