My 4.0L 1994 Jeep Cherokee failed NJ emissions inspection -- they won't test if visible smoke is present -- it started to sometimes blow dark grey or whitish smoke out of the tail pipe -- only when it has gotten hot but it comes and goes -- today it was worse. It also starts very rough and feels like it has a high idle -- though it reads just below 1000 RPM's like .900 on the dial. It also has a lot of vibration when it is in gear and the brakes are being held down. The engine mounts are good and the muffler, all the brakes are brand new. the cherokee has 110,000 miles on it. I checked and I have some oil blow-by -- oil in my oil filter. I also cleaned out the throttle body and took off the ICA it was really mucky with black goo. I didn't take the throttle body off so I don't think I cleaned the place it goes into well enough. I am going to take it off and do better job cleaning it -- I also got a used throttle body with the ICA and the TPS & motor (whole thing) from a running 1990 (it was really cheap) I though maybe I could try it out. I also see some small cracking in one of the molded elbow in the crankcase ventilation {CCV} plastic tube but I don't hear any vacuum leaks. I am off tomorrow and plan to replace the hoses as seen in this web article. I also plan to take off and really clean the throttle body. Any advice or thoughts are greatly appreciated!

Well,   a 90 has a different computer system and a completely different throttle body than a 94 so that spare will do you no good.

Is it actual smoke or steam?

Black smoke is excessive fuel, blueish smoke is oil burning.

Oil in the air filter is commonly caused by the CCV line being plugged with carbon.  The only real way to fix it is to replace it since it gets so caked in there.  The CCV hose is a vacuum line that runs to the rear of the valve cover.

Andy

WOW -- thanks for the info. I did change the spark plugs and wires -- the plugs were light grey and all plugs matched. I also changed a few vacuum hoses. Today, I removed 2 quarts of oil and added 2 quarts of Lucas Oil Stabilizer -- AND JEEPERS -- it stopped ALL the smoke!!! I also added a little bottle of Napa Brand " stop smoke" -- it no longer smokes at all and the oil pressure is still the same as always -- I am now trying to get it inspected again. It still starts very rough and feels like it has a high idle -- though it reads just below 1000 RPM's like .900 on the dial. It also has a lot of vibration when it is in gear and the brakes are being held down. The engine mounts are good and the muffler, all the brakes are new. This cherokee has 110,000 miles on it. I am thinking of pulling the new plugs back out and seeing if any have anything on any one of them - carbon etc. -- I am also thinking of undoing the wires one at a time and seeing if any are not working. I also still want to pull off the throttle body and really clean the area where the ICA goes in. Any other ideas? I have a mechanic who feels that my oil burning can be fixed with a $400 valve seal job. He seems 90% sure that this will fix the smoke issue (though with the additives -- it is no longer there anyway ) -- I am not so sure about this -- I also will wait till the smoke returns -- thrifty as I am -- but is it ok to have 2 quarts of Lucas Oil Stabilizer and a small NAPA stop smoke in my Jeep?? Oh and any idea where I can get a cheap shop manual? Is it made by Jeep?

Watch ebay for a real chrysler FSM (factory service manual) for your year.  Expect to pay as much as $50 or more for one but it is worth it.  They are a few inches thick and very in depth.

Chiltons, haynes, ect.. are all almost worthless.

Andy