Redbaronxj

I have a 99 xj with a 4.0 that is super charged recently it has started to buck and jerk when in boost. With fuel gauge hooked up it has 45 psi when ever it bucks and jerks the fuel pressure gauge flickers down in a blink of an eye then returns to normal. When the seventh injector is unplugged it runs fine.this leaves me to believe it's a fuel issues possibly bad seventh injector or fuel pump. But not sure which way to go. Have had noid light on injector and it pulses fine. I know some people on here have had this supercharger kit using the same split second controller and seventh injector maybe one of them can chime in?

Cummins93

So it has run correctly with the supercharger in the past? What are your short and long term fuel trims? Does it happen at part throttle or wot or both? Have you reset the ECU to see if it returns to normal and over time developes the bucking again? Have you changed out the fuel pump?

salad

I can't comment on the kit but the stock setup uses 49 PSI. Is it the factory pump with this kit?

Redbaronxj

I haven't checked short term or long term fuel trims. It ran fine before with this kit for almost four years. It uses factory fuel pump. It bucks and jerks at partial throttle but only if in boost. It kicks on the seventh injector when it sees boost. It Only does it when you just getting out of vacuum and into boost until about 3-4psi after that 5-10 psi it doesn't do it so wouldn't think it's the fuel pump. if I unplug the seventh injector it doesn't buck and jerk when getting into boost. Seems like the injector is messed up it set for almost three months while building the engine. But several ppl I have talked to said the injector would not likely be the cause and would be rare for them to over fuel. It is also is really rich i believe while in boost because the exhaust has turned everything black behind it with soot. This leads me to think the injector is bad and fueling to much.

Programbo

If you don't mind my asking what fuel injectors are you running with it?

Cummins93

What does your wideband say? If you are sans wideband I would recommend you install one.

Redbaronxj

It has the factory injectors in the engine and uses a 60lb injector infront of the supercharger to help fuel in boost and cool the charge. I don't have a wideband in the jeep use a scan tool to read most of the data. On the scan tool the o2 reads center between rich and lean most of the time. But when in boost it goes into open loop as its suppose to with the split second module using paremeters from it to control timing and fuel.wide band would be wise but I don't like a bunch of gauges and not much room to mount one already have boost gauge on the pillar. I did look at fuel trims today long term was like -7.8 and short term was 3.6 but would go -7 something something while in boost. That means it trying to compensate for being rich correct?

jhc7399

I can tell you from experience that injectors will dry out and lock up, either open or closed.

Cummins93

If you have only changed one variable which is going into boost with and without the injector connected and you have a stumble only with the injector connected that is a fairly good indicator that that is your problem. A few thoughts for you though. 1: do not run into boost without extra fueling on board. You did it to diagnosis the issue and that's fine but don't keep running that way. 2: and I cannot stress this enough IN CAB WIDEBANDS ARE ALWAYS ALWAY ALWAYS NECESSARY. Modern widebands are small, compact, self calibrating, and relatively cheap install one.

So moving on to resolving this issue I would contact the maker of your supercharger kit and see what they think and if they will send you out an injector for replacement or testing purposes. An injector should not have gone bad in such a short period of time.

Lupac

I was having a similar issue in my jeep at around 2000rpm and it turned out to be the throttle position sensor. you can check it in your obdII computer while driving

CobraMarty

Like we discussed, most likely the problem is your fuel pump. Either clogged filter, weak old pump or low voltage.
Testing the voltage with the key on is not accurate. You need to test the voltage when running and when you are having the problem. When the pump is being stressed, like when getting into boost and more is demanded of the pump, the pump will draw more current. All the Cherokees have old wiring.
Just like the headlight issue and using Putco relay harness to fix.

An easy test is to run a sufficiently heavy gauge wire directly from the battery to the fuel pump and go test.

And don't forget your GROUNDS. Did you hook up all your ground cables when you installed the rebuilt engine? Is the fuel pump adequately grounded? Add a battery to chassis ground cable and fuel pump to chassis ground wire.

Cummins93

Its been almost a couple months, what fixed your issue?