I've had a fault p0303 that's a misfire cylinder 3. It only shows up when car is cold basically fresh start of day ONLY if I let idle...I do think outside temp matters cause when engine is below 40 degrees first start it will occur everytime I let car warm up while idling but it also happens in spring/summer with 70+ it still happens but not all the time. I was monitoring with full air boxes and splitter off leaving throttle body fully exposed and one hose open at valve cover on drivers side and noticed as usual engine light comes on after secondary air pump turns off and engine running fine then rough and can feel the misfire type shakiness...car engine temperature was under 40 degrees at that time. I reset code and turned car off then on temperature was around 50-55 eventually warning code p0303 showed again. I waited til car engine temp at 60+ and reset code then turned car off then on again and NO codes. Now IF I just turn car on and drive NO codes appear at all. I've already:

1. installed new plugs rechecked again by me and a shop swapped also

2. Swapped coil pack many time last was from cylinder 6 to 3 and still code in same cylinder 3 (p0303)

3. Changed fuel filter since regulator in it.

4. Checked and still monitor fuel rail pressure when on constant 60 p.s.i.

5. Swapped spark plug wires several times currently have on cylinder 3 factory wires on.

I'm told could be a vacuum leak maybe hose going to valve cover to valve cover but I have full air boxes off including splitter...thus hose from splitter going to second opening from valve cover was exposed but no code at all when engine warm 60+. I haven't checked hose from SC to brake booster but have doubts it's that.

I was told compression but like I said IF I turn car on and drive before secondary air pump finishes NO codes at all and car runs perfect.

I was told injector but this car by what I'm told doesn't have a history of bad injectors and when car engine warm car runs perfect.

I forgot how but even the CPS was mentioned since I guess it controls all the cylinders but my CPS is atleast 2 years since replaced.

I was told Cats but how does car run perfect even at WOT with even a cat going bad




SUGGESTIONS???

 

I was told just recently it could be o2 sensor....If I recall right I believe there are 4...which ones would be the cause?

 

Vic, you still running regular in that thing??? Try some premium....


Seriously, take it to Speedriven in Palatine , Marcin is good people.

 

I am talking to them...yesterday me and my friend Sam did compression test and it was fine

it's only me bothered by this from driving the car it's knowing that's annoying....I'm also working with Nick P(Nick55) he went to us41 with us had a w210 E55 then and based off my explanation from yesterday and his conclusions are maybe failing o2 sensor or an air injection valve because this time the codes didn't come on but the car started running fine then as I let idle as usual it felt rough like it has when I leave car idling from fresh start but this time no codes. Pictures below