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Some 2014 Focus st issues.

1. (completely stock) Every so often the engine starts to lug like it's about to stall but at 3k RPM's. Service light comes on *Engine fault service now* (Scanned it: P0236 Boost Sensor). I park the car to let it idle and the RPM's start to jump between 1200 to 700. I rev up to 1500 and drops to about 200 just low enough where it almost stalls out but recovers back to normal idle. Turn off the car and turn it back on fixes it. (Currently doesn't happen anymore)

2. (MBRP 3" catless downpipe/ MBRP 3" catback) I've been running this exhaust for about 4k miles now. I already know that I'm going to smell the exhaust fumes at stop signs and lights whatever but for about a week now, the smell has changed. It smells MUCH more rich, nothing electrical/clutch/brakes/plastic but maybe oil i don't know. I've tried running 87, 89, and 93 octane to see if it changes but it doesn't. The smell is coming from mainly inside the car and towards the engine, I got behind the car and smelled directly from the exhaust and it smells like the normal fumes from before it started smelling different.

3. (MBRP 3" catless downpipe/ MBRP 3" catback) A little before the smell issue was known the engine was acting weird whenever I filled my tank. I fill it, turn it on, the starts doing the engine rev drop to about 200 like in #1 but this time I'm not giving it any throttle just drops and rises from 200-700. I drive for a couple minutes and between shifts the exhaust backfires incredibly loud. At decelerating in gear 2100-1800 RPM's the exhaust backfires constantly until I give it throttle again then eventually stops.

The only thing I can think of for #2 and #3 is that I installed the exhaust but haven't gotten a tune for it and the car is just now being affected after the 4k miles driving with it. I've read a lot of forums saying it's ok to not get it tuned and I'm aware that tuning it removes the check engine o2 sensor and actually gives it the full potential power but who knows.
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