I have a 2000 GMC Jimmy, 4.3 L v-6, that is giving me a serious headache. I was getting a P0304 Misfire #4 Cylinder trouble code, and the check engine light (CEL) would flash after about 1 minute of drive time. Only flashed under load. I could sit at idle or half throttle sitting still and not have the flash. Being a specific cylinder I didn’t figure it would be a vacuum leak unless it was right next to that cylinder. Also the fuel trim on bank 2 seemed high, at least compared to bank 1. LTFT B2: 17.5 vs Ave LTFT B1: 8.9.
I tried releasing propane all around the intake plenum and heads with no change in idle. Blew cigar smoke into a vacuum line and saw smoke (very little) coming out near where the throttle assembly is attached to throttle body. I’m guessing a bad o-ring around the shaft but I doubt that would cause a specific cylinder to misfire. I also changed the plugs, wires, cap and rotor. The solenoids can be hear clicking with a stethoscope. I checked the solenoid resistance and all are within specks and I checked operation of the solenoids with a test light ( I don’t have a noid light)
To make a long story short I replaced the intake manifold (had to, I broke it) and cleaned the injectors and assembly. I made a bench cleaning system with an old fuel pump in a can of B-12 solvent and some switches to activate the pump and solenoid. When I first ran my makeshift cleaner I noticed that the #4 injector seemed to be obstructed. Eventually I got all injectors “cleaned” or at least they looked like they were flowing better and about the same. I didn’t have a graduated cylinder to do a volume test so I used an old Everclear bottle and ran solvent through each injector for a minute, then marked the fluid level on a piece of paper behind it. Each injector measured out within about 1/8 to 1/4 of an inch.
So I put everything back together, including a new fuel pump and this thing still runs like crap. Now I get a P0300 random misfire instead of P0304. The exhaust doesn’t smell as bad at least. But my Short Term Fuel Trim on bank 2 is now pegged at 50%. I also noticed that O2 sensor 3 bank 2 has no voltage. I think this is the one after the cat, please correct me if I’m wrong. Prior to doing the work sensor 3 seemed to be working voltage wise but the fuel trim was always “NA” so I assumed sensor 3 just reports data and doesn’t actually change how it runs. I also did the propane trick to look for vacuum leaks and found none. I have an ODB2 scantool, does anyone have the normal values I should be seeing on the tool? I have looked all over the web but have only found bits and pieces. Sorry for the long post. Any help would be greatly appreciated.

 

Im not sure what to tell ya but im interested in seeing your injector cleaning tool method

 

I have attached a couple of images that may have the values you're looking for (specs are for a '98 Blazer, VIN "W", RPO L35...I don't think they're different from your truck). Also, I did some looking through the TSM because you issue intrigued me. There were a couple things that caught my eye yo may want to look into...one is the EGR Valve and/or the EGR tube (Note: take a close look at the wires on the EGR valve).....the other was a bad catalytic converter. Both of which could

I could not find a bank 2 sensor 3 for this vehicle (once again my '98). There is the bank 1 sensor 3, which is the sensor to the rear of the catalytic converter in the exhaust pipe.

P.S. I would also be interested in seeing your bench tool.

 

Thank you for your replies. I will post pictures of my injector cleaner soon. rockp2 I think you are right about bank 1 sensor 3. I was going from memory which in my case is entirely fallible. I will check the EGR although I'm not sure if that could lead to a specific cylinder misfiring.