I had a number of senors go in my jeep, and replaced them, did the groove and reported this on another forum topic:
"Just working on my 4L Cherokee. Started with +4 LTFT, went down to -12, but for some reason back up to +4, with power the same: probable new major vacuum leak. Will check hopefully today."
Well, there were no vacuum leaks that I found. The next couple months were spent intermittently checking and changing a few other sensors. The LTFT by now after the groove was fairly constant at +4, and came close to a 16 to 25 % increase (23- 25mpg now verses 21.5 before, with occasional increase to 27mpg, all highway).
Then come fall, I noticed a decrease in mileage, back to the original, and then less yet from stock, all the while with the fuel trims rising to +18 to 21. The fault lay with a bad fuel pump, which was now putting out only 30 psi from the required 49 psi. What I think happened is that when I first did the groove, when the pump was still fairly cool, full pump pressure was achieved and the trims were at -12 on my highway run. After an hour of driving or so, as the pump warmed up and slowed down (as it was faulty), the fuel trims went up to 4 in order to maintain the proper fuel ratio. As the pump continued to degrade, and supplied less pressure,the fuel trims opened up further up to + 21 to get enough fuel to even-out the fuel ratio. The problem is that the injectors were designed to atomize properly at a range of +/- 5psi from 49 psi, so when I checked the pressure (LTFT at +21 and pressure at 30psi), the spray pattern was so poor that the combustion was not efficient and resulted in the higher than stock fuel consumption.
I have since changed the fuel pump, and with stock injectors, the fuel trims went down to -6 with again an average 15% increase in mileage compared to pre-groove status. I am in the process of trying out different styles of injectors. I will update as necessary.
The groove on these are straightforward, but needs the IAC re-route, being careful to stay within the gasket coverage. I would suggest going fairly deep on these re-routes if you live in colder winter areas, as the width is limited to a narrow gasket, and you do need the flow in order to have the proper warmup revs upon startup in sub-0 weather.
I do find the mileage gain erratic, and so I think that this vehicle will be a good candidate for the MAPster. I will report once it is on.
Regards,
Luke
