Golf/GTI VII :: Rough Idle And Shuts Down Sometimes

Symptoms:

- rough start-up, car shuts down

- backfire when releasing after WOT

- OBD-II codes (see below)

Okay so these issues started to appear only 1 day after I got tuned...

Problem History:

Day 1:

- car started mildly rough

- I got an EPC light on my dash 2 minutes after moving and for 2 seconds the car sounded like an STI (the sound you hear with a misfire)

- I turned it off and on, EPC gone car started up normally

- took it straight to dealer, they checked coils and plugs and said that it's a tune issue.

- went to take it back from the dealer and that was the first time the car shutdown immediately after start-up. it started at second attempt with a quite long ignition sound and the STI-like sound briefly once again.

Day 2:

- I went to my friend and we pulled the codes from OBD-II and only got the P0420 00 [175] - Efficiency Below Threshold code only (as the dealer probably erased any other code related to my EPC light)

- we deleted the code and drove around

- every time we turn the car off and on. The car starts up mildly rough and the code comes back up

Every 2 days or so:

- the car would require 2 attempts to start but no EPC light coming up.

Day 4:

- I had a night of possibly 6 WOTs to test the cars performance, and it was performing great. didn't feel any loss in power or anything. (was friendly racing my friends modded 135i with very impressive outcomes)

- car backfires after i release the gas pedal at WOT (not with every WOT but 2 out of 6 runs it backfired)

- After the last WOT i got the EPC light. turned the car off and let her relax for 5 minutes.

- car took 2 attempts to startup. with severe roughness and shakes (as if the car is cammed V8 engine) for 5 seconds and then back to normal.

Now since the EPC light came back on i wanted to check if there is another code from the OBD-II. I was hoping that the issue will only be the P0420 00 [175] - Efficiency Below Threshold code but my fears were true and i got another very annoying code

This is last nights code pull

2 Faults Found:

14914 - Boost Pressure Regulation

P0299 00 [096] - Control Range Not Reached

Intermittent - Not Confirmed - Tested Since Memory Clear

[Code] .....

Day 7:

- The car had the sever start-up roughness again

As I'm trying to find a solution for my problem here are the steps i took/will take:

- took it to the dealer for nothing.

- ordered an O2 sensor extender for the P0420 00 [175] - Efficiency Below Threshold code and i hope it's somewhat related to the start-up issue. it will arrive today or tomorrow.

- once the extender is here, I'll take it to an exhaust shop to install the extender and check for any DP/Exhaust leaks

- I will take it to a mechanic to check any leaks in all intake/air connectors and piping, any leaks in DV (I have forge recirculating DV), Boost pressure sensors (the piggy back was connected to them and one might be loose when we removed it), spark plugs and coils.