i have a problem with my volvo s60 cylinder number 4 misfiring,i have done all possible tests and the problem still its not solved could it be in the cpu

 

What have you done?

 

not sure how you'd conclude the CPU is at fault. In my list of things that can can cause a misfire, I'd start with the coil, the wire harness/connectors to the coils, the plug, the injector, the knock sensor (that should thow a code), then it gets ugly - compression issues due to rings/valves, head gasket issues (and oil/coolant mix or loss? odd smelling exhaust or whitish smoke)? Did you throw in a can of Techron to clean the injectors?

To vet the above, start by pulling all the fault codes (are you only getting a P0304?) you can flip flop coils, replace the plugs, inspect the connectors for hairline cracks, test for HCs in the coolant, check for sludge in the oil or loss of coolant/oil, then do a wet/dry compression test and finally pull the injector out (not sure if your's hide under the intake or not...) You can try reseating the ECU (pull out, use a contact cleaner then reinsert) - not sure if the ECU is easy to get at in your model...so if that's the case, I'd leave for last.

 

You could also use noid lights to see if the wiring to the computer is intact. I don't have a set, but they're cheap enough.