what is causing misfire codes?

[cjben]

2004 4.6 140,000 miles. Here are the codes PO 300,302,303,316. Engine runs great and no check engine light until under heavy throttle. Then check engine light comes on and engine misses. Just had a new cat put on passenger's side for this problem. Thought it was fixed,but it's back again. Really don't want to clog another cat. How can I determine if it's the coils on cylinders 2 and 3 or something else? Really don't want to throw coils on if it's something else

 

[lincolnshibuya]

swap the coils 2 and 3 with 1 and 4, if the code changes to p301 and p304 then you need to change the coils.

 

[cjben]

Did this and drove it for a few minutes. Check engine light came on with codes(302,303) for the same 2 cylinders. What do I do next?

 

[lincolnshibuya]

next is to swap plugs, if result is the same, swap the injectors, if result is the same do a compression test if the compression test showed something (specific to a cylinder - low compression) then do a cylinder leak down test to figure out if you have bad intake/exhaust valve or possibly a head gasket issue (normally misfire between adjacent cylinders is head gasket)