I've never talked about this before, but i really need advice on what could be a crucial decision.

I've suspected for some time now that my girlfriend (Who lives with me) has been cheating on me.

The usual signs...
Phone rings but if i answer, the caller hangs up.

She has been going out with the girls alot recently although when i asked their names she always says.
"Just some girls from work, you don't know them."

I always stay awake to look out for her taxi comming home, but she always walks down the drive.
Although i can hear a car driving off, as if she has gotten out of the car around the corner.
Why?
Maybe she wasn't in a taxi?

I once picked up her mobile phone just to see what time it was she went berserk and screamed that i should never touch her phone again and why was i checking up on her.

Anyway,
I've never approached the subject with her.
I think deep down i just didn't want to know the truth,
but last night she went out and i decided to really check on her.

I decided i was going to park my motorcycle next to the garage and then hide behind it so i could get a good view of the whole street when she came home.
It was at that moment,
crouching behind my bike,
that the valve covers on my engine seemed to be leaking a little oil.

Is this something i can fix myself,
or should i take it back to the dealer?

thanks in advance