Burnt valves on 4-6 cyl, but lean mixture on all cylinders?

Refractory · 10-02-2020, 04:43 PM

Engine: n52b25, huge problems in the lower rpms

Okay, so - Dry + Wet compression test results:
Dry compression test - between 12.5 and 13 bar across the board (180-190psi) which is obviously pretty low, piston rings are worn for sure.

Wet compression test is where it gets interesting - cyl 1-3: 17 bar (250psi), so obviously the few drops of oil help fill the gaps caused by the worn piston rings, but cyl 4-6 with a few drops of oil - between 13.5 and 14.5 (190-210psi), burnt valves seem to be the apparent cause. So I would understand a lean condition on bank 2 (cylinders 4-6), but why would bank 1 (cylinders 1-3) run lean?

TLDR:: Why would the car be throwing Lean mixture codes on ALL cylinders (both banks), if 1-3 only have worn piston rings and valves seem to be in order? Or would the huge compression loss from the piston ring wear also make it burn lean?

Meeni · 10-09-2020, 02:54 AM

I believe that seeing a lot of unburnt fuel in the exhaust (as spewed by your leaking cylinder with not enough pressure to ignite the fuel, witnessed by a high difference in pre-cat and post-cat air sensor readings) would cause the DME to lean it to compensate for what is measured as overly rich mixture?

Or conversely the O2 sensors are covered in soot from the unburnt fuel and readings of lean are incorrect because the sensors are dirty?

10-02-2020, 04:43 PM	#1
Refractory New Member 2 Rep 7 Posts Drives: E90 325i 218hp, N52B25 Join Date: Apr 2020 Location: Plovdiv, BULGARIA iTrader: (0)	Burnt valves on 4-6 cyl, but lean mixture on all cylinders? Engine: n52b25, huge problems in the lower rpms Okay, so - Dry + Wet compression test results: Dry compression test - between 12.5 and 13 bar across the board (180-190psi) which is obviously pretty low, piston rings are worn for sure. Wet compression test is where it gets interesting - cyl 1-3: 17 bar (250psi), so obviously the few drops of oil help fill the gaps caused by the worn piston rings, but cyl 4-6 with a few drops of oil - between 13.5 and 14.5 (190-210psi), burnt valves seem to be the apparent cause. So I would understand a lean condition on bank 2 (cylinders 4-6), but why would bank 1 (cylinders 1-3) run lean? TLDR:: Why would the car be throwing Lean mixture codes on ALL cylinders (both banks), if 1-3 only have worn piston rings and valves seem to be in order? Or would the huge compression loss from the piston ring wear also make it burn lean?
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10-09-2020, 02:54 AM	#2
Meeni Gateropode 332 Rep 2,846 Posts Drives: BMW 330i 06 Join Date: Apr 2012 Location: TN iTrader: (0)	I believe that seeing a lot of unburnt fuel in the exhaust (as spewed by your leaking cylinder with not enough pressure to ignite the fuel, witnessed by a high difference in pre-cat and post-cat air sensor readings) would cause the DME to lean it to compensate for what is measured as overly rich mixture? Or conversely the O2 sensors are covered in soot from the unburnt fuel and readings of lean are incorrect because the sensors are dirty?
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