The E90, E92 and E82 335i and 135i used the N54 first and the N55 later. They look similar in the engine bay and they are not the same engine. Workshops still mix the names up on the phone. The stamp on the block is the name that counts.
Two different sixes
The N54 has two small turbos, high-pressure injectors that were revised more than once, and a high-pressure fuel pump with its own campaign history. The N55 has one twin-scroll turbo, a different intake and an electronic wastegate. You cannot move an N55 turbo onto an N54 and call it a repair, and you should not order an N55 for a car whose computer still expects an N54.
The N54 list
Ask whether the injectors are the later index, and whether the high-pressure pump has been replaced. Wastegate rattle on the N54 is common and does not always mean the turbo is finished, but a turbo that howls or smokes is a different matter. Look at the oil filter housing and the valve cover gasket. Both leak onto the exhaust side.
A used N54 is a fair buy when the code matches and the turbo housings are dry. It is a poor buy when you are also inheriting a failed pump and a set of early injectors with no plan to change them.
The N55 list
On the N55 the wastegate actuator and the charge pipe are the usual extras. The engine itself is simpler to live with than the N54 if the oil leaks are dealt with at fitting. New oil, a new filter and a cooling system that actually holds pressure matter more than a detail on the listing title.
Parts worth replacing now
On an N54, budget for the later injectors and a known-good high-pressure pump if the engine did not come with proof of either. On an N55, budget for the charge pipe and a look at the water pump. Both engines want new oil, a new filter and a cooling system that has been pressure-tested before the first long drive.
Wastegate rattle that was noted before you bought should be written on the job card, not discovered by the customer. It is a different conversation from a turbo that smokes.
The yard is at Andreas Meyer Straße 38, 22113 Hamburg. Call or WhatsApp +49 15510 030835, or email autowerkeengines@gmail.com, Monday to Friday, 9:00 to 16:00 CET. Shipping is 249 € per order. We are an independent seller, not BMW AG.