XJ Power Lag

Hey All,

I have been having an issue for a while that I have been putting off for a while.  98 XJ 4.0 AX15

Here and there I will have a power lag.  It feels like I have about 70% power and then it will snap back in to full power hard.  This will happen starting from a dead stop or when I am cruising along on the highway struggling to make it up a hill.  Sometimes it will backfire through the intake (I think it is through the intake at least) from a stop but most of the time it will run smooth just under powered. Also, it will idle a tad rough when cold.

I have replaced the TPS, up and down stream O2 sensors, Cat, injectors (refurb neon upgrade, this was happening before the injector swap), gas pump, plugs, wires and cap.

My guess is that it could be the CPS, coil or a bad fuel pump.  Has anyone dealt with similar issues?

How does the cap and rotor look? Spark plugs and wires? I know you replaced them, but have you physically checked them recently? Could it also be the heat soak on injector 3 has ruined your new injectors already?

Could be a failing MAP sensor, they commonly go in a lot of Jeeps. I had one go in my Cherokee last summer completely out of nowhere, I was mid-turn and the thing just cut out on me. I couldn't get it to start back up without pumping the throttle and had to keep on the throttle or it would die. I also had one go in my YJ a few years back as well. Here's a description of a bad MAP sensor taken from a Jp article:

"All fuel-injected Jeeps use a MAP (manifold absolute pressure) sensor to determine how much fuel the engine requires. The MAP sensor monitors engine manifold vacuum and translates that reading into an electric signal sent to the ECU. Depending on the voltage from the MAP sensor, the ECU will fire the injector for a longer or shorter burst to deliver more or less fuel, as the case may be. It's a fairly foolproof and simple design, but they can have issues. If after the engine has come up to operating temperature your Jeep suffers poor drivability, surging, rough idle, excessively rich exhaust, or pinging even with mid-grade fuel, chances are the MAP sensor has begun to go out of range or has failed altogether. The thing is, a bad MAP sensor won't always trigger a check engine light or cause the computer to register a DTC (diagnostic trouble code)."

Cap and rotor look good. I will pull the plugs after a project I am working on now (front frame stiffeners).

MAP sensor sounds like it could be a good call.. I seem to remember changing it a while back but it may have been the IAC.

Is there any way to shield the injectors? I have substantial venting on my hood.

The only things I really know of to combat the heat soak issue are heat wrapping the injectors and/or the manifold, wiring the aux. fan to run for a few minutes after you shut the engine off, hood louvers, or any combination of these things. If you poke around online forums it seems the aux. fan timer route is the most popular and most effective, but doesn't really 'cure' the heat soak problem.

In fairness I've only ever experienced the heat soak issue twice on the trail. Both times were after shutting the vehicle off after crawling at low speeds with low air flow. Scared the absolute shit out of me the first time it happened because we were about halfway through the Henniker, NH club run late last year and the thing felt like it was firing on like four cylinders. That's when I learned of the widespread Cherokee heat soak issues. I just did 600 miles round trip from here to New Jersey over the weekend with no issues. I was quite worried it was going to pop up, but because there was flowing air getting under the hood it wasn't an issue. Full disclosure: I popped the hood once we got to our hotel Friday night and sat there for like 10-15 minutes to let the heat out with the heat soak problem in mind. My girlfriend was not pleased about the delay. It's a Jeep thing.

I can't see your problem being heat soak-related simply because there's no way the engine bay temps have reached critical just doing normal everyday driving, especially this past winter when it was so cold. That doesn't mean it's not injector- or fuel system-related, I just don't think it's because you're experiencing vapor lock. I'm planning on going the Neon injector route myself when the time comes.

Yeah so if you check my build you will see how much a piece my XJ was when I bough it. It had no electric fan and the wiring is cut out.

I did recently drop in an aftermarket backup manual controlled electric fan that really moves a lot of air. I use it in traffic in the summer mostly being a Boston area commuter.

I wish there was a simply way to check my fuel pressure as I am cruising along. I need to stop by autozone and rent a fuel pressure gauge to see what my pump is doing at startup anyway.

Speaking of heat soak.. I just remember that I once got heat soak issues with my current CPS in traffic before I redid my whole cooling system. I even got a CEL thrown at me. Can't believe I forgot about it. Ill test the CPS and report back when it is on 4 wheels.

My plan is to wire up my aux. fan to a switch so I can do the same thing in the summer, as well as for on the trail to combat the heat soak stuff. I'm not familiar with all the symptoms of a bad CPS, but I do know it's another sensor that constantly fails on these things and leaves you dead in the water.

When my MAP sensor went I initially thought it might have been the CPS (having read horror stories about it) since the thing just died out of nowhere and wouldn't restart, but luckily it did throw a code and it was for the MAP sensor so that's where I started first. I do recall it sort of hiccuping and lurching on the highway before the MAP sensor went that day (kind of like you've described), but if yours threw a CPS code then I'd start there too. My MAP sensor was original (16 years old at that point) so it was due to let go, as was my TPS which went within a few months of the MAP sensor. Funny thing is the same thing happened with my YJ; the TPS went bad and within a few months the MAP sensor crapped out too.

Well thanks for the user experience. Like I said, my jeep is in pieces right now and I am trying to get it together for the BBQ. Ill keep this updated for future reference.