Well i thought i knew everything about my 97 zj 4.0.(about 170,000) Man was i wrong. Anyways, i noticed loss of power and a rich smell one day while driving. I gave it a tune up, still there. I brought it to get the code check and came up as 02 rich. We also noticed a winding sound coming from the intake manifol area and they told me it could be a manifold gasket problem or leaking vacuum hose, and my hoses were all good so i bought a new gasket and changed it. Problem still there. I changed the intake manifold thinking it may be cracked. The problem is still there! I cant get an inspection and the thing still drives and runs ok. Its driving me nuts. Anything else i can try cuz im ready to get a new motor next. Any help would be great. Thanx

with it running spray some carb cleaner or something similar around seals under hood. If you hear a change in idle that is where the leak is. It will suck in the spray and cause a change in idle.

Already tried that. Sounds like its coming from in the block. I can hear it near the intake manifold. Almost sounds like a fan running at the same rpm as the motor in the intake.

i seen the post of carb cleaner but have you tried a small propane tank with a small hose attached its dangerous but it zeros down the area where a leak would be likely?????? a bit better than carb cleaner?

A vacuum leak would cause a lean condition, not an over-rich one. It could be a problem with the injectors.

I have no cracks outside the motor pulling in air and no bad vac hoses. If its a fuel injector prob, could it be a map sensor? The injectors are all running.

Oh, and I have no gas smell. I get the sooty tail pipe and bad smelling exhaust. It runs pretty smooth when first started(with the smell), and when it warms up it revs almost like it has bad timing

I believe a bad map sensor would set a code.

Sounds like a cracked exhaust manifold to me they crack where the two goes to one at the flange. Its also makes the o2 see to much oxygen so it rich'ns the mix.

I have a header on it thats about a year and a half old with a brand new gasket. Im not getting any loud exhaust sound either.

Is it possible for one of or both the o2 sensors to make it run like this?

I just dont wanna replace every sensor in the thing to find out its an engine problem. Sensors get expensive.

Actually it is.
I wouldn't replace anything until I knew what is causing the issues. JMO
If I had a similar vehicle to use as a donor, I would try switching sensors and see what cured the problem.
Dan

Thanx Dan, Imma start with the pre cat o2.

At least you're trying to be analytical about it. That's good.