P0420, Misfires, etc.

[Custom441]

2015 GTI Autobahn, 6sp manual, 113,000km (mostly highway), always fuel with 91 octane, bone stock.

A year or so ago I had a code for a cylinder misfire. Engine was due for a tune-up anyway so I replaced all 4 spark plugs. Fast forward to the last month or so and I've noticed the engine stumble from time to time while cruising. Nothing aggressive, just smooth-burble-smooth... weird. I recently had a CEL with P0420 code, mechanic replaced rear O2 sensor, didn't work. Was due to have my summer tires put back on so figured I'd wait til then to further diagnose. Before I got the chance, while accelerating to pass on the highway, my engine bogged down, car started shaking below 2800rpm, flashing CEL and EPC light. Pulled over, shut down for a minute, start up, everything is fine. I have since replaced all coil packs and it hasn't happened again but I still have the CEL & P0420 code and the engine will stumble from time to time while cruising. Mechanic wants to replace the forward O2 sensor, and while I can appreciate him wanting to start with cheaper fixes I'm thinking there is an actual problem and not just a sensor issue. Bad fuel pump? Bad/dirty fuel injector? Carbon buildup? Blocked catalytic converter? Not sure where to go from here and looking for some thoughts & ideas. Cheers!

 

[Kdubya]

Carbon clean (walnut blasting) and try PCV replacement

 

[Alex@Stratified]

The most common things that cause misfires with these motors from the cars we tune and see.
1. Spark plugs - old or gap too large
2. Spark plug grounding
3. Coil packs
4. Injectors - dirty, leaking, aged
5. Excessive carbon buildup
6. Exhaust leaks

 

[Custom441]

The most common things that cause misfires with these motors from the cars we tune and see.
1. Spark plugs - old or gap too large
2. Spark plug grounding
3. Coil packs
4. Injectors - dirty, leaking, aged
5. Excessive carbon buildup
6. Exhaust leaks

Awesome - thank you! Plugs done a year ago, coils done this month. Looking to have a carbon clean done and injectors replaced. Hopefully this solves my issue and at the very least I figure it's good preventative maintenance.

 

[aegis]

Hey, will respond with my anecdotal info as well.
While my symptoms weren't the exact same, I went through the same process Alex mentioned above. I was having random misfires and startup problems. No codes or engine problems during cruising, just hot start problems and lumpy idle.

In this order, I've replaced these parts and overall the car has come back to its spirited self. No longer misfires, however idle while parked is slightly lumpy (much less) which I'm told is somewhat normal while parked. I'm going to get a proper grounding kit and I have a new lambda sensor on hand to maybe help with this.

• Replaced spark plugs and coil pack boots. Coil packs were deemed fine.
• Carbon cleaned (media blasted).
• New OEM PCV
• New OEM Injectors.

The above resolved my misfires/rare engine lights. I then replaced my HPFP and that solved my hot-start issues, as my OE one was failing and leaking vapours back into the crank case causing a no-start condition. We tested the pressure at the rail and LPFP sensor which all seemed fine. it's been a long road with diagnosing but so far so good, good luck with the maintenance!