Hello,

I have a 2008 H3x, with some issues. Rough Idle, Stalls on start up somtimes.

The Hummer has 75400 miles on it and just started to throw the P0304 code.

So I changed all the plugs with new ACDELCO Iridium and then I replaced the Coil on Cycl 4 with a new ACDELCO as well. I also cleaned the TB and MAF.

Now the Hummer is throwing a P0300 and P0304 code.

I read all the posts that i could fine with these issues and did the advice there and not sure where to go from here. New Injectors? New Fuel Pump?

Any advice would be great.

 

Here is the list for your symptoms, not the code(s). Run through that entire list, then sea where you are.
https://www.hummerforums.com/forum/h...what-do-33916/

For the random misfire and #4 misfire, check the harness wiring to coil #4 for any cracks, frayed insulation etc. Make sure all the coil pack harness connectors are clean and snapped together correctly. Never hurts to clean those with CRC electrical contact cleaner either. Make sure you got the spark plug boots on straight and all the way before you tightened the coil retaining bolt. Sounds silly, but you would be amazed how many people mess up that simple spark plug connection.

 

Doc is on top of this one. Also check the compression on cylinder #4. You need 4 basic things for proper cylinder fire. Air, fuel, spark and compression. take away one of these and you have a misfire. Take the coil and plug to cylinder #1 and see if the misfire changes to #1. You can also do the same for the injector. Don't do all three at once because you won't eliminate the faulty part, unless it's a wiring issue to the coil or injector or mechanical failure (bad valve?). Something I can't do, but if your good you can feel an injector firing. Probably not your fuel pump but always a consideration. You can do a pressure and volume check on it with the right tools.