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Hello all. I'm working on a W203 C200 Mercedes with a 1.8L I4 engine that begins to misfire on warm-up.
All coils and plugs were swapped previously by mechanic.
Misfire for all cylinders and O2 B1S2 heater circuit codes were in memory. I cleared them but only the O2 code returned and engine still running rough..
STFT is stuck @ 0.0 and LTFT @ 14.1. Car seems not to want to go into closed loop and STFT won't respond to lean condition (brake booster line pulled) or rich condition (adding throttle cleaner). Engine runs smoother with fuel added
I discovered that when the car is running badly it won't enter closed loop, STFT stays 0.0, LTFT 14.1 and B1S1 o2 is 2.3 +/- 0.1. The car runs just fine when the B1S1 is @ 1.5 (normal fluctuations). It'll enter closed loop, fuel trim will adjust LTFT is fixed @ 14.1 and STFT was high pointing me to find the broken vac brake booster line fitting causing lean condition.
Anyone ever met this issue with a car not going into closed loop intermittently? I'm thinking Upstream o2 going crazy or bad ECM.
Would the vac leak cause the PCM to be stuck in open loop? I never saw that happen. One minute it enters closed loop and STFT works normally and the next STFT is 0.0 and open loop seems stuck.
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