Hi Cherokee owners, I hope all of you are having a great day today with it being spring now. Anyways, as the title states, I have a huge, I mean 1/3 loss in gas mileage, I can literally see my gauge going down. Before you say I should, I’ve already given the classic tune up to my Cherokee, this is a problem that is much deeper than a needed tune up. Nevertheless, here’s what happened:
•I was wiring in some lights and I broke the vacuum line going to the cruise control module, so I put a screw into the hard line considering I never use cruise control.
•The same day, I was working on my handbrake and I was around the EVAP charcoal canister a lot with tools since the bracket for the handbrake cable rests next to it.

For the cruise control module, I think a vacuum leak is out of the question as the system lets out a huge hiss, even when the car is off. For the EVAP system however, I have no way to determine if this is faulty, causing my drastic drop in mileage. No CELs have been thrown, and the Jeep doesn’t seem to give a damn, only me for the amount of money I’m wasting on gas. Any help is appreciated, and if there is anything else that should be considered, please feel free to add suggestions. Don’t forget that before I got the drop in mileage, I was working around the EVAP system and vacuum lines, like I said though, I don’t think it is vacuum. Thanks so much!
Liam

 

Were you also working around the upstream O2 sensor? Had one die on me while on a trip. Cut my mileage almost in half.

 

Yeah, my thought exactly Earl. Bad 02 Sensor.

 

To add fuel to the fire (no pun intended) I took advantage of my obd2 port today and took some measurements for about a 40 mile drive, here are the results:
• Coolant temp peaked at 94 degrees Celsius
• Intake air temp peaked at 51 degrees Celsius
• MAP psi was on point, read atmospheric pressure with the car off and follows with the rpm (rules out vacuum leak)
• Long Term Fuel Trim peaked at 0 (around startup), went down to -30 (extremely rich), and ended with -14 (still rich)
• Short Term Fuel Ratio started at 0 on startup, went down to -30 after 5-7 minutes, and smoothed out to around -2 (seems okay)
• Throttle position was 14% when the car was off and at idle, follows the rpm curve respectively
• Timing advance in degrees... this is where it gets weird. Started off at 0 for start up, jumped up to 35 after 2 minutes, and bounced from 30-40 throughout the whole trip. This seems absolutely nuts to me and I think this is definitely a factor.

Thanks for all your guys’ help and I hope you can turn these numbers into solutions for me! Have a good night and happy Jeeping!

Here are graphs of my data:





Edit, there is some some skepticism on my upstream o2 sensor, here is a graph of that as well:

 

Thanks for your guys’ replies! Just the fact you’d come to this thread gives me hope already. Please refer to my last reply in response to the o2 sensor and some more information I found. Thanks in advance!

 

LTFT ending rich could be the O2 sensor. Read down to post #13 at this link. It tells how to test the sensor.
https://www.cherokeeforum.com/f2/o2-...issues-251391/

As far as timing advance goes the variation in readings could be play in the timing chain. Old time test I used was to watch the timing mark on the harmonic balancer with a timing light. In a perfect scenario the mark should stay steady. Most I saw would jump some showing a bit play in the chain.

 

Hi, thanks again for giving me suggestions. Considering I bought a new o2 sensor a while ago (amazing deal so I had to get one), I’ll just replace it anyways as I’m sure it hasn’t even replaced in its 187,000 miles of use. As for the timing chain, I have a couple questions:
1. Is this something I should be concerned about? When do you think I’ll need to change my gears and chain?
2. What about running 30 degrees of timing advance at the lowest? Isn’t that extremely high already on a Cherokee? Should I also test the CPS or camshaft position sensor?
Thanks again,
Liam

 

Since you can't adjust the timing on these motors the timing specs aren't on the sticker under the hood. It refers you to the FSM which I don't have. Hopefully someone will reply with answers to your timing variations. As far as chain replacement goes unless it is the cause of erratic timing readings of which yours could be from other issues or possibly (hopefully) no issue at all or the chain is hitting the cover it doesn't need replaced. I mentioned the timing light test because it's of no cost to do. Wish I could have been more help.

Are you saying you have a new O2 sensor to replace the one now in it? If so what brand is it? NGK/NTK's are highly recommended here since they are used from the factory.

https://www.rockauto.com/en/moreinfo...315943&jsn=396

 

Hi, thanks again for your reply and yes, it is a a NTK. I decided to pull the o2 sensor and after about an hour of wrestling, I managed to get the old one out, it was pretty baked. However, considering working on a Jeep seems to be on par with opening a can of worms, I unfortunately didn’t get out of this free and clear: my o2 sensor stripped the bung. I am getting a 18 mm tap today, so I’m hoping there’s enough metal in there to make a new thread. Me being a student, I really can’t afford to take it to an exhaust shop, that and the status of my catalytic converter, which I’ll leave at that if you catch my drift. I’ll give a thread update on if and when the o2 sensor goes in. Thanks again!
Liam