BMW N-se­ries

Just how bad are the four-cylin­der petrol en­gines?

Car Mechanics (UK) - - Contents -

It doesn’t have a glow­ing rep­u­ta­tion in the trade, but what makes the N-se­ries petrol unit tick? An­drew Everett is your guide.

Those of you who read Steven Ward’s Dealer’s Di­ary col­umn may have seen his di­a­tribe in the De­cem­ber 2017 is­sue about the N43 en­gine pro­duced by BMW, used from late 2007 in four-cylin­der petrol 1- and 3-Se­ries cars un­til the re­place­ment N20 and N13 units ar­rived around five years ago. It has a well-de­served rep­u­ta­tion for spell­ing trou­ble.

The N43 was the fi­nal de­vel­op­ment of the en­gine se­ries that be­gan in 2001 with the N42. Back then, the N42 2.0 was a rev­o­lu­tion­ary unit and was built in a brand-new fac­tory in Hams Hall, not a mil­lion miles away from the Rover plant at Long­bridge, where it was sug­gested it could be used in a fu­ture range of Rover cars – that didn’t hap­pen, of course.

The spec of the N42 was im­pres­sive for late 2001 and it would still be thor­oughly mod­ern if launched again to­day. It in­cluded an all-al­loy con­struc­tion with a lad­der-type block – a reg­u­lar al­loy block but with the main bear­ing caps built into a frame for rapid assem­bly and ex­tra strength – as well as Dou­ble Vanos cam tim­ing, 16 valves with Sim­plex chain-driven camshafts, dou­ble cam sen­sors (one for each cam) and Val­vetronic.

The last of these was BMW’S new con­cept where the lift of the in­let valves – and thus en­gine speed and load – was vari­able thanks to an ec­cen­tric shaft run­ning the length of the head in be­tween the valves, in­let cam and rock­ers. A 12V elec­tric mo­tor bolted to the top of the head turned the ec­cen­tric shaft via a toothed quad­rant to in­crease or de­crease valve lift, to al­low the en­gine to breathe more eas­ily with­out the re­stric­tion of a throt­tle body. The mo­tor was con­trolled by a sep­a­rate ECU that took in­for­ma­tion from the en­gine ECU, based on load, revs and a sen­sor on the front of the Val­vetronic shaft. Fur­ther re­fine­ments over the old iron block/al­loy head M44 that pre­ceded it (E36 and Z3) in­cluded a plas­tic in­let man­i­fold and cam cover – this helped keep the weight down, with the new en­gine weigh­ing 87.5 ki­los as op­posed to more than 100 ki­los for the old unit.

The tim­ing chain set-up was all new and was de­signed for rapid assem­bly. This was a cas­sette-type chain assem­bly, low­ered down into the en­gine with the head fit­ted as a com­plete unit with the chain, the plas­tic guide rail and piv­ot­ing ten­sioner blade, crank sprocket and the two Vanos units. The N-se­ries crank had no ‘nose’ onto which the crank sprocket might slide be­cause it was all low­ered in as one unit and the big crank end bolt tight­ened to 300Nm. This im­mense torque meant there was no lo­cat­ing key­way, re­ly­ing on the torque of the bolt to keep it to­gether.

The Vanos units were not keyed to the camshafts and stretch bolts were fit­ted. The Vanos units also dif­fered from pre­vi­ous BMW prac­tice – on the pre-2005 six-cylin­der en­gines the com­plete oil-pres­sure-fed Vanos unit was fit­ted to the front on the head and lo­cated onto the front of the cams. On the N42, though, sep­a­rate Vanos units were built into the cam sprock­ets them­selves. They were ac­ti­vated by oil pres­sure and reg­u­lated by the elec­tronic so­le­noids in the front of the block that also con­trolled the oil pres­sure go­ing to the units.

The al­ter­na­tor, power steer­ing pump and wa­ter pump all ran off one aux­il­iary belt, with a spring ten­sioner fit­ted to the front of the block. To save space and weight, the wa­ter pump was com­bined with the power steer­ing pump and bolted to the side of the block. Due to the Val­vetronic de­sign, there was no in­let man­i­fold vacuum ( just like a diesel) and so a camshaft-driven brake vacuum pump pro­duced vacuum for the brake servo.

The oil pump was in the sump, in­te­gral with the bal­ance shaft assem­bly bolted to the block and driven by a se­cond short chain from the front of the crank. An Ecu-con­trolled ther­mo­stat reg­u­lated en­gine tem­per­a­ture.

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