What's up guys, my name's Lee and today I'm going to be starting a new thread based around the wonderful 4.2L's infamous misfiring issue "P0300--". I know the majority of us have all seen the discussions regarding this one particular issue before so I've decided to post my own input on the matter and throw a little insight on this out there to you all.
By no means am I a mechanic, so don't take my word on this to heart and please feel free to touch-up on a few things that I may say! I'm a U.S. Marine currently serving in the infantry. Since being stationed here on Camp Lejeune I've had my 2006 Trailblazer LT for the last four years. The misfire issue came up right as I got back from my previous deployment. Here's what happened and what I noticed..
The misfire code trips the check engine light. I can feel a slight to moderate vibration in the steering wheel as well as the rest of the truck as I sit tight at a stop-light waiting to go. I get the green light and progress down the road and decide to roll on over to our shop and get the code ran. P0300 series showed up as soon as I hooked our reader up, it said multiple cylinder misfire. I quickly began to investigate further and attempted to narrow my issue down. I was furious about the shaking. I felt like I was driving a semi down the road not a mid sized SUV. I went on a rage and purchased everything I thought it could possibly be, I would not advise doing this for all you readers out there. I bought a brand new starter, alternator, NGK laser iridium spark plugs, six brand new coil packs, a thermostat and two engine mounts. All of this was installed. It ran okay for all of one week and then began to misfire once more. I then replaced the intake solinoid. Nothing. The check engine light disappeared but the misfire still persisted...
I threw in the white towel after that guys. I took a trip to the dealership and paid for a compression test to be done. They said that starting from cylinders 1 through 6 the compression ratio was 90% 90% 65% 90% 65% 90%.. Cylinders three and five were firing low.. They stuck a scope down into the upper cylinder head and took a look at the combustion chamber. What they saw blew me away..
Now mind you, I don't drive my TB like a bat out of hell even though I did install a CAI and had a retune by PCm4less. I don't push it hard and I accept the fact (unlike others) that I drive an LT, not an SS. Moving on. They saw a ridiculous amount of carbon built up, and it had caused my valves to "stick" almost due to the build up. I purchased a brand new head gasket, and pretty much the entire upper cylinder rebuild kit. I'll be taking the upper cylinder head completely out, taking off the cams and sending it into a machine shop here in town to be cleaned and restored. While this is happening my friend and I will be working with the combustion chamber to clean out any gunk we can find all along keeping the combustion chamber covered and lubricated. We'll see what happens.
The guys at the machine shop know the 4.2L all too well. They told me they get about three to four a week for the same issue. I don't know if it's an issue with the seating or how the engine was built, but I do know that once we're done with the rebuild, all cylinders should be firing at 140% individually.
If any of you have any tips about this, share some word! I take each oppertunity as a learning experience and am open for input from others. Bottom line, once we get rid of the carbon, the cylinders don't misfire, there is no more shake. I'll be posting pictures as all this happens. Thanks guys. -Lee