Used car buying guide: BMW 6 series

Towards the end of 2005, BMW replaced the 4.2 with a more powerful 4.8, producing 362bhp and capable of 0-62mph in 5.1sec. Amazingly, economy was around 2mpg better. Economy? It was becoming a hot topic to the extent that BMW took the opportunity in 2007 to slip its Efficient Dynamics package of economy and CO2-boosting measures into the model’s mid-life facelift. The major beneficiary was the 630i (the ‘C’ was dropped from the model name), now capable of 35mpg. Extra power, LED lights, and subtly tweaked front and rear ends topped the update.

The 635d was introduced at this time, too. A diesel-powered GT was pretty novel then, but BMW didn’t muck about: the 635d’s 3.0-litre straight six produced 282bhp for 0-62mph in 6.0sec, just 0.1sec shy of the now uprated 264bhp 630i. Economy was 40.9mpg.

It’s a strong engine but the good economy and low CO2 emissions that once attracted company car buyers are not uppermost in the minds of used car enthusiasts. For them, the straight six petrol is the model of choice. Just remember to dip the oil every few minutes.

An expert’s view…

STEVE BUCK (A1BN)

“They can leak oil — lots of it: hoses cracking or getting blocked, gaskets failing, gearboxes running dry… these are the problems we see all the time. Would I buy one? ‘Buyer beware’ is all I can say. Go in with your eyes wide open and don’t buy the first you see, or the second. I’m bound to say it, since we can offer this service, but pay for a pre-purchase inspection by a professional. We charge £102. It takes a couple of hours and we go through the whole car. That flushes out the problems.”

BMW 6 Series problems

PETROL ENGINE

A ‘check engine’ light could indicate that the camshaft position sensor needs changing. Vanos solenoids and actuators can fail, leading to poor running. Oil leaks from the rocker cover gaskets, the oil filter housing and the oil lines to valve timing units. V8 engines can suffer high oil consumption, which could be valve stem seals, bore wear or perished breather hoses. Idle for 30sec and then blip the throttle, watching for smoke. The 630i suffers oil separator breather pipe issues. V8s run at high pressure, and older hoses and thermostats can fail. The expansion bottle can explode.

DIESEL ENGINE

Check the turbos’ pressure converters and vacuum pipes. The crank dampener can separate. Inlet manifolds and swirl flaps leak. Checka faulty exhaust gas recirculation thermostat isn’t letting the diesel particulate filter soot up.

TRANSMISSION

Plastic sump cover can leak fluid, requiring replacement since the filter is part of it. It’s sealed for life but not unknown for 100k-milers to have just two litres of oil left (it takes six). Check the shifts for a thump changing down, snatchy and lethargic going up. It could be pressure sensor oil starvation.

BRAKES AND SUSPENSION

Road humps destroy the front control struts (feel for a shudder through the brake pedal) and front shocks, which leak. Lower rear hub bushes wear and springs break.

ELECTRICS

A new battery must be coded to fit.

BODY AND INTERIOR

Should be rust-free. Fabric roof sensors can fail. Check the bulkhead drain holes are clear.

Also worth knowing…

The 6 Series hood can be troublesome but don’t despair: folding roof specialist Cayman Auto Services (01306 885566) should be able to fix it.

BMW 6 Series prices

£3000-5495

Early 645Cis with more than 100k and some with full service history.

£5500-7495

More 645Cis, including cabrios, but also tidy 2005-2006 630Cis appearing now, plus the first 650Cis, with sub-100k cars of all types closer to £7495.

£7500-10,995