Before you buy a used BMW Serie 5 (E39).

Look out for: Coolant loss without an obvious puddle, sweet smell, low-level warning, steam in the engine bay, heater stops warming and the temperature needle rises. Expansion tank failure is usually sudden; the water pump can fail with no visible leak.

Ask them to check: With the engine cold, check the level and look for cracks or dried coolant on the tank, radiator neck and thermostat housing. At a garage: pressure-test the system and inspect the pump. If the temperature rises, stop somewhere safe: a single overheat can warp the aluminium head. Never open the system hot.

Look out for: High oil consumption, bluish smoke, rough idle, whistling from a vacuum leak, engine light with lean-mixture codes and very strong suction when trying to remove the oil filler cap at idle.

Ask them to check: At a garage: smoke-test the intake to find leaks (CCV hoses, intake boots, DISA valve seal, valve cover gasket) and check whether the dipstick tube return is blocked. A cold-climate version of the system exists for cold regions.

Look out for: Loss of torque and pull below about 3,000 rpm, louder or unsteady idle, the engine bogs when the air conditioning is switched on and sometimes VANOS fault codes. On 1998–2000 M52TU engines, jolts or stalling on cold starts.

Ask them to check: Read the engine codes and, at a garage, compare actual and requested camshaft positions in live data. Rule out vacuum leaks and the CCV first, as they cause similar symptoms. On M52TU engines with cold jolts, the kit maker describes a test with the intake VANOS solenoid unplugged.

Look out for: Wiring that cracks with age: the car will not start or cuts out (ignition switch), the cluster, radio or navigation fail and the tail lights, third brake light, plate light or rear wiper go out. Suspension needing constant upkeep (wishbones, top mounts, drop links and thrust arms, with a shaking wheel under braking) and failing air levelling. Rust on the sills, jacking points, fuel flap, door edges, tailgate and bonnet. On automatic diesels, a GM gearbox that fails before 200,000 km and swirl flaps that can come loose.

Ask them to check: Test every switch and light; check the underside and suspension; on automatic diesels, change the gearbox oil often and check the flaps; campaigns by VIN.

Look out for: Usually none until it fails. A common early warning: oil seeping around the flap spindles on top of the engine. If a flap or its screw comes loose and enters a cylinder: sudden heavy knocking, loss of power, smoke and possible turbo damage as well.

Ask them to check: At a garage: confirm whether the manifold has swirl flaps (some versions do not), look for oil around the spindles and play in the flaps, and check the history for earlier replacement or removal. With any new metallic knocking, stop the engine and do not restart it.