Hey I've got an 88 4.0 automatic and I used to get 20-23 mpg hiway which is just about all I drive. But now I'm getting 17 mpg. I had a problem with my jeep just a while ago (not related to the milage problem) that made me suspect the catalytic converter. I removed the cat and that didnt fix the problem I was having so I put the cat back on but I toasted the gasket on it. Would this cause a minor exhaust leak making me get bad milage? I didnt do anything else mechanically to my jeep besidesthat so that's why I think is an exhaust leak, I just wanted some feed back on what you guys think. Thanks!

 

How close is the o2 sensor to the leak? I'm not as familiar with the 80's stuff, but IIRC there's considerable distance between the sensor and the cat...leak should be far enough downstream to not affect the sensor.
But I think you're on the right track..if the leak isn't doing it your O2 sensor may have taken early retirement.
What is the "other problem" you were/are chasing?

 

O2 sensor is in the exhaust manifold, far away from the cat.

They were due to "retire" every 82,000 miles IIRC.

 

the other problem cruiser actualy helped me on a while back, my jeep would sputter but wouldnt die when it got up to running temp but i fixed it with a new crank sensor. theres only 1 o2 sensor on the exhaust in the 80's and the cat is about 2 feet from it so that probably wouldnt be the problem......do you guys have any other ideas about what it be?

 

Bad O2 sensor could cause poor fuel economy.

 

i just replaced my o2 about 400-500 miles ago though, and i was getting good milage up until now.

 

O2 sensor heater relay?

It's the one closest to the front.

 

One problem that I keep seeing pop up is theft; people syphoning fuel outta your tanks. Just some 'fuel' for thought.

 

which one is that? i know were the three relays are (i dont have a/c so i only have 3) if im standing on the passenger side facing the driver side will it be the left, middle, or right relay?

 

haha, no noone has taken fuel outa my tank, i have a locking gas cap and i check my fuel and milage daily. i still need to know wich relay is the o2 heater

 

Found this for an '89 if it helps:

 

See post # 7. It's the one furthest forward.

 

Well all of my relays work just fine and just to be sure, I replaced them all and I'm still getting 17 mpg:/ Hmmmm.... Any other ideas guys?

 

Has the drop in mileage been a sudden thing or a gradual decline?

 

Already check for leaks from injectors or vacuum lines?

Broken map sensor line was giving me 4mpg!

You probably already checked but it's worth mentioning.