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Old 05-08-2019, 02:54 PM   #1
Cwazywazy
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Default Bad misfire in boost..

Over 5-10psi it feels like it's running on 3 cylinders and sometimes it backfires violently.. When it happens I smell raw gas so I'm pretty positive it's an ignition problem. Sometimes the problem goes away for a little while before coming back. I have:

Replaced coil with OEM Bosch
Cleaned and gapped plugs to .030 (I was running about .035 before, thought maybe that was it)
Compression tested (150-155psi on all 4)
Cleaned a bunch of connectors
Cleaned the hall sensor/cap/rotor
Tried a spare MAF

Pretty much out of ideas.. The problem started happening awhile ago on a cold engine but it'd go away after 5-10 minutes of driving. Now it's all the time.



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Old 05-08-2019, 03:06 PM   #2
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Auto? Do a power brake and see if it breaks up.
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Old 05-08-2019, 03:13 PM   #3
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Auto? Do a power brake and see if it breaks up.
M46, but if it was auto what would that tell me?
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Old 05-08-2019, 05:27 PM   #4
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How much total boost? What plugs? I used NGK 8's gapped about .025 to fix spark blow out with lots of boost and tiny 15g turbo.
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Old 05-08-2019, 07:08 PM   #5
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Spark plug wire quality?
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Old 05-08-2019, 07:12 PM   #6
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Plug wire specs are 500 ohms per foot. I would try going to a .028 or even the above mentioned .025 gap to help this if the wires and cap/rotor are good.
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Old 05-08-2019, 07:42 PM   #7
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Wires are almost new ngks, tested fine. 300-400 ohms each. Lowered the boost to wg and it's still doing it. Swapped in a Bosch cap and rotor too. Will try lowering plug gap some more.

Was running 16-17psi, now set to 7 or 8ish.
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Old 05-08-2019, 09:25 PM   #8
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So for some reason the boost suddenly dropped to 1 or 2psi. Then the downpipe broke in half. At least it's not misfiring anymore..
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Old 05-08-2019, 10:16 PM   #9
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Is the cat plugged?
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Old 05-08-2019, 11:09 PM   #10
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Is the cat plugged?
Well, I'm not sure what the cat's constipation issues have to do with my car, but turns out the wastegate rod fell off the flappy bit. Also the turbo was loose on the manifold, even though I used copper locknuts.

DP is welded back together and I guess I'll go buy another set of BPR7ESes tomorrow.


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Old 05-09-2019, 12:09 AM   #11
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If the exhaust is restricted, It creates backpressure which will lead to heat build up in the downpipe. The heat cycles can crack stuff. That increased backpressure will also effect turbo efficiency and also cylinder evacuation. This can lead to misfire. That is where my mind was going with the is the cat plugged comment.

You commented it used to only do it when cold but was fine when warm, Perhaps the cat was partially plugged and flowed some once warm, then plugged worse.
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Old 05-09-2019, 12:32 AM   #12
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Quote:
Originally Posted by NotSoFresh View Post
If the exhaust is restricted, It creates backpressure which will lead to heat build up in the downpipe. The heat cycles can crack stuff. That increased backpressure will also effect turbo efficiency and also cylinder evacuation. This can lead to misfire. That is where my mind was going with the is the cat plugged comment.

You commented it used to only do it when cold but was fine when warm, Perhaps the cat was partially plugged and flowed some once warm, then plugged worse.
This car doesn't have a cat but it does sound like that might be the problem with my other 240 now that I think about it..
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Old 05-09-2019, 02:09 PM   #13
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Popped in a new set of NGKs gapped to .027 and no difference..

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Old 05-09-2019, 03:49 PM   #14
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ECU, ICU, powerstage, and distributor are now all ruled out..
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Old 05-09-2019, 05:59 PM   #15
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Last edited by Cwazywazy; 05-09-2019 at 10:40 PM..
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Old 05-09-2019, 07:29 PM   #16
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So as usual the problem was simple and stupid. Someone suggested it could be an injector leak so I looked at the rail and one of the bolts was just gone..
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Old 05-09-2019, 10:34 PM   #17
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Good find! Ignition or fuel computer ground being open I am surprised it ran at all.
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Old 05-09-2019, 10:47 PM   #18
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Quote:
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Good find! Ignition or fuel computer ground being open I am surprised it ran at all.
Whole car needs a bolt check. Turbo was falling off too.....
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Old 05-10-2019, 11:37 AM   #19
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Stumble going into boost is commonly an air leak.

Spray around with brake cleaner to find air leaks.
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