I have a 2008 Mustang GT 4.6 3 valve I got a check engine light for a cylinder 5 misfire so I changed all the coil packs I changed the injector for cylinder 5 now it say random misfires This happens when I hit like 4800 to 5000 RPMs now the check engine light comes on flashing then goes off Driving me nuts HELP!!!!!!!!!
I'd also recommend checking the plugs and if they haven't been replaced lately just replace them all. They are relatively cheap compared to other things and easy to replace. If they haven't been changed in a long time there is a chance of them breaking off in the heads with this particular engine though. There are a number of threads here about that issue.
If his 2008 model was built after 11/30/2007, it'll have the later style 12mm one-piece plugs that aren't break prone. The clue is the color of the coil boots. If they're black, you have the earlier style two-piece plugs. If they're brown, you have the later style plugs.
Hows it do in the lower rpms? an how many miles is on the spark plugs?
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