I have a 99 cobra and when I start it up cold there is white smoke and a strong gas smell. It allso makes a rattaling sound when it first starts up.I know this sounds dumb but it seem to have started after I had the oil changed(yes I have checked the oil)I have a check eng light on I think its the 02 sen. I have the back cats cut out.
Thanks

 

It might be something more serious then that. Get the code read.

 

how bad is the rattle?

 

The rattles not to bad but I can here it

 

Do all the usual maintanence checks. Look at all your vaccum lines, hoses, fluids. How many miles are on the car? Does the smoke stop after the car is warmed up?

 

I've checked the oil don't see anything that has come lose and yes the smoke stop after the car is running. The smoke is white and smells strong of gas

 

is it smoke or steam? does it have a blueish tint?

 

I really do believe your car is just showing some steam out the tailpipes. Mustangs do it more than other cars in the cold. Has this been happening recently when the cold weather hit? If, so I wouldnt worry about it.