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Another misfire problem

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Hi all, 

I have a 2001 Boxster S, 6MT, US-spec. The car has been owned by either my brother or me since 2005. The car has received more than recommended maintenance over its lifetime. In December, the car started to set misfire codes and fuel trims indicated a vacuum leak. I don't recall which cylinders indicated a misfire, but I'm pretty sure it was cylinder 1 and 5 and maybe 4.  


To address the misfire, I first unplugged the MAF, which had no effect on how the car ran. I replaced it with new one, but no change in symptoms. 


Over the past few Saturdays, I investigated the symptoms. Coils were original, so I replaced all six, as well as the plugs. Plugs had only a few thousand miles on them but showed some carbon buildup. It appeared to be fuel, not oil. A couple of plug tubes showed minor leakage, so I replaced all six. 


I bought a smoke machine and tested the intake tract. I found no signs of leaks, but noticed signs of oil leaks from the intake. After removing the intake tubes, I saw a LOT of oil in the intake manifold. That indicates a failed Air Oil Separator, so I ordered and installed a Porsche-branded AOS and cleaned up the pools of oil. Before putting the engine cover back on the car, I started it to see if it was fixed. It started easily enough but didn't idle smoothly. It would run fine above 2000 rpm, so I concluded the engine was probably burning off oil in the intake tract that I couldn't reach. There was no CEL, no smoke and no unburned fuel smell, so I figured it was safe to put the covers back on. 


When I started it this afternoon, it would barely run. It ran rough at idle and would barely rev. I could smell unburned fuel and the CEL came on. I have Durametric so I checked the codes - P0300, P0301, P0304 and P0305 - general misfire and misfires on cylinders 1, 4 and 5. Durametric chose to lock up then so I couldn't get any live data. 


Now I'm stumped. It seems I'm back to square one. The misfires seem to be consistent on cylinders 1 and 5, which is suggestive. I suppose my next step, other than ensuring the plug connectors are securely seated, is to check the wiring to the coils. Any suggestions for how I go about that, or any other ideas to pursue? I haven't noticed anything that leads me to think there is a mechanical issue. Although the IMS is original, I've heard no noise and I always check oil and filter for anything metallic. Oil was changed about 1500 miles ago with absolutely no metallic particles in the oil or filter. Any ideas welcome at this stage. 



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Welcome to RennTech :welcomeani:

I would do two things: Remove the coil packs on the misfiring cylinders and look at them for any signs of cracking; if you find any, they need to be replaced. Second, pull the plugs and "read" them, looking for carbon build up, signs of oil or fuel. If highly carboned or covered with fuel, you may have injector issues. 

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The problem with misfires after replacing the AOS turned out to be an intake leak. It seems I didn't get the crossover tube perfectly fitted into the rubber boot. After adjusting those, it started and ran perfectly. 

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