Cracked exhaust manifolds are very common for the 4 liter motor. There are a few aftermarket manifolds that are less prone to cracking, but if the it's not leaking much I wouldn't worry about it. You might need to replace it if it gets worse.
A cracked manifold can potentially introduce oxygen into the exhaust stream, which will be detected by the oxygen sensor. This can lead to decreased mileage, carbon buildup, and damage to the catalytic converter in the case of a bad crack.
I left the manifold alone on my cherokee. Once the manifold warmed up the thermal expansion was enough to close the crack. I put about 20,000 miles on mine before I sold it without any change.
I would just see this as an opportunity to haggle the price down. Call a couple shops and get a rough estimate how much it will cost for the manifold replacement, and deduct that from the price.
Saturday, May 9th, 2009 AT 8:24 PM