Since I recently had the opportunity to take a closer look at the fuel injectors on my own 4.0L, I figured I'd update this thread with a little more feedback.
Over the past month or so I was getting a stumbling at higher rpms that slowly got worse until the CEL finally came on. I pulled the codes and got a P0304 (misfire on cylinder # 4). Naturally, along with the ignition system, one of the things I focused on was the fuel delivery system. Fuel pressure checked out fine, the fuel filter is new, and so after doing a full compression test (wanted to make sure cylinder # 4 was OK), I swapped the fuel injectors around to see if the problem followed the suspect injector. What ended up happening is that the misfire did seem to follow the injector, but I also got an additional misfire from another cylinder. So I then decided to pull all the injectors and have them professionally cleaned.
Well, it turns out my injectors (130k) were pretty darn dirty, and a professional cleaning of them was a very good call. Overall, it resulted in an average of over 12% increase in fuel flow rate. That’s quite an improvement if you think about it. Moreover, the spray pattern of the injectors is also much improved as a result of the process, resulting in better fuel atomization and therefore more complete combustion.
One of the injectors measured over 5% deviation in volume of fuel delivery from the others after being cleaned, so I had it replaced with a new one.
Click Here to View the Injector Test Results.
Long story short, I reinstalled the injectors, buttoned up the engine, and after clearing up a spark plug gap issue, I fired up my 4.0L and it started purring like a kitten. Problem has been solved!
And to top it off, my 4.0L is running better than ever! I really can feel a difference in performance, especially when under load. It just seems like I don’t have to give it as much throttle to maintain speeds. Acceleration is as smooth as silk. I fully expect to be getting much better gas mileage, and I’ll be keeping an eye on that.
I’m very happy with my nice clean injectors.
P.S. For those of you who are wondering, I run a bottle of fuel inejctor cleaner through my fuel system about every oil change...