1999 gmc jimmy 4x4. i had a engine code saying to change my 02 sensors. so i went safe and replaced all 3 at once ( disclaimer... I bought cheap sensors online) few days after i changed them out, jimmy started running rough and when driving, i noticed large plumes of black smoke from the exhaust when i speed up after stopping at traffic lights. engine sounds very bad now. when idling at lights, engine starts shaking and stuttering. i also had a full tank of gas. only drove like 30 miles over the course of 3 days and now my gauge is showing almost empty. it is unbelievable how much gas it has burnt. when i rev engine it acts as if it is starving for fuel. also when reving engine i can hear a really deep rumble in the exhaust and it sounds as if it is about to backfire. i took it to advance auto today on way home and got new trouble codes. i had 3 codes saying P0300 random misfire. so they recomended a new distributor cap and rotator. well, that didnt fix it. still just as bad as before. can anyone help or is it time to go to the shop?

 

You could have a massive fuel leak from the fuel injectors under the upper intake manifold. In my old 92, I had a large fuel leak that caused the O2 sensor codes. It also coated them with black carbon deposits. That you also have having black smoke come out of the tail-pipe could indicate a very over-rich mixture, i.e. gas flooding the cylinders. I could also see how a misfire could arise if you have lots of carbon build up in the cylinders causing the spark plugs not to work properly. Your large fuel consumption could easily be related to this too.

If you take off the upper intake manifold and see that there are areas of the top of the lower intake manifold are clean, that is bad. That likely would indicate that there has been gasoline leaking directly onto the surface dissolving the years of varnish.

With our year Blazers/Jimmys, we can install a later version of the fuel injection "spider" that is supposed to work better. If only replacing that part, it is not very hard to do as all of the work is on top of the engine and only a few things have to be moved out of the way. I replaced my fuel injection spider when I also replaced the lower intake manifold (LIM) gaskets. I replaced the LIM gaskets when I learned how they could catastrophically fail and dump raw coolant into the lifter-valley ruining the main bearings and thus totaling the motor.

 

Wouldn't I smell gasoline? Seems strange that the engine ran perfectly until I changed the 02 sensors.

 

You might have had the O2 sensors originally go bad because of the over-rich condition. With continued driving after the O2 replacement, there could have been further degradation of the fuel injectors causing worse symptoms.

You could recheck the O2 sensors that you installed and see if they are connected correctly. But if you remove them and they are covered with black soot, you are getting too much gas into the cylinders somehow.

As for smelling gas, yes and no. You would have to open the butterfly valve on the air intake and smell there.

You could try installing new, good quality O2 sensors to see if the problem was with the cheap ones you installed.

I would suggest you keep on the list of culprits that the fuel injectors are bad. But if not them, it sure sounds to me that you are getting way too much fuel into the cylinders perhaps by bad inputs to the engine computer, e.g. from bad O2 sensors.

 

I pulled the o2 sensors again and switched them out, no change, also, the o2 sensors were covered in soot or charcoal looking residue. I also tested the air I take temp sensor near the air filter, no change .

 

What could have been going on is that the O2 sensors giving errors were an early indication of them getting fouled buy an over-rich fuel/air mixture and that the issue has only gotten worse. In other words, the O2 sensor issues are not the source of the trouble but rather only a symptom of the real issue.

Perhaps someone else can give another possible diagnosis but at the moment, my money is on your fuel-injection spider having gone bad.

BTW, if you check the oil and it smells like gasoline, then it definitely is a gas leak. A possible issue with such a large leak is that it would clean off oil on the cylinder walls increasing wear between the piston rings and the cylinder. However, I can imagine you still having a bad leak from the fuel injectors with no or little smell of gas in the oil.

Taking off the upper intake manifold to replace the injectors is not very hard to do and would provide evidence of a bad leak in the form of the interior surfaces of the lower intake manifold being very clean from being washed by raw fuel.