I've been getting a p300 random cylinder misfire code the last few days. I changed all the sparkplugs and plug wires today and the right bank-cylinders 2,4,and 6 were all black whereas cylinders 1,3, and 5 were good? I had a no activity o2 sensor code too, but couldn't get that o2 sensor out yet. Can a bad upstream o2 sensor affect just the right bank of sparkplugs? Any help would be appreciated. Thx

 

Faulty O2 sensor will not cause one bank to foul plugs. There is only one injector used on 95 CMFI, and it is batch fired, not sequential. Check fuel pressure and leakdown. Key ON, engine OFF, fuel pump running: pressure must be 55psi to 61psi and must remain above 50psi for 10 minutes. Even numbered cylinders fuel fouling is usually caused by the fuel pressure regulator leaking. It can be confirmed with a leakdown test, or remove the plenum, turn the key to the run position and watch for a leak.

 

Fuel pressure regulator was the problem. Can take the vortec cover off and pull the round sensor to verify the regulator is leaking before pulling the plenum. Thx for the help