stummble and hesitation under acceleration

i have a used 4.2L with a 4.0 head i bought off a guy who rolled his jeep, with stock everything in my 88 cherokee. after replacing the old motor i found that around 1100-1900 rpm the engine sputters and spits, with the occasional backfire. its not trowing a code and i checked the new plugs and they were really white, i replaced the fuel pump and filter. now it is still doing the same thing. some how i am still getting a lean condition. i replaced the intake gasket b/c it had a wicked exhaust leak and that turned up no resolution to the problem. i am getting the proper fuel pressure and the regulator is fine. there is no cat. on the exhaust and it idles fine. I am clueless now, i have been told there is a vacuume leak, but i cant seem to find one. i have also been told that the timing is off, but i dont understand how that could happen if the timing is fixed? and it seems to run fine once im at normal driving speed. this problem has been a pain in the butt, and my jeep has some new dents in the fender because of it. any comments are welcome.
my specs are
4.2 block, stock 88, 4.0 head
stock injectors
stock distributer
no cat.
dynamax exhaust
stock manifold
stock TB

To check for a vacuum leak use some Brakekleen (works good) or some kind of carb cleaner and spray around any hoses, gaskets or anywhere vacuum is involved. Just be cautious if the engine is warm, especially of the exhaust manifold. Unless you wana torch it to the ground.

well i couldnt find any vacuume leaks, and i checked the O2 sensor and it was all gunked up so i sprayed some carb cleaner on it then burnt it off. it still does the same thing. so i am very confused and i do not want to keep throwing parts at it.

Re: stumble and hesitation under acceleration

Check the EGR valve. If it's stuck open the engine will run very lean.

egr is good i tested that yesterday, at idle, i pumped it up with a vacuume thing and it shut the engine off. so its good. could the o2 sensor be bad even if it isnt throwing a code?

I'm having a bit of stumbling/bucking at 2000 rpms on the highway, and it seems to be either the cps or the upstream O2 sensor. I haven't confirmed either one yet, but my mileage is is going down simultaneously so I'm leaning toward the O2 sensor.

well i put an O2 sensor in yesterday and as soon as i started the jeep it did the same exact thing. so now i am lost...again

Check the distributor indexing and maybe use an ohmmeter or voltmeter and check the TPS.

rehab said:

well i put an O2 sensor in yesterday and as soon as i started the jeep it did the same exact thing. so now i am lost...again

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A bad O2 sensor usually causes a rich condition and since your engine seems to be running lean, your old sensor's probably still good so keep it as a spare.
You might wanna check the wiring to the sensor though. It passes very close to the exhaust manifold and if the insulation is fried, the wiring could be shorting out causing the sensor to send an inappropriately high voltage to your ECU. When this happens, the ECU is fooled into thinking the engine's running too rich and it shortens the injector duty cycle to lean it out.

that sounds good, ill check that today. i replaced the injectors with the ones i had cleaned and it didnt help either, it made it worse untill i realized i didnt plug one of the injectors back in! lol

i just snooped around under the dash and found two wires that werent connected so i conected them and now i have a emissions maintenance light. does anyone know how to retrieve trouble codes off an 88 cherokee? i looked in the manual but they only had the 84-86 V6 models and the 91 and higher.

Different motor, same scenario. On my old XJ (89), the vacuum line from the MAP sensor to the throttle body had "sagged" due to heat and caused a stumble under acceleration. I never pulled the plugs to see what it was doing, though. Worth a shot anyway!

MIke