Z4. 6Cylinder most reliable engine.

[deltasierra]

All need proper maintenence, the S54 most of all, warming up properly is important, N52 has an electric water pump that can fail otherwise similar to M54, which is still a complex engine, that does need above average DIY skills to avoid big garage bills.

 

[Mr Tidy]

There was never a 3.0i with the N52 engine in the UK but I seem to remember that there was in the USA, and maybe some other countries.

I don't think the 2.5Si offers any significant improvement in fuel economy over a 3 litre IME. I've had an E46 325ti Compact with the M54, an E91 325i with the N52 in Si tune and currently an E90 330i also with the N52 and they all averaged 32/33 mpg - much the same as my 2 3.0Si Coupes did.

A 3 litre may cost more to buy, but you're always likely to get the extra back when you sell!

Running costs for all of them were pretty much the same. I've had two electric water pumps replaced on N52s, but costs weren't much different to the cooling system my M54 needed when the radiator was bent as a banana.

But "deltasierra" is right about the S54 in the Ms needing much more regular TLC, which obviously costs more. Well worth it though.

 

[Chris_D]

M54, which is still a complex engine, that does need above average DIY skills to avoid big garage bills.

nah