single cylinder misfire when cold at idle

[nmalenovic]

Hello. Like the title says.

i hooked up the car on VCDS and looked at misfire advanced measurements. car at start runs misfires on cylinder 2. once it hits 100 misfires per minute, dashboard CEL lights up. At 250 it disables the cylinder (dashboard EPC icon). While driving I get *0* misfires. as soon as I stop and sit at idle, I get misfires again but no CEL. once engine is at operating temp, i get 0 misfires - either driving or at idle.

so far performed:
1. new coils
2. new spark plugs
3. walnut blasting valve cleaning
4. new injectors
5. new oil separator
6. cylinder pressure test ok

next: smoke test to look for vacuum leaks.

key questions: why cylinder 2 only, why only when cold, and why only at idle?

correct answer winner gets a lifetime lifeline in return and good karma to boot!

 

[Diggs24]

Take you charcoal cannister out and check to see if it is full off fuel. Alternatively, disconnect the EVAP from the PCV and block it off.

 

[nmalenovic]

Take you charcoal cannister out and check to see if it is full off fuel. Alternatively, disconnect the EVAP from the PCV and block it off.

any reason why fuel supply (charcoal canister) or engine air return (oil separator) would cause misfire on a single cylinder only, and only at cold start idle?

 

[Diggs24]

low flow of air thru throttle, more stays in center of manifold. cold start is when it starts metering fumes from the tank

 

[nmalenovic]

low flow of air thru throttle, more stays in center of manifold. cold start is when it starts metering fumes from the tank

so... thank you @Diggs24 - you were directionally correct and thus earn the lifeline brownie points. turns out it was an airflow issue - smoke test showed that throttle body hose was leaking air because hose clamp escaped the metal railing in the rear of the hose and didn't make a good enough seal. Once this was fixed, there are still a few misfires on the real time data advanced measurement screen but far, far, far fewer and certainly not even close to trip CEL (or EPC - cylinder disable). I read somewhere that cold misfires (that don't trip CEL) are normal - or should I go full compulsive obsessive and check the charcoal canister? if so - any recommendation for good DYI for charcoal canister - I haven't done this one before. also, your answer is the briefest and most precise explanation I have ever read - anywhere.

 

[Diggs24]

a few misfires are normal

 

[swcrow]

Diggs FTW

 

[GTIscream]

Diggs FTW

Yeah, that was just awesome.

 

[nmalenovic]

oh no, @Diggs24 is even more correct than I first thought. After fixing the boost pipe clamps on to the throttle body, I still get an occasional EPC/CEL because of cylinder 2 misfires, albeit at much lower frequency (say from every couple of minutes to maybe every other day at start). I replaced my K&N reusable filter (read somewhere that it throws oil into air intake that confuses the MAP measurements) with a disposable one and EPC/CEL went down in frequency even more (e.g. to once a week), but there were still cylinder misfires that wouldn't trigger EPC/CEL. Then I read N80 EVAP / Purge Valve Issue and so I disconnected my purge valve solenoid. I hooked it up on VCDS and checked IDE01976 Numb. of misfir.: cylinder 2 (along with IDE01975/IDE01977/IDE019758 for cylinders 1,3, and 4) and I had grand total of 2 misfires on cylinder 1 and 3 misfires on cylinder 2 at *cold* start during first few seconds and *0* misfires at any point after that - and that *0* includes subsequent non-cold starts (e.g. stop/start while engine is at operating temperature). I ordered a $40 replacement solenoid from fcpeuro and that should be the end of EPC/CEL misfire on cylinder 2 and the "heartbeat" shaking on this car.

 

[nmalenovic]

new N80 valve didn't solve the problem - fully - still get misfires/slight shaking albeit not enough for an EPC/CEL - more misfires when cold (eg 50 a minute on cyl 2), less when warm (eg 5 a minute). So @Diggs24 you recon it's time to take it to the dealer under 150K/15year PZEV CA warranty?