i have a 2000 xj 4.0 front 3 cylinders spark plugs are black with dry soot back 3 are white with little light brown any ideas what could cause this

 

I'm just guessing here soo someone else please correct me if I'm not making sense...

Lean/rich fuel thanks to your o2 sensors. My guess is one of them is either running rich or lean. Your 2000 should have the split exhaust manifold with x2 o2 sensors in the down tubes.

Do you have any check engine codes or is your exhaust manifold cracked around any of the welds?

 

You have not really provided enough information (such as any symptoms) to even guess

However I would check the OBD2 codes and report back

 

codes are 171 174 running lean bank 1 and 2 but it is actually running rich i have replaced rear exaust manifold and gasket all o2 sensors maf tps temp iac throttle body gasket and a lot of other parts if you clear the codes it will run good for about 5 miles when the light comes back it runs rough at idle and you can fumes i also replaced fuel pump spark plugs injectors coil pack

 

intake manifold/gasket ?

vacuum leaks ?

Your OBD2 tool should be able to display live data such things as vacuum and timing advance them, along with STFT & O2, check them

If you are using the key trick, spend $30 for an OBD2 tool, pay for itself x over 1st time

I would also buy and engine vacuum gauge, old skool way of telling what is wrong with your engine, maybe $30 or less

You should be able to accurately diagnose the engine fault with $60 worth of tools for life, save a huge amount of time and money, dont throw away any of the parts you took off, bag and tag them

 

replaced intake/exaust gasket had it smoke tested fixed all leaks still get same codes no matter what i do