My sons 04 d2 was looking like a diesel and getting about 4 mpg.

replaced the front o2 and still the same issue.

had recently replaced the maf due to water injection...

 

Is the engine running rough? Got any trouble codes?

 

That's probably going to be more than 02 sensors. Need codes and more descriptions. Sounds like it's dumping fuel into the engine if its getting 4mpg and running rough like that? Or are you meaning its smoking as in rolling coal? Best to stop driving it before you do more damage.

 

perhaps a bad iat and/or coolant temp sensor?

 

Or a injector that is pouring fuel into a cylinder