Actually a lean condition can cause a misfire. If you checked the elbow that everybody automatically assumes is the culprit and it is ok then there are a few other VERY COMMON spots that like to leak vacum. The first one is the intake plenum where it bolts onto the lower intake manifold. The seals are just o-rings and over time they get hard and start to leak vacum, usually these are more of a problem with a cold engine but when they get bad enough they are a problem all the time. Another problem area is the actual egr valve itself or the tube from the egr that runs to the intake. The diaphram inside gets hard over time and starts to leak vacum and the tube seal at the intake does the same thing. The lower intake manifold also likes to leak vacum but it is not as common as the other 2 symptons I mentioned. Also don't forget about a dirty maf sensor however this doesn't usually cause a misfire condition. One other thing to check(actually the first thing you should verify) when you have a lean condition and a misfire is the fuel pressure and volume, low pressure and volume=not enopugh fuel in the cylinders and presto you have a lean misfire. If your problem is happening while you are on the gas then fuel pressure and volume may be your culprit being that a vacum leak usually only causes a misfire while at idle, or perhaps you have a lean condition and you have a misfire not being casued by the lean condition. Either way get the lean condition fixed first and then go after the misfire if it still persists
