I don't feel like typing it again forgive the copy and paste:
They're old BMWs who appeal much more to the sideways hat crowd than members of the BMWCCA they're pretty well confined to low resale value. Thankfully this means you can have a truly incredible car for next to nothing. They're also not nearly as crummy as people suggest and there is no reasonable service that requires drivetrain removal.
Common failures and solutions:
Early car (07-08) ecu failure: Injector drivers can be replaced by any electronics repair place. Alternatively the later MSD81 ECU is an easy retrofit with the right software.
Injectors: Common and expensive fault, if your car has never had a FULL SET replaced under warranty/recall action BMW will install and program a set at no charge. If you have to pay for 'em FCP Euro is the go to, not the cheapest but close and lifetime replacement.
High Pressure Fuel Pump: The later revision and aftermarket pumps seem pretty stout this honestly isn't a big problem anymore. The sub $200 aftermarket pumps are surprisingly okay.
Water pump: Again the early pump was kinda junky but the aftermarket seems better. Currently over 100k on a VDO pump on my personal car.
Carbon build up: It's a DI engine walnut blast every 30k if stock, 50k with a good catch-can setup, or never with reworked PCV and/or port/methanol injection
Leaks: Yeah they leak from everywhere. Valve cover is very common but it's not a gasket failure, replacement plastic covers are out there for $100 and work for awhile, the BMW plastic cover is the best but close to $500, M18 and VTT both now make cast aluminum valve covers expensive at $600ish but no more issues. Those covers also allow you to rework the PCV system helping to prevent blowing other gaskets out. Oil filter housing gasket is common and important as it pukes oil onto the belt drive, if the belt fails on one of these it will almost certainly be sucked into the timing chain assembly. Oil pan gasket is another very common one. It's hot under the hood of these things gaskets fail. Replace neglected gaskets with good replacements and ignore for a long time.
Misfires: Stock ignition system is garbage N55 Eldor coils are the drop in and go upgrade, I prefer for the money the Precisions Raceworks coil kit
Turbos: This again is largely over with, the wastegate arm bushings in the BMW turbos aren't great and you end up with rattling wastegates and boost control problems. Lots of non BMW turbos out there fairly inexpensive that cure this.
The new one seems to be mysterious low compression on a cylinder or two, the fact is these things are cheap and make big power easily they routinely get the tar beaten out of them without mercy.
Rod bearings: I simply disagree with the rod bearing fault... I cant say I recall an N54 having rod bearing failure without cause. That said given the age and mileage of most of these cars many have worn (note I didn't say failed) bearings it's reasonable to replace them if you're in there
If a stock stock N54 car came into the shop (few are) with an enthusiast owner asking for it to be bulletproofed" my recommendation would likely be as follows
M18 Valve cover
Atmosphere vented crankcase
Precision Raceworks Coils
Front main seal retainer/guard plate
New tensionser and idiler pullies, Gates Racing belt
VDO water pump and thermostat housing
Mahle upgraded oil filter housing gaskets, Burger motorsport oil cooler valve. Factory oil cooler at an absolute minimum
Index 12 fuel injectors
Carbon cleaning
Redline fluids in differential(s) and transmission. Liquimoly 5w40 in the crankcase.
Aluminum cylinder head water fitting
Replace as much under hood rubber as budget will allow.
Metal charge pipe and cone intakes
MHD tune (necessary for the coil upgrade)
This is an easy 350hp car you could drive to the moon. Intercooler, charge pipes, and downpipes make a nearly 500hp car you could drive to the moon.
My car is close to 200k early last year it was driven from PA to NH without so much as opening the hood, in the fall I drove from PA to The Dragon and then straight across NC to a wedding (yes my best friend from college is a dolt who got married during COVID) the only thing I didn't feel uneasy about all weekend was the car. In the six years I've owned mine it has done three years back to back as a sales repmobile driving all over the state everyday, took a break to be my toy and spent a couple years being railed on every weekend (drag racing, half mile, autox, track day you name it) then my ex used it as her repmobile for the better part of a year. They are fantastic everything cars a little heavy and the open diff is a bummer but you can fix everything wrong with them relatively easily.
