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Bad misfire in boost..

Cwazywazy

Cwazywazy

Single jingle
Joined
Aug 20, 2016
Location
Granby CT
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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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.
 
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.
 
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..
 
NotSoFresh said:
Is the cat plugged?
Click to expand...

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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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.
 
NotSoFresh said:
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.
Click to expand...

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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Last edited:
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..
 
Stumble going into boost is commonly an air leak.

Spray around with brake cleaner to find air leaks.
 
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