Sooo, out of nowhere today my s4 started acting up. It started surging/bucking and idling rough in the middle of a drive. It feels like a fuel delivery issue because it's popping and crackling out the exhaust on return to idle and it smooths out and acts normally above about 4000 rpm. Any suggestions on where to start before I shotgun parts? If it's low compression I'm jumping off a bridge because there's 1200 miles on this fresh rebuild.

 

if it starts driving fine right above 3800 rpms id imagine a primary injector has become partially clogged,came unplugged, or is just not flowing right. you could try swapping your primary and secondary injectors, and seeing if it runs normal below 3800. if so then id look into figuring out which injector is bad and replace it. or get them all flow tested to determine how they are.

 

I swapped around the injectors to no avail, then checked my plugs. They were fouled to hell and back. That was probably from the 2-stroke I was running but I switched ti Idemitsu so I should be good. Thanks for the input!