1998 Vauxhall Astra high reves after going through puddle

Hi I drove my 1.6 8V T reg astra through a very deep puddle and have had problems since.
The engine at idle is roughly 5000 rpm now and my mechanic friend is not sure what is wrong with it. The puddle was as deep as the wheels and I think water must have gone into the exhaust and knakered that as well. When I shake the back box it sounds like a biscuit in a biscuit tin. My freind has disconnected one of the air valves and the o2 sensor to bring the revs down to roughly 1500 at idle. My freind has allready checked the majority of the sensors and pipes ect in the enigneer and can find nothing obviouse. He has a fault reader which he has used on my car before but since going through the puddle it will not register any faults, but the engine managment light is on.
My question is does anyone have any idea what could be causing this or what i should try replacing first?

Thanks for any help