I have an '08 Sport, K&N Typhoon short ram intake and Tanabe Concept G muffler. Spark plugs and coils have all been replaced recently. I've been getting P0304 Cylinder 4 Misfire code, hesitation, misfires that I can feel very well if I push it into the 4-5k rpm range, and CEL flashes when it misfires along with idling so rough it feels like someone doing a potty dance in the back seat. In lower gears with high RPMs it backfires through the intake. A mechanic friend said it sounds like a valve issue and is looking into the parts for me, but I'd like to see if anyone has had exactly this issue and had a valve job remedy the problem. Only major replacement so far was when my VTEC pressure switch failed 5-6 months ago. Car is over 150,000 miles, has had regular full synthetic oil changes, and I've run RXP and Techron fuel injector cleaner through it a few times too often according to another mechanic pal. Intake filter and hoses, and throttle body were cleaned around the same time as the spark plugs and coils were replaced.

 

I don't see valve adjust in that list anywhere,, its a maintenance item no matter what Honda says..

 

They've never been adjusted. Stupid move, I know, but I've only recently (past 10-20k miles driven) started looking into more major maintenance besides oil and all that.

 

I'd DEFINATELY do that before you look at replacing valves, a lot of mechanics are out of the habit since so few cars need valve adjustments these days. The engine in this car seems to behave a whole lot like a bike motor as far as wear and maintenance..

 

Backfire through the intake is classic of valve adjust OR a weak Crank position sensor.

 

I'm planning to turbo it at some point so replacing the valves and all that would need to be done for it to handle the boost anyway. I'm fine putting some money into it to make sure it runs as long as possible, I love that car. Problem is with a lot of engine components to diagnose them costs a ton, I haven't found anyone near me that can do it for cheap. The friend of mine who will be doing the valves lives well over 40 miles away and I don't have time to drive out there anytime soon