PROBLEMS, PROBLEMS

The N-series unit has a catalogue of potential troubles, with the later N43 being by far the worst. Frequent oil changes can help, but while the N42 and N46 can be kept plodding on, the N43 really is a disaster area.

TIMING CHAIN

Chain issues are infamous. The links wear, causing chain stretch, resulting in the chain thrashing against the rails and eventually breaking them. Look into the oil filler cap and if the guides are a dark brown or burnt orange colour (they were ivory coloured when new) then they will be brittle and need changing. Replacing the chain isn’t difficult, but involves locking tools and a ¾-inch breaker bar and socket to get the 300Nm crank bolt undone. It requires a clean working area (not out in the road) and patience – getting the locking pin into the flywheel is very frustrating but essential as you cannot time it up on Top Dead Centre down a plughole. If the guides have broken up, the sump needs to be dropped to get the bits out. Ignore this and the stray bits will block the oil pick-up and the engine will seize. If the guides are all in one piece then you can do it with the sump still attached by avoiding turning the oil pump chain and thus disturbing the balance shafts. A new chain kit from Febi Bilstein with new cam/crank bolts (you cannot reuse the old ones – they are stretch bolts) costs around £120, but be careful with cheap locking tool kits as they can require work with a file or a grinder to get them to fit. For some reason N43 chain guides don’t last as long and we’ve heard of them failing at as little as 30,000 miles.

VANOS

Vanos units can fail, although N43 ones are better. Bad ones can rattle and throw up cam timing fault codes. Use a 16mm socket on a breaker bar and see if the Vanos units can be moved in relation to the camshaft (they should be locked), because they’re scrap if they do. With the timing chain tensioner plunger removed and the crank and cams timed up and locked in place, it’s simple enough to fit a new or secondhand Vanos unit, but you must fit a new bolt. Vanos units are the same for N42, N46 and N45 engines, as is the chain assembly; N43 ones are different.

VALVETRONIC

The eccentric shaft can wear on the tips of the lobes, affecting the amount of valve lift so that the engine misfires at idle but smooths out over 1000rpm – if the misfire reduces with the Valvetronic sensor unplugged, that’s the issue.

This will cause an MOT fail on emissions. Unplugging the sensor can often scrape a pass, but repairing it is an involved job that is rarely worthwhile on a £500 banger. The Valvetronic motors can fail and, to be honest, are best replaced by new units as they cost less than £100 online. You will need proper diagnostics to both reset the Valvetronic and Vanos adaptations, as well as to make sure it’s the motor at fault and not the sensor. Replacing the sensor requires extreme care to avoid dropping the three 10mm bolts into the engine.

HEAD GASKET

Beware coolant leaks from the back of the head gasket. You might think it’s leaking from the plastic coolant elbow, but Sod’s Law says it won’t be. Alloy corrosion of the block around the bore liners will cause gasket leaks and pressurising, but forget about repairing it – a good used engine is the only way to go.

OIL BURNING

The N42, N45 and N46 can get smoky in old age. It’s a combination of deteriorating valve guide oil seals (rare) and, more commonly, worn piston rings. The acid test is to leave a warm engine idling for two minutes before blipping the throttle. If a small cloud of blue smoke issues from the tailpipe then the engine hasn’t much of a future. Anything over 100,000 miles is prime for a bit of ring wear – not an issue on a £400 E46 winter hack, but not something you want on a £2500 car.

OIL LEAKS

The N-series can really leak oil, particularly the N42, N45 and N46. The main leak points are the plastic cam cover’s rubber gasket, the oil filter housing’s rubber seals, the vacuum pump O-ring where it bolts to the head, the dipstick tube O-ring and the rubber Vanos solenoid O-rings. By 100,000 miles you’ll find that they have all been baked hard and become brittle and crack-prone. Replacing the whole lot – it’s not worth skimping – will cost you £50 in parts and you’ll need to degrease the engine and undertray before blasting it with a serious pressure washer. But it’ll transform a car that marks its territory, as well as stinking of burnt oil. The really nasty one is the sump gasket, which requires removal of the front subframe. On the E46 it’s not so bad, as once the engine is supported on a brace, you can unbolt the steering rack and drop it down with the wishbones, but it’s more involved on later cars. So do the easy bits first, clean everything up and maybe you’ll be lucky.

