My 94 gmc jimmy exhaust leaves a black mark on the ground?

I own a shop, and a 94 S-10 Blazer with a 4.3 V-6 engine. The first thing I would take a look at would be the temperature sending (sensor) unit. It may be bad, and this rings true if the engine runs worse when it warms up. The other thing I would do is replace the O-2 sensor. You could have an injector that is sticking an not closing all the way off. Add a bottle of STP (black bottle only) to about 3/4 of a tank of fuel, and this should help. Mine has done the same thing for a long time, and the injector cleaner has worked for me. I know its the central port injection, but it still has injectors, they just work different than a direct injector does. Do you smell fuel at times? If so, then you may have a leaking pressure regulator. The black mark on the ground certainly suggests there is far too much fuel getting to the engine than it should have. If the O-2 sensor is bad it will do this. A 94 is an OBD-1 computer system, and does not give codes or let you know if a sensor is bad sometimes. A couple of other things to look at is the MAP sensor may be either not getting vacuum to it, the hose is split where it plugs on, or there is not the right amount of vacuum on the hose for some reason. A bad throttle position sensor can make one sluggish, and run bad a times. These engines are really bad to have the EGR valve stick on them, and if its sticking this too will make it run real sluggish. I have had to replace the one on mine several times, but I have 250,000 miles on it, and I guess that would explain that. Check your air filter, if its plugged up, it will in fact make this exact condition.

Glad to help out, Good Luck!!!

if it is a vortec v6 it most likely has a bad fuel pressure regulator seeing alittle black smoke when you pop the throttle are ya it runs OK cold because the fuel enrichment isn't enough to cause a problem cold a cold engine needs more fuel anyway after its warm it will black smoke and run poorly.

you are burning oil, cold oil thick, warm oil thin. You probably have some bad piston rings. Cold it has compression and warm it doesn't because the thin oil is passing past the piston