I've been fighting the p300 multiple cylinder misfire for a couple weeks on my 08 with 155,000 miles. It started after I replaced the egr valve and found that it was nearly clogged with crap. Did plugs and wires while I was at it and now it starts fine and is smooth up to about 20mph, then it starts to stumble and throws the multiple cylinder code and runs rough as hell. I did just replace the intake manifold gasket as well as there was some leakage around it. A few months ago I remember hearing a lot of rattling sound from underneath until it got warmed up. I figured it was the exhaust rubbing on something, but it went away and I forgot about it. Now I'm wondering if it isn't a clogged cat which is limiting my ability to breath especially with a new clean egr, and cleaned throttle body and intake gasket. So, any suggeations to a better exhaust other than going back with stock? I figured now would be the time to do something with it, but I have no idea what one to look at. I know it won't make much, if any, power difference but I need it to at least run better than it currently is.

 

Clogged cat will show up like cvt all rpm no power. I’d disconnect 02 to see if it clears up if not you got other issues. Front sensors are air fuel down stream clean up check cat. Most times random and a multiple are plug or fuel related (wrong plug) I’d be rich of the amount of times I’ve seen motorcraft plugs installed in non Ford causing miss fires in 20 miles. Some time’s adaptive memory causes issues. So I’d check fuel trims see how ecu is doing. Possible but unlikely you could have bad heads and there leaking coolant in the engine. If asd fuse isn’t blowing your driver wires should be fine to coil pack however Coil could be getting hot and failing also. Random are not enough information you need to find more data if your intake gasket was leaking you would get lean code as well typically. Random are time intensive to diagnose as they are not enough to warrant a comprehensive diagnostic including compression test it’s throw plugs at it unless just done. Still have to check to see if they were gapped and correct length could be short sparking to high carbon fouling. Step 1 check plugs ( did this resolve the problem if not go to step 2), check coolant level, check fuel, check pids graphs and fuel trims, pull front o2 out of exhaust, check coil pack on hot soak,

 

how do I do a hot soak