gargantuan

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Aug 30, 2022
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Hey everyone recently bought a jeep TJ. Since it's nearly 20 years old and a gas guzzler I didn't think much of the heavy gas smell when It sits idle for a couple of seconds even. My question, is this normal? I am suspicious that there may be something wrong since it is so strong when I turn it on and it idles for even a coue seconds. There is also no smell when the engine is off. Please share possible causes and fixes if you have any.

note: I searched for my specific issue in this forum and other but couldn't locate the topic related to while the jeep is running/idling, feel free to link it if you know of one. Thanks community.
 
A common cause of that is the fuel mixture having been set to full-rich by the PCM. A common root cause for the PCM doing that is a bad O2 sensor that is falsely indicating to the PCM that the fuel mixture is too lean so the PCM keeps richening the fuel mixture trying to get the mixture where it wants it to be. Eventually the mixture ends up and stays at full-rich.

If that turns out to be the cause make sure to ONLY (!!!) install NTK, NGK, or Mopar O2 sensors. The NGK and Mopar O2 sensors are made for them by NTK. Don't even consider using Bosch or store-brand O2 sensors.

O2 sensors do eventually go bad so it wouldn't be unreasonable to believe that could be causing your problem.

But a bad ECT (coolant temperature sensor) can also cause the PCM to think the engine is running cold and cause the mixture to be too rich. If your instrument cluster says at or close to 210 when the engine is warmed up it's ok.

This shows the correct NTK part numbers for your '04.
O2 sensor locations.gif
 
Seriously awesome response! Thank you so much for the response. What I find interesting is that it passed it's smog about a week ago despite smelling like fuel when running. I was frankly surprised. I was tossing around the idea that the fuel injectors might be another cause but the O2 sensors makes much more sense. Sure enough I'll be replacing both for good measure. Again, great response and direction on parts! This will save me a lot of time, money and headaches. 🙏
 
You're welcome and to be sure your '04 has four O2 sensors. :)

Oh interesting, are you saying there is a chance not all the 02 sensors are in? Also I ran a scan and there aren't any codes for O2 sensors or anything else, can a O2 sensor be bad without throwing a code? From what I've read into so far the O2 sensor matches up with the smell when idling and the absolutely terrible gas mileage. I think I'm right around 10 mpg or less. Thanks for your help!
 
The PCM isn't always smart enough to know when an O2 sensor is providing accurate information or not. If it's getting data from an O2 sensor saying the mixture is too lean it trusts that data, it can't always cross-reference data from elsewhere to determine the truth. There are times it will say there's a bad O2 sensor when it's really a bad pre-catalytic converter that is causing the O2 sensor to look bad when it's not.

And 2004 4.0 engines have four O2 sensors, especially those made to pass California smog requirements. Not sure about those built for export but I'm assuming you have a TJ that was made for California. The above diagram shows you where the four are located.
 
The PCM isn't always smart enough to know when an O2 sensor is providing accurate information or not. If it's getting data from an O2 sensor saying the mixture is too lean it trusts that data, it can't always cross-reference data from elsewhere to determine the truth. There are times it will say there's a bad O2 sensor when it's really a bad pre-catalytic converter that is causing the O2 sensor to look bad when it's not.

And 2004 4.0 engines have four O2 sensors, especially those made to pass California smog requirements. Not sure about those built for export but I'm assuming you have a TJ that was made for California. The above diagram shows you where the four are located.

you are spot on, I am in California. Thanks for the info that makes a lot of sense. I am going to go ahead and just replace all of them. At the very least it can't hurt. I appreciate your advice and direction immensely. 🤟
 
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