MPG loss and rough idle
 

Couple of weeks ago, my 1996 Cherokee went from getting 16/17 mpg putting around country roads and the farm to 11 mpg. I have changed spark plugs, wires and the distributor cap. Those things needed to be changed so I didn't mind spending the money. None of those things fixed the problem. I also took the throttle body off and cleaned it all.

When driving around 45 to 55 mph with the torque converter locked up I can feel a miss. After it warms up, the idle is rough. I don't notice any loss of power and it actually has more power after new plugs. I keep getting the code P0171. Tomorrow I will try a new IAC.

It might not be related but once I noticed my loss of MPG, my fuel gauge started to become more accurate. Before when full, the needle barely went past a quarter tank but would make it 250 miles before I needed to fuel up. Now it'll read almost 3/4 of a tank but I need to be looking for fuel around 150 miles. What's my next step?

Given that you are going through more fuel, more quickly, but the computer still thinks the mixture is lean...it would point to your upstream oxygen sensor. Its not the only cause of a problem like this, but it is the most likely.

If your 96 is OBD2 capable, I would highly recommend getting a cheap OBD2 scanner for your Jeep. It will show you what is going on with your fuel flow and oxygen sensor voltages.

My guess is that the upstream O2 sensor is faulty and stuck reading lean and the computer is reacting to that.

Normal life for an O2 sensor is about 110k.

Another candidate is an air leak (vacuum leak) that gets worse as things warm up. If unmeasured air is getting into the combustion cycle it will fool the upstream O2 sensor into thinking there is too much air and not enough fuel. It will measure a lean mixture and the computer will react by progressively adding more fuel, until it hits a limit and then throws the P0171 code. But typically air leaks like this will present themselves more at idle when vacuum is at its strongest (pulling air strongly from any leaky area), but at speed and higher RPMs there is less vacuum, and the majority of the air is being measured by the MAP sensor at the throttle body. The leak is still present, but makes up a much smaller portion of the over-all air volume.

So of course, the MAP sensor can also be a candidate if it is not measuring the incoming air correctly. I don't know the specs for measuring its signal off the top of my head.

Is the up stream sensor the same as the down stream sensor? I know I have a spare down stream one laying around. I have checked for vacuum leaks and have none. I have a cheap scanner but I have a friend with a high dollar scanner I'll try out.

I do not believe they are interchangeable on the 96.

https://cimg1.ibsrv.net/gimg/www.che...60f54a7d98.jpg
https://cimg6.ibsrv.net/gimg/www.che...8ddcdc127e.jpg
https://cimg3.ibsrv.net/gimg/www.che...9c97659218.jpgSpark advance jumps from 9 to 21.
Here's the the scanner says. The downstream sensor (P0138) has always been present since I have owned the Jeep.

Timing Advance will move around. That is not unusual, not related.

Oxygen Sensor Bank 1 Sensor 1 at .04 is what you should be focused on.
Typically it would cycle back and forth between about .1 and .9
It is currently pegged in a position that tells the computer that mixture is extremely lean.

That is why the Long and Short Term Fuel Adaptive Factors are pegged at 50%. The computer is maxing out the fuel trim to try make the mixture more rich. The net result is that your engine is running very rich right now. Because the sensor is crying "lean" and the computer is reacting by commanding full "rich".

That O2 voltage level of .04 is suspect. However, a screenshot will not do the trick. Typically they will oscillate between about .3 and .6 every second or so. If it was doing this correctly, the fuel trims (adaptive fuel factors) would be hovering more closely to 0%.

The O2 sensor never did jump around. I might go ahead and order an up stream O2 sensor then.

As suggested by Jordan, I repaired some vacuum leaks and it went from black smoke, raw gas, and pass out fumes to clear exhaust, no fumes, close to 20 mpg, and a far more steady idle. Probably not what's going on w/ yours, but what he described is what I was experiencing to a T.

https://cimg7.ibsrv.net/gimg/www.che...a557ae6659.jpg
Installed new IAC and noticed that the old one the plunger was stuck out compared to the new one. Went to go fuel up and test it out. Cleared the codes before I left and P0171 popped up again once I got to the gas station.

Didn't you think that the issue is the O2 sensor? How/why did you decide you'd replace the IAC - just being thorough?

My uncle has had a slew of Cherokees over the years and that was his recommendation. I had already ordered the IAC through the parts house before yesterday and just stuck it on to try it. My Cherokee has 234,000 miles on it so some new parts on it won't hurt anything. The O2 sensor will be in tomorrow.

I've been using my truck lately during harvest so the Jeep has stayed parked for a few weeks. When I'm getting 10/11 mpg it doesn't make much sense to drive it when it's costs 8 cents more per mile.

New front O2 sensor fixed my problem. Put it on last Thursday. Drove it this weekend and filled up with gas today. Averaged 17 mpg which is good for the roads I was on. Each part I put on helped but didn't solve my problem. I don't feel like I wasted money on parts since all of those needed to be changed. It runs better than it did when I got it. Thanks for all the help.

my 97 also has all his problems. codes are, p0301, p0441, p0171. I have replaced bot 02 sensors. it will get better for a while when I disconnect the battery, then back to the same in about 45 mins. I deliver a weekly free paper out of it, the gas mileage is killing me. It has bad gas fumes and runs rough at idle and rattles. clears up at speed...HELP

You should start a new thread. There's a button on the top left of the main posting list (folks have trouble finding this for some reason).

And lose the begging. The whole point of this forum is to provide help.