Fix whatever is leaking, regardless of what the mechanic may have told you. The only place air should be entering the engine is thru the intake tube. Anywhere else is a vacuum leak, and needs tending too. A minor leak can cause big problems.

 

I'm by no means an expert but I'm not sure if just exhaust leaks alone would cause only 8mpg though.

Since I've drove my jeep it's had a big piece of EX manifold gasket blew out. It was a massive leak. Also I've had no cat, muffler, or tailpipe. And my down pipe didn't even make it to the cat flange branket.

I still got/get 16-19 mpg (stock 235/75) and I tested it many many times. I couldn't stand the thought of horrible mpg but it actually wasn't to bad.

I do not have any suggestions for a fix that nobody else hasn't said. It just seems like it would be something other than just an exhaust leak, based on my experience. I could be wrong.

PS: I'm currently fixing my issue just because I want it as right as possible.
Before I put a 2" lift on. I'm interested to see my mpg when all is fixed with 746 injectors and cleaned TB and IAC.

And I only had these problems due to an idiot cousin.

 

My manifold leak caused my exhaust to fry. Cost me the Jeep. Fix it before it causes major damage.

 

Pretty sure Cruiser was talking about checking the bolts. Mark was pointing out that if you are not compensating, those 33's will make your odometer under read. Mine didn't come with 235's so I need to add 4% to my mileage, (and speedo).

Be double sure that tube to the MAP is flawless. Vacuum getting to the map is what makes it rein in fuel flow. The less the vacuum getting to the map, longer injector pulse width.

There is a basic MAP test here> http://www.lunghd.com/Tech_Articles/...iagnostics.htm

 

Just my 2 cents: My 98 Cherokee Sport 4.0 gets 22 mpg on average, and it has 211K miles on the OD. I think I would be tearing the engine apart if I suddenly got 8 mpg...

 

Two door, manual trans, lots of "regular road?". Sweet! I can get 22 in my 90 4 door with 4WD and a 500-700 lb, load, (then add me and it's 1/2 a ton), at 280K....On the highway.

It's tough to get accurate numbers. One angle at one pump and another at a different station. It takes multiple tanks to get a real handle on it. Then the tires, I didn't care for the calculators I found. Thinking originally my 90 had 225's. Anyway what I did was, using long chunks of county mile markers, 15 to 40 miles, record three or four, then average that out and compare that to my odometer. 4% is what I add for my slightly larger tires.

Because I normally thump around on little gravel, and small roads, but sometimes do 300 miles of 101 from ORE to Santa Rosa Ca, (600 miles), I was able to log a BIG difference. Let's say (conservatively), 6 MPg. 14 or 20. (12 to 22 is also plausible) I HAVE been keeping pretty accurate records for years.

 

It's a 4 door and automatic AW4 tranny. My numbers are by no means as scientifically calculated as yours though but still, I get consistent 20 MPG with a mixture of city and highway. I use the gas calc method.

Is is my DD and I never go wheeling as everything is stock and I do not have 4WD.