TJ Runs Rough When Maintaining Speed

[mrwjr2003]

My 97 TJ 4.0L runs rough when I'm trying to "cruise" along. I am still trying to figure out how to describe the situation in a short phrase.

The jeep sounds strong, and runs strong for the most part. However, when I am just out driving somewhere, and I shift up threw the gears, and get to that point where you are just giving it enough gas to keep your speed, not really no gas, but not really pushing on the pedal either, the jeep starts to run rough. It feels like its not getting enough gas. If I press the pedal to the floor, the jeep comes to life and no more sputtering. If I go back to "cruising" she acts rough again.

I have done the following:
New fuel pump
New injectors
Cleaned out the fuel rail when replacing the injectors
Fuel pressure at 40 with key on, 50 when running.
New TPS installed

I am getting a P0141 code, but that might be because I don't have a cat anymore, and have gone to a Flowmaster super 10 muffler.

Any thoughts, this thing is driving me crazy.

Thanks!!

 

[Tob]

Welcome to the forum!

Could it be that the engine is lugging? What's your "cruise" RPM? And what's your tire size and differential gear ratio?

 

[mrwjr2003]

Welcome to the forum!

Could it be that the engine is lugging? What's your "cruise" RPM? And what's your tire size and differential gear ratio?

Thanks for the quick response.

I am on 35s, with 4.56 ratio.
Cuise is usually 1.5k to 2, there abouts.

I don't think lugging is the issue, the engine isn't struggling, its sounds more like starving.

 

[Tob]

I'm not sure then. I usually cruise at 2000 RPM at the lowest, especially at highway speeds.

 

[Jerry Bransford]

I am on 35s, with 4.56 ratio.
Cruise is usually 1.5k to 2, there abouts.

I don't think lugging is the issue, the engine isn't struggling, its sounds more like starving.

Your engine is CLEARLY lugging when cruising at 1.5K to 2K rpms. Your TJ was undergeared with that 4.56 axle ratio for 35's, you would have been a lot better off with 4.88 or 5.13. But you might also have a bad O2 sensor which can cause a problem like that.

 

[ItchyDong]

You've literally troubleshot the problem to being under geared. The minute you raise RPM's, it goes away. 1.5k is insanely low for our TJ's. If you want proof, go look at a gear calculator for your jeep to see what stock set up had your RPM's at whatever speed it most commonly acts up at.

 

[mrwjr2003]

I promise everyone it is not under geared. I've had this set up for years. Until this problem started, she ran just fine, no issues, perfect use and response out of all 5 gears.
I apologize to everyone, I took her out today just to verify my RPMs since I was going on memory, and it does seem that cruising along is usually 2k to 2.5k.

I took her out for a longer drive than I have been, and after she was good and warm (after 30 minutes) the issue sort of went away, but other issues came up, such as, sitting at red light she would start to idle low then high and back and forth ever few seconds until I rev'd her, then she would go back to idle a little under 1k.

Starting to really wonder if its the O2 sensor. Also, after driving her for a while, she smelled rich.

 

[Jerry Bransford]

I promise everyone it is not under geared. I've had this set up for years. Until this problem started, she ran just fine, no issues, perfect use and response out of all 5 gears.
I apologize to everyone, I took her out today just to verify my RPMs since I was going on memory, and it does seem that cruising along is usually 2k to 2.5k.

4.56 is still undergeared for 35's and it is still lugging or at the verge of lugging at those rpms. A Wrangler with 35's doesn't have the ability to run well at those low rpms like a car can.

 

[mrwjr2003]

I appreciate your opinion. I'll give an update when I replace the O2 sensor.

 

[Tob]

What is not acceptable in California terrain may be passable in Florida - there's next to zero elevation change. Regardless, according to post #7, inadequate gearing is not causing the issue at hand.

I've had this set up for years. Until this problem started, she ran just fine, no issues, perfect use and response out of all 5 gears.