So I’ve been having an issue with my 99 XJ for a couple of months with fuel mileage. My mileage dropped from 18mpg to 15mpg over the course of two tanks of fuel and it’s been sitting at 15mpg for about the last 8 weeks.
I drive the same route to work. I was buying gas at Costco like I have been for the last 18 months or so. I’ve tried a different gas station for a couple tanks in a row and still have the same mileage issue.
I changed spark plugs (NGK coppers), plug wire, distributor cap, and rotor. No change in mileage. I’ve got the updated 4-hole Bosch injectors now and still no change.
IAC and TPS have been changed previously because I have this annoying stall issue that comes and goes. When I first start up in the morning and start driving I feel a dip in power around 1500rpms like the truck is bogging, then a second later it’s back to normal. It does this a couple times and it will be fine for the duration of my drive to work. It might do it in the afternoon when I leave depending on how hot it is outside. Warmer weather seems to keep it from happening.
The reason I’m posting now is this morning I suffered a power loss at an intersection. I pull out and it feels like it dies. I’m still rolling forward, clutch completely engaged at about 1800rpms in first. After a couple seconds it takes off at full power. No lights came on the cluster like it died. Just no power. I make it a few blocks to another red light. Same issue. Only this time I was still letting the clutch out and it died.
I don’t like shotgunning parts at a vehicle. TPS/IAC were changed about two years ago. Plugs/wires/cap/rotor were changed by me for the first time in fours years of ownership.
Where do I go from here? I’m thinking maybe the CPS is starting to dying and the no change with random power loss after changing the IAC and TPS point to this. I have no stored codes that I can read from Torque after these incidents this morning. I had no issues after the second stall this morning.
This is a copy/paste from a forum post I made about my Jeep. I’ve been slowly working through this process and just wanted to document it here as well. I’ll follow up with what I try in the comments. I’m pretty sure it’s one of two sensors that are failing. Either the O2 sensor or the CPS. O2 isn’t really mentioned here because I think that would normally throw a code. However, when logging my vehicle through Torque on my phone, I see the fuel trims are way out of whack in my opinion. Long term fuel trims show the ECU trying to pull 33% of the fuel that is being delivered. That coupled with the short term fuel trims always showing about 6-10% fuel pulled makes me think something could be up with the O2 sensor. I’ve had a bad CPS on my old Jeep Comanche, but it basically just died when it got hot. I paid someone to diagnose it since I wasn’t very good with vehicle diagnosis at that point in time. I’m much better now, but sometimes I want someone to verify my thought process before I spend the money.
Since I posted this on the forums, I have changed both O2 sensors on my Jeep and I have re-installed the factory injectors. I went back to stock injectors because I was starting to get rich codes. I looked at the part numbers on the new injectors and they were not Bosch injectors. They are some kind of knock off injector. I should have checked that before I purchased them. Oh well. Live, learn, spend money.