BMW 4 Series: N20 2.0T timing chain guide and tensioner failure (420i/428i)

The four-cylinder N20 in the 420i and 428i drives its timing chain off the crankshaft through plastic guide rails, with a second chain driving the oil pump. On engines built before BMW's revised parts arrived the guides go brittle and break up, the chain stretches, and the engine either sets a camshaft correlation fault or jumps timing and bends the valves. The oil-pump drive chain on the same end of the engine can also fail, which starves the engine of oil with no warning at all. This is the single most expensive thing that can go wrong with an early 4 Series, and the noise that gives it away lasts only a few seconds after a cold start.

• Rattle or whirring from the front of the engine for a few seconds after a cold start
• Engine warning light with a camshaft/crankshaft correlation complaint
• Loss of power, rough running, or a refusal to restart
• Oil pressure warning immediately before a failure
• Plastic fragments found in the sump or oil filter at a service

This one is decided on a cold start, so insist the car has not been run that day and be standing there when the key is first turned. Stand at the front of the car with the bonnet up and listen to the first five seconds for a rattle or light whirring from the front of the engine — anything more than a brief tick is a walk-away on a car this expensive to fix. Ask for every oil-change invoice and check the intervals, because N20 chains that fail almost always sit behind long intervals or the wrong oil. Ask directly whether the timing chain, guides and tensioner have been replaced, and whether the revised parts were used. On the drive, watch for any warning light and for hesitation as you accelerate hard from low revs. If a workshop inspection is possible: have a BMW specialist read the camshaft-to-crankshaft correlation values, which show a stretched chain before it becomes audible.

Parts involved: Timing chain.

$3,000 to $12,000 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has the timing chain, guides and tensioner been replaced, and with which parts?”
• “How often has the oil been changed, and at what kilometres?”
• “Is there any rattle on the first start of the day?”

The failure described above is documented in the sources below. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them.

• Euro Premium Parts — 'BMW 4 Series F32 Reliability Guide: Which models to avoid and years to buy'
• Euro Racing Parts — 'Buying guide: used BMW 4 Series F32, which engine to choose?'
• Engine Patrol — '12 Common BMW 4 Series Problems'