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Originally Posted by Sschwab
Great work!
Would you say that running thicker oil contributes to the rod bearing failure? If so, would using a thinner oil resolve much of the issue?
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Nope has been pretty throughly tested. Haven’t seen any objective evidence to support using other than the factory spec oil.
Today driving around and coasting downhill noticed pressure dip to about 20 psi. That’s the problem. BMW decided to allow way too low of dip in pressure when not under load.
It’ll work ok until it doesn’t. I suspect parts wear enough at rod bearing interface. Example: the crank opening around the rod journals wears a bit or the sides of the bearing shells wears and the slightly bigger opening gets you a thinner oil film on the journal.
There’s also a known issue on initial fill of the oil circuit that BMW developed a first start procedure of cranking for 3 sets of ten seconds with no gas or spark to prep the oil circuit before starting after any job that interrupts the oil circuit.
It's a band aid for something intrinsic to certain motors that introduced a problem. This procedure is new to the automotive world.
I discovered the feedback on N55 is a simple pressure transducer that maps 0.5 to 4.5v and 15 to 150 psi (numbers may be slightly different from memory)
Anyhow a simple voltage bridge will tell the DME pressure is lower than reality and it’ll make more.
Ideally adding a little extra on the bottom end I want minimum pressure to be at least 30 psi.
For several of the n55 motors I've been connected with that failed, some situation developed that caused a disruption in the oil volume flowing though the rod bearings including: boost pressure in the crankcase, OFHG seal blowout.
There's no room for error. When some disruption happens, a small safety margin could be saving many motors.
There's a whole "cottage industry" insisting that preventive replacing of rod bearings is the solution. It's a band aid. When things are working properly rod bearings are not a wear item.
The ones i recently replaced at 92 kmi do show a wear pattern I'm confident is from start stop and I'm glad i replaced them (and disabled default start/stop).
Certain BMW motors are known for rod bearing issues and to me it's no surprise that aging motors with DME controlled fuel pumps are at the top of that list.
–awr–
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