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DIY: Do It Yourself
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Old 07-29-2015, 09:58 AM   #1
Registered User
Join Date: Jun 2015
Location: Detroit
Posts: 10
My Ride: 2000 328i
Misfire troubleshooting help--coils next?

Hi all,

First, I want to thank this community. I just bought my first BMW about a month ago. It's a 2000 328i with about 194k on it, and joined this forum that week. My car is in great shape especially considering its age, and I really love driving it, and have enjoyed working on fixing the few lingering issues I've been trying to track down. I've relied on this forum constantly--searching for data on my problems, trying the fixes proposed, etc. I know generally these sorts of questions get a "Just search more, noob" response, and I'm certainly not an expert at site searching. I really have tried to research as well as I can, but I'd absolutely be glad to have links to other threads that might help. Again, you folks are just great. I'm really glad to be here and to know there's a group of folks out there helping each other with these questions. I'm pretty confused by the circumstances I'm experiencing so I'm hoping folks can help direct me.

Some history (sorry for the long post, want to be thorough):

Before I bought the car, I took it to a local indy German specialist mechanic for a pre-purchase inspection, where they diagnosed a lean condition likely caused by a vacuum leak.

I also had a rough cold idle which based on my understanding of the codes I was seeing and all the info this community provided, seemed to point to a vacuum leak. I read several threads, inspected for a vacuum leak, and ended up replacing the lower intake boot using instructions from this forum.

That made a major improvement in the idle, but after replacing, I'd still get occasional misfire codes--generally a 300, a 0302 and 0305, plus occasionally other codes in the 300-306 range. I never got another type of code after replacing the intake boot--only 300-306 range errors have come up since I made that switch. And 302 and 305 were the most common. I took it to a mechanic for a smoke test at that point but they were unable to find any vacuum leaks or indications of poor vacuum. So it appears I'm leak free, or at least leak-that-shows-up-in-professional-smoketests free. At that time the mechanic noted that there were misfires in 1 and 6, so they moved those coils to 2 and 5 to see if that changed things.

When I bought the car I also knew the VCG had to be replaced, and that that could ALSO cause misfire codes due to oil seeping into the plug wells and shorting the connection to the wire. So I replaced the VCG, plugs, and plug boots this weekend. Now the car runs very very well most of the time.

But I'm still occasionally getting misfires, in pretty odd circumstances.

My car starts, drives and idles well in the cold, and every morning it starts perfectly. But I tend to get misfires right after getting off the freeway. I'll either get off the freeway on the way in to work and once I'm slowed down and on surface streets, I'll get a CEL, or on the way home i'll get one on the surface streets. I've very rarely gotten one while on the actual freeway or going at freeway speeds. It also tends to be more severe when it's hotter and more humid out.

Also, since the VCG, I've almost always gotten 2 codes, 0302 and 0305, which I generally see while they're pending. I . I've been clearing these right away (I read another thread that suggested that so that you know what issues crop up first) and keeping a log. I've just got a cheapo code scanner at the moment, but I ordered a wifi/iphone one so hopefully I'll get that in the mail soon.

I have also on two occasions gotten other codes, a pending 306 and pending 301. That was right after the VCG, though, so I think it might have been caused by leftover oil in the plug holes.

The question I have is--I know from researching the forums and past threads that the coils are rarely the actual culprit for misfires, that the culprit is generally something else. But in most of the cases I've read about, there are either a variety of codes happening or the issue is when the engine is cold, as opposed to when it's hot.

Does the fact that my codes have all been 302 and 305 lately suggest that those coils may have been damaged by oil seepage from before I did the VCG, back when they were 301 and 306, before the mechanic swapped them? Is there any indication from the fact that the misfires start as 302 and 305 which could tell me what else to check? Do misfires in certain cyllinders suggest certain issues?

I'm basically curious if I should bother starting to replace coils when it seems from the searches I've done that they're rarely the culprit. If they're not, and if I'm not getting any other codes and have not been able to find the issue with a smoketest, what's next?

Thanks in advance for any insight,
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Old 07-29-2015, 02:13 PM   #2
Registered User
Join Date: Apr 2015
Location: Denver
Posts: 81
My Ride: 323ci
Tom, very good description. This is very helpful to understand the problem..

What I would do is swap again coils 2 and 5 to position 3 and 4. If the codes move you found your culprit. I assume that you cleaned the spark plugs/coils free of oil when you redid the VCG, so that should not be a problem.
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Old 12-21-2015, 05:51 PM   #3
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Join Date: Mar 2015
Posts: 17
Any update on your issues? I also have vacuum leak related lean code and cylinder 5 misfire code not triggered on startup but at cruising speed
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