Posted by jim on 11/4/2007, 6:44 am, in reply to "engine missing but not all the time"
71.60.72.188
This can be something as simple as a sparkplug or a plug wire breaking down under load. Yes- even new ones can be bad.
Make sure you don't have any Platinum based plugs in that engine. The surface area of these plugs is too small and will "blow out" under boost or with nitrous. I have a blown S4 and run the cheapo $1.99 standard type single electrode Bosch plugs with no problems. My other car runs up to 28 PSI boost and I also use these plugs. On an intermittent misfire, it's almost always an electrical issue.
After the electrical is ruled out, start looking at:
injectors and injector electrical connection
mechanical condition (do a leakdown- not a comression test)
caps and rotors (also check rotor phasing)
improper routing of secondary ignition wires
injector driver circuit failure such as a faulty transistor or SCR (inside of PCM)
all computer connectors- everywhere
coil drivers/modules (ignitors for the non-USA crowd)
Remember- All intake manifolds are required by the EPA to have a positive internal pressure at all times.
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