My 97 5 liter V8 normally gets 16 - 18 miles per gallon, then all of the sudden, I start getting 12mpg. about the same time that happened, I know have a pretty bad smell, and suffer from headaches after driving for 15 minutes. I think I am getting exahust fumes in the cabin.

I had a mechanic look at it for 2 hours today, and they couldnt find anything. They are normally great. No Check engine light either.

Could this be a problem with the Catalitic Converter, or maybe some other exhaust issue, aslo causing bad MPG?

 

Could be plugged exhaust, but you'd probably hear the exhaust hiss as it escaped through whatever hole it finds for itself. 5.0 Explorers tend to have problems with the exhaust manifolds on both side. They crack, warp, blow out gaskets and sometimes only make noise when it's cold. Another exhaust related component that fails alot is the EGR tube, which is corrugated tubing about 1" thick running underneath the throttle body. They crack and can also leak fumes and enter the passenger compartment through the climate control system. If there was a problem with your converters, it should have tripped a catalyst effeciency code by the time it got so bad as to effect gas mileage so badly.

Take a close look at both exhaust manifolds, make sure all the bolts are tight and not missing. Check that EGR tube for cracks. I'll bet the fix is in one of those.

 

I've ran into several that the engine coolant sensor had failed , Sending a incorrect signal to the puther . There was a severe decrease in fuel mileage , An had a rich smelling exhaust . Both times it never tripped the check engine lite.

What was happening the senser was telling the puther that the engine had not reached operating temp. an had increased the fuel

 

interesting. I will check out the exhust stuff first.

How would I check Engine coolant sensor? I dont know if my exhaust smells rich, it may a little, but I dont think I would normally be taking fumes into the cabin before it got rich.