89 4.0 - power loss until 3000rpm

[uncleben03]

I picked up my first MJ today. It's my 7th Jeep, 5th with a 4.0, but first one old enough to have a distributor.

 

Jeep idles great, tools around town fine and cruises down the highway like a dream. It has a longer crank time than I'm used to with 4 liters, but I've read that's normal.

 

My problem is I have probably about a 15-20% power loss below 3000 rpm. If I'm driving at any speed, below 3000 rpm and I hit the gas hard, the engine seems to stutter VERY slightly. I can tell it's not accelerating like it should. If I keep accelerating, when the RPM's hit exactly 3000, it's like a turbo kicks in. There's an immediate increase in power like I pushed the ignition on a rocket booster or something. That's when the motor starts feeling like a 4.0 to me. Something's wrong below 3000 though.

 

Thoughts?

[HOrnbrod]

Edit your signature stating what year, engine, and transmission is relevant to the MJ you are referring to. It makes things easier.

[Eagle]

I picked up my first MJ today. It's my 7th Jeep, 5th with a 4.0, but first one old enough to have a distributor.

 

Jeep idles great, tools around town fine and cruises down the highway like a dream. It has a longer crank time than I'm used to with 4 liters, but I've read that's normal.

 

My problem is I have probably about a 15-20% power loss below 3000 rpm. If I'm driving at any speed, below 3000 rpm and I hit the gas hard, the engine seems to stutter VERY slightly. I can tell it's not accelerating like it should. If I keep accelerating, when the RPM's hit exactly 3000, it's like a turbo kicks in. There's an immediate increase in power like I pushed the ignition on a rocket booster or something. That's when the motor starts feeling like a 4.0 to me. Something's wrong below 3000 though.

 

Thoughts?

Two thoughts, for starters.

 

First, a slight stutter under load is often caused by bad spark plugs and/or bad spark plug wires.

 

Second point is that the early (non-HO) 4.0L engines don't have the same power curve as the new ones. If you haven't yet owned one with a distributor, that means you haven't owned anything older than a 2000 (or maybe a 1999 if you had a Grand Cherokee). The Renix era 4.0L (1987 thru 1990) is not the seat-of-pants rocket ship that the newer ones are. There's a reason why Jeep called the second generation "High Output" when it was introduced in 1991. If you are expecting a Renix-era 4.0L to give you the same seat-of-pants feel as the 2000+ versions ... forget it.

[uncleben03]

I'm definitely aware the power band isn't the same, but something is certainly not right. It literally feels like flipping a switch at 3000rpm. Not a gradual increase or anything. And I lied, I did have a 94 ZJ. that was a long time ago. And not a renix

[uncleben03]

In case anybody cares, this has been resolved.  It was a bad fuel pump that eventually failed completely.  Just replaced it this weekend, the power loss is gone and it runs like I know a 4.0 should run now.

[Kickinmule]

I care and am glad you resolved it. Thanks for posting it gives those with similar probs a place to look.

[cruiser54]

In case anybody cares, this has been resolved.  It was a bad fuel pump that eventually failed completely.  Just replaced it this weekend, the power loss is gone and it runs like I know a 4.0 should run now.

Thanks for the update. Since you have an 89, have you ever perused my Tips?

 

In my signature below.

[uncleben03]

Cruiser, hadn't seen it til now.  Bookmarked for *ahem*, reading in the "facilities".  :D

[cruiser54]

Cruiser, hadn't seen it til now.  Bookmarked for *ahem*, reading in the "facilities".   :D

You're $#!&tin me. Pun intended!! 

 

Print them out and take em into the garage. 

 

Completely perform Tips 1,3,4, and 5. No shortcuts.