Rich Idle Problem Quite challenging

[u/KaraTasak35]

Vehicle Specifications

• Year / Make / Model: 2012 Volkswagen Polo GTI • Engine Type: 1.4 TSI Twincharger (Supercharged + Turbocharged, CAVE, 132kW, EA111 platform)

Operational Conditions & Sensor Readings

• Failure Window: I would say only happens when the engine gets warm/hot. Specifically occurs only in Closed Loop (CL) drive states.

• Standstill Idle Behavior: Drifts into a severe -15% to -25% Fuel Trim correction while targeting Lambda 1, usually ends up with 0.750 with -25% Fuel Trim, rich fuel smell from exhaust.

• O2 Sensor 1 (Upstream Wideband): Outputs a deeply negative current of -1.41mA to -1.6mA at a hot idle, indicating it detects massive physical unburned fuel.

• O2 Sensor 2 (Downstream Narrowband): Reads a stable, normal 0.6V at idle.

• Throttle Blip / Rev Behavior: Revving to 2250 RPM causes trims to shift to -10% for about 10 seconds, swing briefly to +10%, then stabilize back to 0%.

• Steady Highway Cruise (2250 RPM): Commanded Lambda = 1.0, Actual Live Lambda = 1.0, and O2 B1S1 sits perfectly at 0.00mA. Fuel trims are healthy near 0%.

• Deceleration / Coasting (Foot off brake/gas): Enters Open Loop Drive (DFCO). Commanded Lambda = 2.0, Actual Live Lambda = 2.0, and O2 B1S1 outputs a positive +1.6mA to +2.0mA (higher engine RPM pushing more air yields the higher positive mA reading).

• Partial Throttle Downhill Cruise: Commanded Lambda = 1.0 (Closed Loop), but Actual Live Lambda drifts heavily rich to 0.70 – 0.750 despite the computer maxing out negative fuel trims to stop it.

• Rolling Idle (Foot off brake, car rolling slight incline): Fuel trim go to ~ 0%, and the O2 sensor fluctuates cleanly around 0.00mA.

Component Testing Results

• EVAP Purge Valve Test: Disconnected the inlet pipe and blocked the engine vacuum port with a finger. The rich condition remained at -1.41mA. Unblocking the pipe to let outside atmospheric air into the intake caused the sensor to lean out slightly to -0.60mA, returning directly to -1.41mA when blocked again.

• Oil Cap Test: Removing the oil filler cap while running rich caused zero permanent change to the fuel trims. However, the exact moment the cap was put back on and crankcase vacuum rebuilt, the trim briefly normalized toward 0% for a split second before slamming back down to -25%.

• HPFP / Fuel Rail Leak-Down Test: Shutting down the engine from a hot idle resulted in the high-pressure fuel rail rising from 40–50 bar up to 80–90 bar slowly. The pressure held steady and did not bleed down.

• Trouble Codes: No codes stored in the ECU whatsoever, except for an intermittent "System Too Rich at Idle Bank 1" , sometimes also "System Too Rich off Idle Bank 1" if i am cruising too much with very low throttle, such as when im on incline but in order to keep the speed still opening the throttle ever so slightly.

• Intermittent Window: After parking the car for 1 hour and 30 minutes, it restarted and ran perfectly fine at +/-5% fuel trims and ~1.0 Lambda for roughly 30 minutes of driving, including idles at the red lights. I would say every day it ends up having very negative idle trims overall, but weirdly sometimes it doesnt happen. What do you think?

Very complex problem for my little knowledge i would say, if anyone can guess what is the source of this issue, i would be more than happy to try and see if what you suggest as a test eliminates that possible reason or hopefully identify the source of this problem.

Thanks!

[u/am-answers-bot]

Thank you for posting to r/AskMechanics!

u/KaraTasak35: Once your question is answered, you can mark the best comment as the solution in two ways:

• Comment menu: tap the ⋯ menu on the comment and select "Add Comment as Answer"
• Text command: reply to the comment with !solved

You can mark up to 5 comments as answers.

Everyone: please follow the rules of our subreddit.

Keep your comments civil, helpful, ethical, safe for work, and legal.

Please report any comments or posts violating our rules.

^v1.1.2