Q. I want to buy a used convertible BMW 3-series and have seen an 04-plate model for sale with only 12,000 miles on the clock. I’m attracted by the low mileage, but my friend insists that an old car that hasn’t been used much will suffer mechanical problems when it starts being subjected to more normal driving patterns. Is he right?
A. Low-mileage cars have, generally speaking, suffered less wear and tear, but there are exceptions. The mileage may be low because the car has travelled only short distances at low speeds, never getting fully warmed up. This means unburnt fuel that leaks past the pistons gets into the oil sump and, rather than evaporating when the engine is hot, stays there. This dilutes the oil and creates a potentially harmful sludge — an emulsion of oil and the water produced as part of the combustion process.
Thinned oil does not properly protect engine parts, so wear ensues, and the sludge can cause corrosion and block the engine’s breather system.
Problems can also occur with the exhaust system: the low engine temperature allows water vapour to build up, corroding the pipes from the inside.
So if you buy any used car that has rarely been driven hard, first take it on a long, high-speed run to “blast away the cobwebs”, being careful to accelerate gently, then get the oil and oil filter changed.
Because this BMW is a convertible, it may owe its low mileage to being used for occasional long leisure trips, followed by months of not being used at all. If this is the case then your inspection checklist changes.
Get the tyres looked at — they may need replacing because of damage caused by not having turned for a long time, or degradation by UV light if the car stays outdoors. Rubber bushes in the suspension and steering can also deteriorate, as can shock-absorber seals. So test-drive before you buy and listen for clunks and squeaks from the suspension.
The internal seals of an air-conditioning system can become impaired if the A/C is not used for long periods, needing costly repairs, so check it still works fully.
TIM’LL FIX IT
Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring – read more from Tim here.
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