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I have a 2009 gl 450 (4.6 272 engine, i think) that now has 170,000. I recently replaced had an engine misfire in 2 and 5 and some other codes related to the fuel being too rich/lean. I began by replacing the coil pack for what I thought was cylinder 2, was actually 1 and then switched it about a week later. Disconnected the battery to reset the computer and it did not clear check engine light. I waited about a week and decided to replace all the spark plugs and the coil pack in 5 also.

I found the proper diagram for the engine and replaced spark plugs in this order: 4,3,2,1,8,7,6,and then 5-but that's where there was a problem- I didnt look into the cylinder before putting the socket in, and when I pulled the plug out, I had oil on my socket and the threads and bottom of the plug. Initially, I was worried that cylinder had oil in it, now think that it was a leak from the valve cover that I didn't notice, since the socket had oil on it and couldn't possibly have oil unless it was already in the well....But, that oil got into the piston.

When I put the new spark plug in and coil, still had a misfire- was hoping that the oil would burn out quickly. That not being the case, I sprayed break cleaner into the cylinder and let it evaporate out to remove the oil. The car runs better, but still had a rumble to it. Sounded like it was coming from back of the engine- 5 is from of engine driver's side. So now I checked code and still getting 5 and fuel/vacuum issue from being too lean. I checked air filter and replaced. Then, replaced Mass Air Flow Sensor. Reset code- still getting 5.

I suppose a compression test and valve test is next? I did use NGK Iridium Plugs pre-gaped, but checked them at .32 as well. Any other thoughts? Obviously still need to have valve cover gasket replaced. But any other help would be greatly appreciated. Clearly I am not a mechanic, but like to resolve matters if possible myself.:smile
 

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2007 GL450
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I have been having the same issue. Replaced the intake manifold, all spark plugs, coils, O2 censers. What codes are you getting. I have a P2004 and multiple misfire. I am at the same mind as you it must be one of the two. The prior owners did no upkeep on the car at all. I was told by the dealership that it was the intake manifold due to the runners not moving. I ordered a new one. When we pulled it out nothing was broken it was so filthy and gunked up that the grime was literally keeping them from moving. You might give a look at the intake manifold to make sure nothing is broken as well as the fuse.
 

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I have been having the same issue. Replaced the intake manifold, all spark plugs, coils, O2 censers. What codes are you getting. I have a P2004 and multiple misfire. I am at the same mind as you it must be one of the two. The prior owners did no upkeep on the car at all. I was told by the dealership that it was the intake manifold due to the runners not moving. I ordered a new one. When we pulled it out nothing was broken it was so filthy and gunked up that the grime was literally keeping them from moving. You might give a look at the intake manifold to make sure nothing is broken as well as the fuse.
I'm getting P0305 and P0300 errors. swapped out ignition, no help. Swapped out #5 injector, no help.

Getting some help from fuel injector cleaners (PEA stuff, mixed in gas) but it's not cured. I was having a hard starting problem, which has gotten better. Will probably try some more, and might swap out all the injectors. Those misfire problems are a bitch.

You guys know if there is a way to pull the left side upstream oxy sensor without taking out the exhaust?
 

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Have you tried replacing the fuse. That did work for me. I still had the manifold error as it was prior to me swapping out the manifold. It could solve your problem it's on the passenger side door.
 

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In trying to troubleshoot shoot my miss firing issues. Since they are all on the driver's side we decided to take two coil packs and swap them out to the other side then take two coil and spark plugs and swap them out to the other side to see what was the problem. The errors came back one the driver's side. Any ideas? I cleaned the MAF and it looks fine.
 

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Fixed. It turned out to be the ignition coil after all. That was the first thing I tried, but maybe you have to clear codes before confirming. I figured it would self reset, but maybe not.
 

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Fixed. It turned out to be the ignition coil after all. That was the first thing I tried, but maybe you have to clear codes before confirming. I figured it would self reset, but maybe not.[/QUOTE

I replaced all the coils at the start of my project. I have also replaced all four magnets, sencers, rebreather and cap blugs as part of maintenance.
 
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