SOLVED! Intake vacuum

Ilikemy3s

Active Member
I have a 4.2l 05 trailblazer. Got a P0304 error while driving the other day. Truck ran rough while pulling out but not to bad on highway home. The CEL alternated between off, on and flashing I figured since the highway was clear and no traffic and the truck was running better on the highway, I drive the 40 miles home Once home, I disconnected the battery after confirming the code with a scanner Changed the coil pack and now I get a P0300. Truck still runs rough but a bit smoother. Did some searching and read it should be a vacumm leak. I ma pretty sure i did not undo any vacuum lines but wanted to be sure. Are they any going to the air box i may have missed.
 

Mooseman

Master Blaster
Moderator
It could be a vacuum leak, could also be a bad injector, bad coil or injector wiring, bad compression on that cylinder. And P0300 usually happens until it decides which cylinder is missing and sets a code, like P0304.

You have to do proper troubleshooting before assuming one issue. Are you sure the coil you put in was good/new? The usual initial troubleshooting step is to swap coils between the one that is missing and another cylinder and recheck. This confirms if it's a coil issue or something else.
 

Mooseman

Master Blaster
Moderator
If you want a good read, this thread relating to my own misfire issue goes through a lot of the troubleshooting steps.
Very Slight Miss at Idle

Mind you, my situation is slightly different but the troubleshooting steps for misfires are the same.
 

TequilaWarrior

Active Member
I have a 4.2l 05 trailblazer. Got a P0304 error while driving the other day. Truck ran rough while pulling out but not to bad on highway home. The CEL alternated between off, on and flashing I figured since the highway was clear and no traffic and the truck was running better on the highway, I drive the 40 miles home Once home, I disconnected the battery after confirming the code with a scanner Changed the coil pack and now I get a P0300. Truck still runs rough but a bit smoother. Did some searching and read it should be a vacumm leak. I ma pretty sure i did not undo any vacuum lines but wanted to be sure. Are they any going to the air box i may have missed.

Airbox? If you mean the box that the air filter resides in: No. If you mean the intake resonator that sits on top of the engine: Yes. There is at least a tube that goes from the bottom of it to the top of the valve cover. I believe on some there is a line that goes from the right side of the resonator to the fuel pressure regulator. Mine doesn't have that, so forgive me if I'm wrong on routing/positioning. Also, make sure that you're MAF harness is plugged in (if equipped with MAF) or that your IAT harness is plugged in (if equipped). I'm unclear on what years have which.

This seems to be a fairly common occurence on the forums lately. Start over with your troubleshooting. Remove intake tube & resonator so you can easily access the coils. Start the engine and unplug each coil - one at a time - and note any changes in idle. If you unplug a coil and the idle ~does not change~, that's your misfiring cylinder. You may have more than one. I've had 3 unplugged at the same time (for troubleshooting purposes and to see how bad the idle would get) and the engine kept running - poorly, of course. Once you've identified the cylinder(s) that misfire, it's time to move some coils around. Turn the engine off and move a misfiring cylinder's coil to a good cylinder and vice versa until all misfiring cylinder's coils have been swapped into good cylinders and good coils swapped onto misfiring cylinders. Start the engine and unplug each one, one at a time, all over again. If the misfire(s) stayed with the coil(s), the coil(s) is(are) bad. If the misfire stayed with the cylinder it's time to check plugs/injectors/compression, starting with the plug. Replace with new, only ACDelco 41-103 and do not gap it. Genuine 41-103's are pregapped at the factory. If you get to the point where changing the plug(s) doesn't fix the problem, we'll have to troubleshoot injector(s) and compression. Someone else will certainly have to assist as I have not had to go that far... yet.
 

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