Ok guys I'm tryin to figure this out, my Cherokee is louder than a jet airplane I really need to quiet her down. I've replaced the muffler already. I've got a friend that thinks the gasket in my exhaust manifold might be bad or missing. Any other info or common problems on a 1990 Cherokee 4.0 would be great though. Its really starting to get embarassing. Any help would be appreciated.

All pre 2000 4 liter Jeeps were NOTORIOUS for cracked exhaust manifolds.
The only remedy is to replace the manifold.
If you are lucky its just the gasket, but the crack is more common.

Its such a serious problem there is a class action lawsuit on this. Check these links:

http://www.gtelaw.com/jeepclassaction/jeepclassactioninformation.html

http://www.pixelplayhouse.net/clientforms/gtelaw/classaction.html

My question is though for my 1990 do you still think its a cracked manifold or is that just for 91-99? Any info would be helpful. Also I looked up around the valve cover today and I saw that around the exhaust manifold the gasket is kind of deteriorating,I'm thinking that may be some of the problem but still any info back would be helpful

Yup, sounds like something sprung a leak around the exhaust manifold area.

Do yourself a favor, check your motor mounts. If they are at all sagging, cracked or deformed, this puts extreme pressure on the exhaust manifold as the engine torques on acceleration and deceleration, resulting in either a cracked manifold or a damaged seal between the manifold and the exhaust pipe (or both). Been there, done that, got the T-shirt.

To prevent it happening again, replace the POS OEM type mounts with more substantial aftermarket mounts, such as M.O.R.E. torsion rubber "bullet-proof" mounts - not cheap but well worth the investment. I did it and I am very happy with the results.

My question is though for my 1990 do you still think its a cracked manifold or is that just for 91-99? Any info would be helpful. Also I looked up around the valve cover today and I saw that around the exhaust manifold the gasket is kind of deteriorating,I'm thinking that may be some of the problem but still any info back would be helpful

All the 4 litre Jeeps are notorious for cracks. The lawsuit had to draw the line somewhere, but the pre 1991 Jeeps were just as bad.

Got a question guys can anyone tell me what my exhaust manifold is made of? I assume steel but dont know. Thanks a lot

Thanks for all your help guys but yeah unfortunately my exhaust manifold is cracked real bad. Atleast I pinpointed the problem though. Thanks again for all your help.