Misfire under load Cylinder 1, P0171 P0301

[JoeysIm]

Hi all,

I have a 2006 Mk5 Gti, and before I start buying bunch of parts I may not need, I thought I'd get a few insights on people's experiences.

Problem -
Normal Driving no issues, but under heavy acceleration and going uphill, the car shudders and CEL flashes, which turned out to be P0301. When checking the OBD, this is accompanied by P0171 Bank 1 Too lean.

I have replaced the spark plugs, this hasn't helped. I did briefly get a P0441 EVAP system incorrect code, but this hasn't shown up in a while.

I'm thinking along the lines of fuel related? I hear the fuel injectors on these are pretty ordinary. Coil packs apparently have a similar reputation but doesn't really explain the lean code.

Another culprit could be a vac leak somewhere but the car idles perfectly and drives absolutely fine normally while cruising around..

[Lucas_R]

Don't rule out the high pressure fuel pump (HPFP) cam follower which is a known wear item on the Mk5 GTI engine but rarely replaced. If this is worn then the HPFP wont be delivering the max fuel pressure. Genuine VW Fuel Pump Cam Follower Kit – 06D109309C – AutoInstruct

Also ignition coil packs are another known weak point on these engines. If your car still has the original coil packs then its very likely they will be part of the problem. I suggest swapping your coil packs around and then seeing if your P0301 changes to P0302 or P0303 etc meaning it has moved to another cylinder. Audi R8 Ignition Coil Pack (Latest Version) – 07L905115B – AutoInstruct

[JoeysIm]

Thanks for the reply Lucas, I have a few updates to report:

- I noticed a alarming hissing sound from the PCV valve, upon investigation I found a noticeable air leak in one of the joints of the plastic. This has been replaced with genuine VW valve, and the running feels much smoother, BUT problem of jerking and shuddering under load persists.

- I then noticed another wierd noise coming from the purge valve, this was again replaced with a genuine unit and the only effect this had was removing the EVAP system code.

- As I was mucking around in the engine bay, I noticed my idle didn't change when I accidentally unplugged the MAF connector. This caused some suspicion and upon testing I found the voltage between ground and signal while engine is idling is constant at 2.5V. Resistance of the signal to other pins are all open, which tells me this may be the cause of all my issues.

I'll be replacing the MAF tomorrow and will report any changes. I think I'll also grab the better coils as you suggested, it's only a matter of time before that goes.

[Lucas_R]

Good work on the diagnosing and fault finding - good on your for having a go. The PCV's on the Mk5 GTI engine are terribly unreliable so at least with that replaced, its one less thing to worry about.

A faulty MAF (either the MAF itself or the wiring/plug leading to it) could certainly be causing the engine to run poorly when pushed as it will not know the volume of air being sucked in to then tell the ECU how much fuel to add. Cant say this is a common issue on these engines, but I guess they are 12+ years old now so some things will be starting to pack it in and will need replacing. Hopefully a new MAF will sort it out for you.

When I had my Mk6 Golf R (basically same engine as your Mk5 GTI) I could unplug the MAF and at idle it would make no difference, but if you tried to drive the car with the MAF unplugged it was a dog.

[saabman]

That's a good outcome and a lot easier than replacing the cam follower

The combination of multiple faults also makes trouble shooting a bit more difficult.

If you have VCDS you can display the manifold pressure/vacuum as well and confirm your diagnosis

[JoeysIm]

A bit of an update since the last post... issue is unresolved, BUT improved dramatically.

- The new Bosch spark plugs were pre gapped at 0.030", I dropped this down to 0.024 and it made a dramatic difference, I can step on the gas a little harder all the way up to 3rd gear before the misfire appears.

- New genuine R8 coils installed, this helped with the rough cold start but no luck with misfire issue.

Turns out my MAF might be fine, maybe my testing method wasn't correct?

I'm also getting a frequent DTC for Boost Pressure Solenoid Valve, and when I checked Ross Tech it looks like the DV. I'm assuming mine will have the old diaphragm type and might be cracked but not sure if this will cause the misfire?

I'll have to invest in VCDS it seems

[Lucas_R]

In a Mk5 if its the original DV still fitted to the car then it will definitely be the old diaphragm type which split/rip and then leak boost. You can buy this part which replaces the crappy bits but retains the electrical solenoid bit of the DV. I had one of these on my MK6 GTI and Mk6 R and will soon be buying one for my Mk7 GTI. They are a nice and neat solution and not too expensive.

GFB DV+ Diverter Valve T9351 – VW MK6 / MK7 GTI / VAG – AutoInstruct

Not sure if this will cause a misfire, but if the diaphragm is split/ripped then it will be leaking boost so you will be down on power and you may hear the air leaking when giving it some.

[JoeysIm]

ISSUE FIXED!!

I can't believe this but it was the BRAND NEW Bosch plugs! My second ever time using Bosch spark plugs and BOTH times, they have been faulty from the pack.

I threw in some NGK Platinum plugs this evening and no more misfires! Super easy cold starts, super smooth idle and pulls all the way up the rev range. Should of stuck to NGK plugs, thinking Bosch might be decent since they are OE manufacturer.

Thanks everyone for their input! Even though I only needed to replace the plugs, I managed to find and fix several issues along the way, for the first time since ownership I can now say I'm 100% CEL and codes free.

[Lucas_R]

Good to hear its fixed - im now wondering if the Bosch plugs might have been china fakes as this is really becoming a big problem now.

[guile]

yep, the fake spark plug thing is a real issue and I think that's what potentially toasted my motor in the S3...

Be careful as there are also fake GFB diverter valves getting around too, ask me how I know. I bought one cheap because I only wanted the spring to replace my old one and that's when I worked out it was fake as the parts were all very different and had a terrible fit...