VW Mk5 Golf GTi (TFSI) Misfire When Engine Is Warm

This post summarised for anyone in a rush...

Car
  • 2005 VW MK5 Golf GTI 2.0 (AXX engine)

Symptoms
  • Slight misfire on idle when engine is warm
  • No symptoms when engine is cold

Fix
  • New coil pack for cylinder 1


More information


I recently sold my 2005 Honda Civic Type R (EP3) and bought a 2005 VW Mk5 GTI in the hope of moving to something a little more torque-y and refined.

I was going through that first month or so of owning a new used car which I hate with a passion. You know, when you're constantly thinking:
  • Have I bought a lemon?
  • Was that a knock?
  • Is that sound normal?
  • Is the engine going to explode
Well anyway, during that enjoyable period I noticed that when the car was up to temperature it would slightly stumble occasionally on idle, most "non-car" people wouldn't have noticed it, but anyone into their motors would.

I did not experience any hesitation or loss of power, but after scaring myself with stories of compression loss and head gasket failure I decided the first sensible option would be to change the coil packs and spark plugs.

So I ordered the following parts:
  • Genuine VW coilpack - 07K 905 715 G (previous revision 07K905715F)
  • Bosch spark plug - 101 905 631 H (Bosch part number: FR5KPP332S)
Before I had a chance to fit these the misfire got much worse and finally conked out and wouldn't run at all.

Luckily I had an OBDII bluetooth adapter and Torque for Android which enabled me to read the OBDII fault codes to get a better idea of what was going on.

OBD Scan Result: 
  • P0300 - Random/Multiple Cylinder Misfire Detected
  • P0301 - Cylinder 1 Misfire Detected
  • P0302 - Cylinder 2 Misfire Detected
  • P0351 - Ignition Coil A Primary/Secondary Circuit
Coil A = Cylinder 1 so at least that seemed consistent. However, still not conclusive that this is a coil or spark plug issue.

I changed the spark plugs first but this made no difference. The old plugs and coils were pretty filthy though with lots of corrosion build up pretty much everywhere where you don't want corrosion build up.
bad bosch spark plugs from golf mk5 gti

After changing the coil pack on cylinder 1 the engine fired straight up and the misfire disappeared.

Problem solved.

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