The N55 was a welcomed replacement for the potent but problematic N54, but in recent years, it’s developed a reputation for eating connecting rod and main bearings. Even though the internet cites frequent failures, it got me wondering: Is that reputation justified, or are a few isolated cases being referenced across the internet? Before getting into that, I want to dive into a quick back story of my first experience with an N55-powered BMW and how a specific service bulletin may clear its potentially undeserved reputation.
In May 2020, I was in the middle of purchasing a CPOV 2017 BMW X5 xDrive35i from a BMW dealership near my home. It was during peak COVID, and I was not allowed inside the showroom to do paperwork, but it was a nice day, so I hung around outside by all the technicians. I struck up a conversation with a few of them while they were talking about E36s, S52s, and other things I’m well versed in. Along the way, I mentioned my new purchase, leading to further shop talk about the N55 and things they’ve seen and experienced while working on them (a true free wealth of knowledge).
The shop foreman let me in on some details about a few engine failures after repairs, like oil filter housing gaskets, oil pan gaskets, oil coolers, and VANOS adjuster replacements. It struck me as odd, considering the N52 and N54 engines never had any problems (that I’ve seen/heard about, at least) with eating bearings after those repairs. He then told me that BMW issued a service bulletin in 2015 about the importance of priming the engine’s oil system. It applies to all BMW engines having had any services that interrupt the engine’s oil supply circuit.
Common Services Applicable To BMW SI B11 09 15
- Removal and replacement of engine bearings
- VANOS sprockets/gears
- Bed plate resealing
- Engine oil pump removal/replacement
- Oil filter housing
- Cylinder head
- Engine oil cooler
- Anything that interrupts the engine oil supply circuit
The million-dollar question is this: why do the above repairs seem to affect the N55 more than other engines? I believe the answer lies with the modifications BMW made to the N55’s oiling circuit, which reduced overall oil circuit volume thanks to fewer oil ducts in the cylinder head. The update was to accommodate revised and improved VANOS solenoid valves. However, evidence points to the change harming oil flow after a major repair in which the oiling circuit has been disrupted in some way. I’ve also seen some theories about cavitation issues in the oil pump, but I’m not sure if that’s the case here.
The fact of the matter is it’s never going to be a bad thing to prime an engine after a major repair in which a lot of oil has been drained. It’s also not that much more work to follow a priming procedure either, so as a matter of due diligence and care for a job well done; the standard practice should be to prime the engine and ensure that oil has circulated throughout the engine with good oil pressure.
How To Prime A BMW Engine After Repair
BMW’s oil priming procedure is exceptionally straightforward and can be applied to other vehicles. It involves deactivating the fuel system (disconnecting injectors and cutting off the power supply to the fuel pump) and cranking the engine a few times. Here are the steps for an engine that has been repaired (the procedure for a new/rebuilt engine is different).
Modified verbiage from SI B11 09 15:
- If the engine has been drained before the repair remove the engine oil drain plug again to remove any residual oil that may have settled in the engine oil pan during the repair if the engine oil pan was not removed entirely.
- Reinstall and torque the engine oil drain plug (with a new seal ring) per the applicable repair instruction.
- Remove the oil filter housing cover and verify the oil filter is present. Reinstall the oil filter housing cover and torque it to the proper specification noted in the applicable repair instruction.
- Fill the engine with the proper type and amount of engine oil, as specified in the applicable repair instruction.
- Connect a battery charger to the vehicle. (I have also removed the spark plugs before to remove the load off the starter and aid in reducing battery draw/drain)
- Remove the electric fuel pump fuse. Refer to the applicable wiring diagram using the VIN number of the vehicle in ISTA/D.
- Crank the engine for 10 seconds.
- After 10 seconds have elapsed, stop the starter, and allow the starter to cool for 20 seconds.
- Repeat steps 7 and 8 two additional times.
- Reinstall the electric fuel pump fuse and start the engine. Verify proper engine operation.
- After the engine has reached operating temperature, check the engine oil electronically or with the dipstick, and top up the engine oil as needed.
While the service bulletin doesn't center around the N55 specifically, it exists due to the issues BMW has had with that engine in the service environment. The reality is that any major repair that involves opening the engine oiling circuit, which disrupts the oil supply in the engine oil galley, should be followed by an oil priming procedure as a due course of action to ensure adequate oil supply to the entirety of the engine. It does not take much to oil starve camshafts/camshaft bearing ledges, connecting rod bearings, or main engine bearings. An engine running with disrupted oil flow will quickly destroy those components. A few extra minutes of work up front can save you several hours (or days) replacing an engine that damaged itself from lack of lubrication.
Is this procedure important for your N55 engine after a repair that disrupts the oiling circuit? Based on a few horror stories I’ve heard and first-hand information shared with me: my answer is yes. However, I believe (and reiterate) that the procedure outlined by BMW is a good practice and should be followed for any engine that has had major repairs on the oil supply or has interrupted the oiling circuit.
