[2005rubicon]

Just wanted to let you guys know this is a great site. I drive a 2005 rubicon unlimited with the six speed manual. Right off idle when lightly accelerating and when I want to transition to full throttle at around 2500-3000 plus rpms it hits a flat spot then picks itself back up. I have driven carb vehicles in the past so a hesitation with the motor temporarly falling on it's face might be acceptable but a fuel injected motor should have no excuses. Its been to the dealership multiple times and yes they reconize the lag but say theres nothing availible for a fix. The computer was reflashed the third time in with no results. Everything on the motor is stock. I have tried using the stock airbox with a high flow filter minus the snorkel with no positve results. Any help would be appreciated. By the way there is a tsb on the autos for this problem but it does not apply to the manuals.

[BlackJeephunter]

It sounds like a typical 4.0 to me, they are very flat on HP and RPMs but very high on torque, almost like a desiel engine. I would not expect it to wind up like a V8, they just aren't made like that.

[2005rubicon]

Thanks for your response. I know the characteristics of the old beloved straight six but the hesitation or flat spot I am talking about occurrs when attempting to transition to a heavy throttle input from a low one. The motor starts to accelerate fine but when more than 75% is needed it falls on its face(short time) then accelerates strong. This could be related to a misadjusted cam timing sensor or bad timing. I also hear that the 4.0s run lean at wide open throttle. I drive alot on the highway and the exhuast looks like a discolored red, i.e. very hot exhuast. I know the computer controls the timing advance but are there any known fixes?

[BlackJeephunter]

I don't know, I never really get mine that high when driving on the road, I just shift when it flattens out. I will have to look but I think I shift in normal driving at around 2500, I know I don't make it to 3000. I have a Golf that I wind up and it does just that, winds up. The Jeep motor is flat and I just drive it that way. Have you driven any other Jeeps for a comparison?

[stathunter]

hummm.......no air flow problems?

[ieatchickens]

Since it's a 2005 maybe you should take it to your local Rubicon dealership. It should still be under warranty.

[BlueFlame]

Seems that you're describing a normal thing, these engines don't have a great throttle response, but...
This is one thing that does seem to be helped by a lower restriction intake/filter.
On mine, that lag mostly went away by simply removing the intake horn on the airbox. Slightly crisper throttle response, and it pulls a little better under load (don't have to downshift now on hills on my daily drive where I had to before.) A friend did a TB spacer and K&N intake - night and day difference in response. I'm not convinced it made it more powerful but it drives better. He then added a header and 2.5" exhaust and woke the motor up considerably. Again, all are small gains, and might not have improved it's 1/4 mile times, but it feels much better and accellerates instantly. He added a chip too, and made another small gain.
For myself, I'm not spending all that money to achieve so little, mine does just fine the way it is.
For what it's worth.