MPG drop and rough idling

2000 XJ stock except for K&N FIPK. 5 months ago after I filled up my gas tank, I started the Jeep and gas was flowing out of the gas filler neck, out from around the cap, cap was bad and was replaced. What would cause a pressure builup in the tank??? The tank before this happened, my MPG dropped from around 16 city to low 13's. I parked it for a few months and just started using it again last month. MPG still in the 13's...not good. It will idle fine and then start idling like crap, like it's missing, can feel it and hear it. No CEL light is on, no pending codes either. I did have one pending code for high voltage for an O2 sensor, I erased the code, thought my scanner would save the code but it didn't so I don't know what code it was. I would think if it was a bad O2 sensor, the CEL would have come back on, I have had my scanner plugged in when it was idling poorly and it showed nothing.
I need help as I got a new job and I was 4 minutes from work now I am 30 minutes so the crappy MPG is really hurting now.
ANy thoughts on any of the issues. If o@ sensors where $15, I would just replace them both but they are are a bit more than that, not looking to throuw money into it if that isn't the issue. What brand O2 sensors do people like, Bosch I think is thought of as best(I think I read they invented them??)

Pressure in the tank is normal. If it gets too bad id think the evap is restricted or something, but also think it would trip a cel. The running like crap could be because of too much pressure, o2 on its way out or something as simple as plugs. Will your scan tool allow you to view the injector and other sensors while running? As far as bosch o2, ive seen jeep owners stay clear of em amd go oem. I havent replaved o2s on any of my jeeps but have used bosch on other vehicles with no issues some vehicles are picky I guess. Im sure someone with more knowledge will chime in

20jeep00 said:

Pressure in the tank is normal. If it gets too bad id think the evap is restricted or something, but also think it would trip a cel. The running like crap could be because of too much pressure, o2 on its way out or something as simple as plugs. Will your scan tool allow you to view the injector and other sensors while running? As far as bosch o2, ive seen jeep owners stay clear of em amd go oem. I havent replaved o2s on any of my jeeps but have used bosch on other vehicles with no issues some vehicles are picky I guess. Im sure someone with more knowledge will chime in

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No, the scan tool I have now will not view live info, I want to get a better one, I picked it up at HF(#99722) to help get my GF beetle up and running again. HF website says:
Views freeze frame data and graphs data
Reads O2 sensor data
...Maybe I need to look into more of what it can do?

According to hf website it can read o2 data and freeze frames and graphs. May want to look into your scanner manual .

20jeep00 said:

According to hf website it can read o2 data and freeze frames and graphs. May want to look into your scanner manual .

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Box doesn't mention O2 data, gonna actually read directions(don't tell my GF:shhh

Have a smart phone? Not sure about others, but if android, you are in luck. Pick up an ELM237 BT reader apparatus from amazon for around 20$ and download the APP called Torque, free version is fine, paid is better.\
The BT apparatus comes with software, but not sure how good it is.

Something that can happen is that the 02 sensor will internally short the heater element to the signal wire intermittently, causing funky running issues like you are having. Rare but happens.
My 98 does the same thing and, over the years, it has been an ongoing intermittent issue. I'm really starting to think it's just a bad tank of gas, which feels like a cop-out but just when my Jeep was getting really bad, almost bad enough to have a symptom to diagnose at the shop, I lent the car to a guy & he adds gas, it comes back, no problem.
Post up with results!

run the cheapest plugs u can. the 4.0 loves the cheapest stuff thats wat its made to run on

An unexplained drop in your gas MPG's of 30-40% or more is often an O2 sensor issue. Rough idle, hard starting, bucking, and stalling can also occur. O2 sensors have a normal service life of approximately 100-150,000 miles and their replacement is normal routine maintenance.

Weak or nearly failed O2 sensors often will not trip a CEL.

The Jeep 4.0L seems to run best with NTK O2 sensors and the 00-01 with distributor-less ignition seem to like AutoLite APP-985 plugs best.

Start with your base tuneup items, (plugs, wires, cap and rotor (if applicable)). Try getting your fuel injectors cleaned. Getting a look @ the live data is a must nowadays. If your O2 sensors are off, it will probably show in the fuel trim # before it throws a code.

+1 on the fuel trim--02 sensor not functioning properly and the fuel mix is rich, sending you fuel mileage tumbling...also watch your temperature gauge, if it's not getting up to operating temperature (around 210) it will burn more fuel also.

everyone is right. Do a good tune up , I actually include an o2 sensor in my yearly tune up now.
Plugs, cap, rotor, plug wires, o2 sensor, fuel filter, oil change, sea foam it, test tps, check out cps, re fresh all grounds and batter terminals. clean all contacts for every connection under the hood. air filter. dont forget to fiddle with the injector ev1 connectors to make sure they are working well.

my money is on the o2 sensor, tps, or a bad ground/connection somewhere.

Thanks everybody, looks like I will be getting some O2 sensors soon. plugs only have about 35k on them, I have new ones so I might as well put them in.

blondejoncherokee said:

everyone is right. Do a good tune up , I actually include an o2 sensor in my yearly tune up now.
Plugs, cap, rotor, plug wires, o2 sensor, fuel filter, oil change, sea foam it, test tps, check out cps, re fresh all grounds and batter terminals. clean all contacts for every connection under the hood. air filter. dont forget to fiddle with the injector ev1 connectors to make sure they are working well.

my money is on the o2 sensor, tps, or a bad ground/connection somewhere.

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How about donating those 1 year old O2 sensors to a good cause?

ME!!!!:laugh3:

pajeepman said:

Thanks everybody, looks like I will be getting some O2 sensors soon. plugs only have about 35k on them, I have new ones so I might as well put them in.

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35k is right around the time to change copper plugs.

Most older motors spec a 30K plug change. I think the Jeep is included there. Most newer engines spec 100K change interval, which is fine, but you want to remove the plug every once in a while to "inspect" it, more to make sure the threads wont seize than anything.