cracked manifold and O2

Ok I've read the prior posts on O2 sensors and exhaust manifolds and still am not able to answer my question. My exhaust manifold had been cracked for about a year before I decided to get around to welding it up. The check engine light was on before I did that. I unplugged the battery cables to reset the light. Everything went fine for about 10 miles. The light came back on and so I had it tested. The test said I needed a new O2 sensor. I replaced the sensor that's on the downpipe from the exhaust manifold. My cherokee is a 98 with about 100,000 miles. From the mileage I figured it needed the O2 sensor anyway. The light then came on again after I reset it. What is my problem here? Do I need to change the sensor after the cat too? Since I ran it with the manifold cracked could that have sent unburned fuel to my cat and ruined that? If that's the case I didn't think that would set off the check engine light. Also I read in the manual that there is a mantenance timer to change. Is that just on the older engines because I sure couldn't find one. Last note my gas mileage sucks. I'm getting 15 to the gallon with 33's and 4.56 gears. And yes I have changed the speedo gear so it's not that.

15 MPG is not that bad compared to other XJ's... Like mine for example. I have no lift and 31inch tires. This is my DD and I drive mostly freeway. And I get about 12MPG. SO dont complain!!!

Definitely change the other sensor. The timers are only on the older models (like my 90). I think it was discontinued shortly after that. My 94 has to be reset with a special tool or code reader or something like that.

But when I know that they can get 18-20 mpg with the kind of driving I do, I would like to be able to get that. If it's not then there is something out there that can fix it.

kirby said:

But when I know that they can get 18-20 mpg with the kind of driving I do, I would like to be able to get that. If it's not then there is something out there that can fix it.

Click to expand...

18 to 20 with 33s? you must be running it lean and goin downhill
above info correct- maintenance timer aborted in 1991, pre HO years/renix.

a cracked manifold will cause the o2 sensor to read a lean condition-forceing the management to richen the mix- rich mix kills the sensor- the cat should be fine for now, but I`d replace the downstream o2 anyway- that`d be my guess for a fault light/code.

dunno about your setup, but my 89s that had cracked manifolds also melted the clutch line from the res. to the internal slave. The heat just ate the line up like it was made of paper. Check around where yours was cracked and spewing heat out, make sure nothing else was melted.

Checked all of that plus it wasn't a very large crack so I don't think a lot of heat was escaping but I'll have to get that rear O2. Thanks