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P21F9 - Too Rich At Idle

Mk7GolfRK

New member
Location
UK
Car(s)
Mk7 Golf R
Hi All,

Been trying to solve an issue for a while now with no luck. Wondering if someone on here has some advice

Symptoms :
- P21F9 code on VCDS
- Rough Idle when up to temp
- Missfires after 30 or so mins idling
- Excessive overrun pops due to running rich
- Negative ST and LT fuel trims
- Drives fine once on throttle, no sign of lost power

Replaced:
- All 4 Direct Injectors
- Intake Manifold
- Port injectors (MPI) on the Intake Manifold
- PCV
- Both O2 sensors
- All 4 Spark Plugs
- Multiple Oil Changes

Everthing replaced is OEM

Any Ideas anyone ?
Both HPFP and LPFP actual values are matching specified values
Also done the fuel rail pressure test and pressures are correct

Mods
- Resonator delete
- Intake Airbox Mod (panel filter)
- Turbo Technics intake pipe
- Upgraded intercooler
- Standard, plans are to go stage 2.

Any help would be appreciated
Thanks

Dave
 

Mk7GolfRK

New member
Location
UK
Car(s)
Mk7 Golf R
@Diggs24. Tried that but car ran the same.
Cleared codes to reset fuel trims. But still in the negatives when I checked.

Also noticed when off throttle my post cat o2 sensor reads at like 3v+. Normally sat at 1.9v. Is this normal ?
Have had the car leak tested too. No air or
Vacuum leaks present.
Clueless as to what this issue is. Could the hpfp or fuel rail cause issues like this.
Or even a clogged downpipe cat. Current one is standard but does sound a lot more raspy than it did 6 months ago.
Any help would be appreciated
I’ve taken it to numerous vw specialists with no luck sourcing a fix.
 

Mk7GolfRK

New member
Location
UK
Car(s)
Mk7 Golf R
Update, I carried out another oil change and put a new hpfp as it may have been leaking fuel into the oil.
Checked on VCDS but still reading negative on the short term fuel trims.
Also had it smoke tested again, and no leaks found.

Could a blocked catalic converter cause it to run rich.
I have a cat less downpipe to put on, thinking it maybe worth putting that on to rule out the cat.
 

AceHammer

Go Kart Champion
Location
NY USA
Car(s)
2015 A3
You don't have either of the MAP sensors listed as replaced, have you checked them? Relatively easy and cheap and can affect afr if one is bad or dirty

You are also MPI so i'm assuming a non-US car, you should also check the low-pressure fuel sensor as well. Will be different from the high-pressure rail reading.
 

Mk7GolfRK

New member
Location
UK
Car(s)
Mk7 Golf R
Thanks everyone. My issue was got sorted, I rechecked my fuel trims after a few days and it looks like the hpfp sorted the issue.
Both ltft and stft are within spec.
Yeah I have a UK spec, and both hpfp and lpfp were both meeting specified specs.
I guess the seal on my old hpfp may have been leaking fuel into the oil.
Also I replaced the map sensor on the intake manifold prior to the manifold change.
 
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