4.0 running lean and feels weak.

[camarors8992]

My 95 jeep used to run great. Then I hydrolocked the motor and we replaced it. We used to motor from a 94 jeep xj that was rebuilt 20k miles ago according to the owner. Sometimes it just doesn't feel like the old engine. The old 4.0 had 207k miles on it and the thing SCREAMED. This motor just doesn't seem to have the grunt. My engine light is on and when the codes are checked it says it is running lean. What should I do to solve this ? I have replaced the plugs, wires, cap, rotor, air filter, oil, oil filter, tranny fluid and that's about it. What can I do to wake this thing up but not risk damaging it ? Why is it running lean ?

[Blue XJ]

O2 sensor bad?

[old_man]

Most likely is the O2 sensor. Second is the MAP sensor, third is a big vacuum leak like an intake gasket or brake booster.

[camarors8992]

The motor ran great in the donor jeep. No codes no nothing. We had to cut the exhuast with a torch somewhat near the o2. Could that have killed it ?

[camarors8992]

We hooked the jeep onto a snap on scanner and the o2 checked out fine. I was told the small crack on my manifold could suck in fresh air causing the o2 to think there is a lean running condition. could anyone reccomend a good replacement manifold.

[j99xj]

We hooked the jeep onto a snap on scanner and the o2 checked out fine. I was told the small crack on my manifold could suck in fresh air causing the o2 to think there is a lean running condition. could anyone reccomend a good replacement manifold.

Sure can....the pacesetter header I installed last week. It's under the street performance section somewhere.

I think I'm running lean too....just from mods.