Hello i just put a new/used engine in 96 jimmy. engine is 98 4.3 W vin. starts good but when it warms up it chuggs between 30 and 55 mph goin at steady speed when i step on it runs fine, when i floor it black smoke comes out. the tailpipe is all full of blk soot.before i swapped engines my old engine did the same thing until i spun a bearing. i did a fuel pres test it was gd. I do smell gas sometimes from fuel tank area cld be related??? I looked in throttle body i dont see any clean spots. I cleaned egr valve.new plugs wires,cap and rotor. can someone point me in a direction please .

 

The black smoke means its running rich, do you have acess to a scan tool that you can check your o2 sensors to make sure there not getting lazy? That would be the first thing I would check since your previous engine did the same thing.

 

where are the o2 sensors? how do I check them? Thanks

 

There are 3 o2 sensors. 1 after the cat that only monitors the cat efficenty, and 2 are upstream that help control the air/fuel mixture. If you look at each of the exhaust manifolds and follow them down you will find them. Get underneath the passenger side and look up and follow the pipes. They look kinda like spark plugs with a electrical connection.
The only real way to check them is with a capable OBD2 scan gauge that can diplay the o2 sensor readings.

 

ill have to get a scanner. ill check it out tomorro.