One cylinder not firing

[rdobirdman]

Hi Guys, I have a 1996 GMC Sierra 2500 that started running rough and giving misfire codes. It mostly says cyl 4 misfire or random multiple misfire. short Fuel trims are high, around 30 on the passenger side bank. Strangest thing that although it says cyl 4 misfire, cyl 2 seems to be not firing at all, I can put my hand on the manifold at cyl 2 when its running and it's barely warm, if I pull cyl 2 plug wire it makes no difference to the truck running. It has spark, compression is 150, I've changed a lot of parts to try and fix this.
New fuel injectors
New plugs and wires
New distributor
New ECM
New ignition coil
New EGR valve
New fuel filter
New O2 sensors
did a smoke test to check for vacuum leaks
Tested to see if injectors are getting power and they are

I'm pretty stumped

 

[Schurkey]

WHAT ENGINE???

Misfire can cause false "lean" signal from the O2 sensor(s). Not surprising that you have rich-command on the short-term fuel trim numbers.

Is the distributor installed within spec for "offset"?

Does Cylinder 4 actually misfire? What are the misfire counts for each cylinder?

 

[rdobirdman]

5.7 vortec engine. My scanner doesn't give misfire counts for each cylinder I need to take it somewhere to get an accurate reading. Not sure about distributor offset. I marked it's position at cyl 1 TDC before I swapped it so I think it's it's installed correctly. It starts up and seems like it's running great until it's in gear. It started doing this suddenly when the original distributor was still in it

 

[Schurkey]

Connect a real scan tool. Get the misfire counts, AFTER you assure the offset is within 2 degrees at ~1100 rpm.

The offset has to be right, to assure the computer knows which cylinder is misfiring.

 

[GoToGuy]

You changed all four O2 sensors right?

 

[Carlaisle]

Injector wiring harness and cap will round out the list of things that can cause a single cylinder misfire.

 

[L31MaxExpress]

Did you cross the injector tubes on 2 & 4 correctly?

 

[Erik the Awful]

Are you running the factory Vortec poppet injection, or the upgraded MPFI? If you upgraded to the MPFI I'd suspect that you have the #2 and #4 injectors crossed and the injector physically on the #2 cylinder isn't firing, either because the plug is bad or the injector is bad.

Injectors run on duty cycle, so having their leads crossed won't affect the engine's operation. Ignition is timed, so it can't be an ignition problem.

Edit: The more I think about this the more questions I have. The computer is sensing an rpm drop on the #4 ignition event. How is it misfiring on #2 but seeing #4?

 

[L31MaxExpress]

Are you running the factory Vortec poppet injection, or the upgraded MPFI? If you upgraded to the MPFI I'd suspect that you have the #2 and #4 injectors crossed and the injector physically on the #2 cylinder isn't firing, either because the plug is bad or the injector is bad.

Injectors run on duty cycle, so having their leads crossed won't affect the engine's operation. Ignition is timed, so it can't be an ignition problem.

Edit: The more I think about this the more questions I have. The computer is sensing an rpm drop on the #4 ignition event. How is it misfiring on #2 but seeing #4?

Actually having the injectors out of order WILL effect the running. The injection is sequential and if the injector fires at the wrong time it will cause a misfire.

 

[rdobirdman]

I replaced the two upstream 02 sensors because they are the ones that affect the ECM unless I'm wrong. I have the upgraded MPFI and I was very careful to install it correctly, the problem is exactly the same after the new injectors were installed, the previous ones were also the upgraded ones. I unplugged the big connector on the spider and tested it with a light, all the injectors were getting 12V when the engine is cranking