I was getting the P0300, Multiple Cylinder Misfire for about 6 months, on and off. Nissan had me replace spark plugs, fuel pump, idle air relearn, etc. etc.-- had to pay a $149.95 diagnostics each time and they failed to fix it three times!
Turned out to be an internal HG leak...only way I found out was there was oil in my radiator reservoir. Got the HG replaced at a local family-owned shop and couldn't be happier with the results. All is good-- no more CEL, /knock on wood/ and didn't realize how much power was being sapped until I got it fixed...luckily no other damage was found.
04 Spec V
Nismo: Cams, Cat-back, FSB, RSB. JWT Valve Springs, BSR. AEM CAI, MMI, B&M SS, UR pulley, 2JR Plenum, Fidanza 11lb. flywheel, Spec Stage 2 clutch. BR coilovers, 6K swift springs, Nismo wheels, Projector HIDs @ 6K, LED tails and LED turn signals
~170WHP w Cats intact, CARB-legal.