Backfiring issue

[the_mechanic]

Tried searching and didn't come up with anything. Been having issues lately with my '95 XJ, 4.0. A while ago, I had upgraded to the 19lb Bosch 4-hole injectors and it ran great until a few weeks ago.

It started when I was driving home and I had a vacuum boot split on the fuel pressure regulator and once I got it home, discovered one of my injectors was stuck open (or just plain broken). I'm assuming the pressure spiked when the regulator lost vacuum and it blew out something internal in the injector. I ordered new injectors and made the ill-advised decision to try 22lb injectors, Jeep ran like crap. I just got another set of 19lb injectors installed and it ran better but still has problems.

When I accelerate any more than lightly and gradually, it will backfire through the exhaust. Seems to mostly happen between 1000 and 2000 rpm but happens at no particular throttle position. Normally, I'd think TPS but that is new and it happens with light or heavy throttle. EFI is not my strong suit for hoping for some suggestions on what sensor might have gone bad. Or maybe it's still readjusting to undo the adjustment it made to try to accommodate the 22lb injectors.
Going to check the plugs this weekend and see if that tells me anything.

Let me know if any more info will help.

 

[lawsoncl]

I doubt the vac line to the regulator issue caused an injector failure. It's not like you have vacuum before you crank the engine.

Are you using new or refurbished injectors from a reputable source, and not getting crappy fake chinese bosch knock-off from ebay or Amazon?

 

[the_mechanic]

The vacuum boot actually split while the Jeep was running on the freeway.

Genuine Bosch injectors, all new.

 

[8Mud]

For the first few minutes after startup on a cold motor, the fuel trim is from the PCM, open loop. After it warms a bit (happens quicker than you'd think) it trims from sensor input, closed-loop. If the bucking never happens when cold but only after a few minutes of driving I'd suspect the front O2 sensor. Most sensors when they are out of the envelope will throw a code, not so with the O2 sensor. The O2 sensor often only codes after it is completely dead. They tend to mess up most when accelerating (on-ramps) and again when cruising near peak torque. Near peak torque the motor trims to fuel saving, sometimes the trim is too lean from a faulty or lazy O2 sensor. Have a really good look at the O2 sensor harness, if routed wrong it can cook on the exhaust manifold, where it typically cooks is almost impossible to see.

Could be a vacuum leak, double-check your intake/exhaust manifold bolts and injector seals.