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Discussion Starter #1
I mainly get this error at idle (which makes me think vacuum leak), but I've swapped the plug, coil, and injector from another cylinder. Still getting the error on 4.

Anybody have any luck with this code?
 

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Maybe you've done some online research but if not here's a thread: Error Code P0304: Cylinder 4 Four Misfire Detected

So it could be a lot of things. Switching the plug and the coil was clever, but if it was me I'd just install new plugs. In this day of 100,000 miles plugs, many people don't realize that the Patriot plugs are only good for 30,000 miles.

While you may not have as good luck as I've had, the coil packs in my 2008 went to 275,000 miles without failing. I sold it to a friend last year and he hasn't mentioned any problems.
 

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Discussion Starter #3
My plugs aren't that old. And if it was the plug, I assume the error would have moved to the cylinder I swapped. Unless during the swap the good plug went bad too which is highly unlikely. Same goes for the coils.

I took my throttle body off today and cleaned it. I noticed oil in the intake manifold... I know a little is normal, but I didn't like it. I might replace my PCV valve while I'm at it.

Valves have carbon deposits, esp. on cylinder 4. I might get some seafoam to clear that up. While I was under the hood, I thought I heard a hiss coming from the throttle body. Will replace the gasket and see what happens but it's just weird that a vacuum leak would cause ONLY cylinder 4 to misfire at idle.
 

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Discussion Starter #5
I did. Gaps are fine. If 4 was bad, then the cylinder I swapped it to SHOULD be getting the misfire now, but it's not. I'm scratching my head.
 

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Hmm, this makes me fear it could be something mechanical. Bad valve? Gasket? Do your oil & water look OK? You didn't mention oil fouling on the plug so I'm guessing there wasn't any. I never had any engine problems with my '08 but yours is an '07. It is getting elderly but I applaud anyone who keeps an older vehicle running well.
 

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Hmm, this makes me fear it could be something mechanical. Bad valve? Gasket? Do your oil & water look OK? You didn't mention oil fouling on the plug so I'm guessing there wasn't any. I never had any engine problems with my '08 but yours is an '07. It is getting elderly but I applaud anyone who keeps an older vehicle running well.
Thanks. I do what I can. I'm just a DIY mechanic that's learned a lot over the years.

So after cleaning the throttle body, installing a new throttle body gasket and intake manifold gasket, changing the plugs (which did not have oil fouling), and running sea foam through the intake, I think I fixed it... Time will tell. After inspecting my intake manifold gasket, it was brittle and I suspect this was the problem. We'll see.
 

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Thanks. I do what I can. I'm just a DIY mechanic that's learned a lot over the years.

So after cleaning the throttle body, installing a new throttle body gasket and intake manifold gasket, changing the plugs (which did not have oil fouling), and running sea foam through the intake, I think I fixed it... Time will tell. After inspecting my intake manifold gasket, it was brittle and I suspect this was the problem. We'll see.
That bad intake gasket could mean the engine wasn't getting the air/fuel mix it was asking for.

You are a more adventurous mechanic than I am. Jump starts, filters, changing tires, oil, and spark plugs are about all I do anymore. I did a little more in the old days.
 
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