N43 DRAMAS

The N43 is notorious for running faults and misfires. It’s not really BMW’S fault, as they were forced down the direct-injection route by emissions laws and the bits that fail were made by other companies. The original Piezo injectors are dire and should have been replaced by new ones by now – look to pay around £170 each, but forget used ones and remember that they will need coding to the car. Coil packs were another problem area and four new Bosch ones at around £30 each is the solution, unless you enjoy chasing misfires. Once you’ve fitted new coils and injectors, you then have the NOX sensor, which can fail and make the car run massively rich. It costs £434 from BMW with no aftermarket alternative, and used ones are rare – would you want one anyway? Thankfully, the three Lambda probes are pretty reliable.

N43 plus points? Well, they don’t suffer piston ring wear like the older units and oil leaks are less of an issue, but overall the N43 is worth avoiding.

OTHER ISSUES

The crank sensor is positioned under the inlet manifold by the starter, making it impossible to see, let alone change, until the inlet manifold is removed. Removing the manifold is not that bad, although the two bolts that secure the base of the manifold to the steady bracket (not fitted to the N45) are a pain to undo and replace. The crankcase breather assembly lives under the manifold and, if the engine uses oil, you might as well replace it. With crank sensors, only use a genuine part or one from Febi, as cheap ones rarely work for long, if at all.

Also, the brake vacuum pump can fail (the servo won’t work) and it’s not so easy to replace.

Here’s the exhaust camshaft position sensor. Both inlet and exhaust sensors are the same part number, but N43 ones do not interchange with those from the older N52 (six-cylinder), N45, N46 engines. As with the crank sensor, avoid cheap aftermarket items.

This is the rear exhaust cam locked – the inlet one slides in first, then the one pictured above. The cams are locked, then the chain is pre-tensioned with the threaded tool and, once set up, the Vanos bolts tightened and the locks removed.

On these units, there are Vanos control solenoids mounted on the front of the head to control the flow and pressure of oil to the Vanos units. These need periodic removal and cleaning, along with new O-rings to stop annoying engine oil leaks.

This is the Valvetronic motor, a 12V unit that spins at great speed to open and close the Valvetronic shaft. These can fail but often hamfisted mechanics break off the end when disconnecting the plug. New aftermarket ones are £80.

On the N42 unit, the Valvetronic system was operated by this separate ECU that sits outside of the ECU box. On N46 engines, this was discontinued and VVT control handled by the main engine ECU.

Here is the N46 engine in a 2007 320i. While technically improved, the N46 seems more prone to oil leaks – it has been said that they run hotter for emissions and the rubber gaskets have a harder time.

Here is an N42/45/46 timing tool set. Top is the gold front locking bar and below that, on the right, are the black tools that lock the back of the cam – my cheap set needed a mod with a grinder! On the left are the crank locking pin and chain tensioner.

Here we can see the reluctor plates and the holes drilled in. When you remove the cam cover and see this for the first time, the fact that the plates aren’t a mirror image of each other makes you think the cam timing has jumped when it hasn’t.

The front locking tool located in situ. It has two dowels that lock into the small holes on the cam sensor reluctor plates fitted to the front of the Vanos units. If these aren’t in the right place, cam sensor errors are unavoidable.

Here is a typical shattered chain guide. By the time it’s a dark brown colour it’s absolutely finished and must be replaced. It will be brittle and break up too easily. The sump must be removed to clean up the debris.

Here is a new and old timing chain kit from an E90 320i – same parts for N42 and N45. When new, the plastic guides are a light ivory colour, but end up a grotty dark brown or orange, at which point they must be replaced.

This yellow arrow shows the position of the crankshaft position sensor and why the manifold has to come off. Above the starter is the wiring junction box and the crankcase ventilation system, secured to the inlet manifold by two 10mm nuts.

Here is an early N42 as fitted to the E46. It’s been said that the N42 suffered less from oil burning than the later engine – possibly due to running cooler? Regular 7000-mile oil changes are the key to a long life.

Oil filter housing leaks are common, but later engines suffer more from this because the rubber gaskets get hot and become too brittle to seal effectively. Replacing all of the gaskets is a good day’s work.