I need help with my straight six 4.0 I have tried so much on this so any help would be greatly appreciated. Cherokee starts up and idles fine. I can rev it and its fine. I drive down the road and its fine till I hit near 50mph. That's when the bogging, bucking and losing power starts. I've changed the Camshaft and Crankshaft position sensors and that did not help. I've done throttle position sensor and that did not help. New MAP sensor did not help. New plugs. Neutral Safety Switch
​I just can't figure this one out. Can anyone out there help me on this please? Thank you, Neil V from Maine.

*****Its a 2004 Grand Cherokee WJ Special Freedom Edition 4.0 straight six. Yes it runs fine from 30-50****

 

How exact is the 50MPH thing? Like have you tested drove it at 35-40MPH for extended periods of time without having this issue?

 

First, let's get the year correct. If it's a 2004, it's not an XJ. If it's a typo, please correct it so we can endure any suggestions are applicable to you.

You mention MAF so I'm leaning it's not an XJ.

 

no cherokee after 99 had spark pug wires, so you need to confirm what model and year you actually have.

 

Its a 2004 Grand Cherokee WJ Special Freedom Edition 4.0 straight six. Yes it runs fine from 30-50****

Just changed Neutral safety switch as well. That did not help.

 

So, sustained load at 50mph is at least 2000 rpm… I’m leaning toward a bad coil pack. Maybe the wiring is weak to the coil pack?

Could be a fuel system restriction too.

 

what is your fuel pressure? How is it climbing hills?

 

Hey Neil, I think you found the "limp" mode but it's not setting up the MIL (check engine).

We need to eliminate the question of Open- or Closed-loop operating status. Are you driving-off with a cold engine or are things warmed-up to normal operating temperature (190-210) before taking it out on the road? (I like to warm-up mine with the tranny in Neutral, to run the pump and warm the fluid.)

If it warms fully and the system transfers to Closed-loop operations while on the road then this may explain why it's stumbling when you bring it up to speed. (Not yet the "what" but at least the "why". The "what" might throw a code, but it could take too long to happen and waiting could make things worse.)

A bad or intermittent O2 sensor can cause this behavior without throwing codes, so get yourself under there and inspect them and their wires. Also, check oil and coolant for the "milkshake" or creamy foam deposits, which suggests a head-crack or a head-gasket gone bad.

 

Neil, How many miles on the 04 WJ?