Application is a '97 Jeep Cherokee 4.0L straight-six

I installed EV6 fuel injectors to replace the outdated EV1 injectors. The new ones are slightly higher flowing I think.

  • old: 23.2 lb/hr @ 49 psi
  • new: 22.7 lb/hr @ 43 psi (but they're being operated at my Jeep's 49 psi fuel pressure)

Watching my Long Term Fuel Trim levels on an obdII scan tool, they can get as low as -12% or -13% after accelerating, and they're around -8% or -9% while idling, and they can get just above 0% if accelerating hard.

Assuming everything is functioning perfectly fine with my Jeep, is it ok for the LTFT levels to be less than -10%? Like, could that be harmful on the injectors or will it cause the check engine light to come on?


2 Answers 2


By using this fuel injector calculator, your new injectors are flowing 24.2 lb/hr @ 49psi, which is a bit more than stock (by 1.0 lb/hr). This could definitely be why you're seeing LTFT levels being pulled by over 10%. 10% is the nominal maximum you want to run your fuel system (whether + or -). Realistically this is borderline maximum of where you would like to run at, but it's still not where you'd like to be. Your computer can probably handle it, but it would mean you are at the maximum it can handle and it would have no room to go anywhere if it needed to. With that in mind, you want to see about getting your Jeep tuned so it's running a lot closer to that magic zero number or getting injectors which will run at the stock value. It isn't something you'd have to do immediately, as the ECM is keeping up, but it's something you'll want to do going forward.

There is one other solution which might work for you. That would be to get an adjustable fuel pressure regulator to replace the stock one. You could then dial the pressure back a bit and your flow would be where it should be and the computer would once again be happy. 45psi would put you right in the ballpark.

  • Thanks! Injectors have a max duty cycle, but do they have a minimum duty cycle? Is there a maximum negative LTFT before the PCM throws a CEL? Commented Jun 26, 2018 at 20:37
  • 1
    This really has nothing to do with the duty cycle. It has to do with how much the computer can allow for through it's tables. CEL's are based on max/mins setup in tables within for every CEL available. If any of the CEL's max/mins are busted due to it being out of range, it will throw the code. Every manufacturer sets all of their engines' CEL's differently, so who knows what those max/mins are unless you know the tables inside and out. Commented Jun 26, 2018 at 20:48

I know this is an old post but my answer to this question might help future users. The new fuel injector that flowed 22.7 lb/hr @ 43 psi will become a 24.23 lb.hr injector at 49 PSI. That is about 8% bigger than the stock injector. Since the new fuel injector is about 8% bigger therefore the negative fuel trims by about the same percentage points.

Fuel trims can never be harmful to the injectors only to the motor itself when a continuous and excessive lean or rich condition is experianced

I used this fuel flow calculator.

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