M271 Running rich and running lean
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Joined: Jun 2025
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M271 Running rich and running lean
Hi guys, I have a 2005 slk r171 with m271 engine and I have this check engine light, when I got it checked by a local mechanic with an obd scanner. He said that the fuel mixture was lean and rich at the same time and he wanted to check the valve timing. After that I took it to a STAR diagnosis and the only thing that wasn't in order was the air intake temperature. It was reading high... So the guy who ran the STAR diagnosis thought about it (he tought it was sucking air from somewhere) and he told me to change my MAF sensor to see if it is gonna fix itself, he also said that the timing was off by one degree which is nothing. After changing the MAF sensor (I bought a brand new HELLA one) the check engine light didn't fix itself so I erased it and I am currently waiting if it is going to appear again... I also have this noise on cold starts which seems to be some kind of grinding then after 3 seconds it stops (I think the oil viscosity might be the reason because it is currently 10w40 which is bad oil for this car). Any advices what could be the reason for this check engine and what to check?
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Joined: Jan 2026
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Check if there are leaks after the MAF (cracked PCV/breather hoses, intake boot, loose clamps, vacuum lines). A small leak can cause lean codes and unstable trims. The intake air temperature sensor itself and its wiring/connector. On many setups it’s part of the MAF, but on others it’s separate. If it reads unusually high, the ECU will miscalculate fueling. O2 sensor live data and fuel trims. If trims are swinging, it helps point to an air leak vs sensor vs fueling issue. Also check fuel pressure (weak pump/regulator can cause lean under load).
About the cold-start grinding: that’s often related to the M271 timing chain/cam adjuster/tensioner area. Oil viscosity can make it worse, but a brief “grind/rattle” on cold start is a known warning sign. I’d avoid driving it hard until you confirm chain/tensioner condition and correct oil spec.
About the cold-start grinding: that’s often related to the M271 timing chain/cam adjuster/tensioner area. Oil viscosity can make it worse, but a brief “grind/rattle” on cold start is a known warning sign. I’d avoid driving it hard until you confirm chain/tensioner condition and correct oil spec.
