A Pinking occurs when the fuel/air charge is ignited in the cylinder too far in advance of the piston reaching the top of its stroke. The rising piston is opposed by the early detonation of the mixture, and the resulting clash of forces creates the metallic noise.
There are two main reasons for pinking. First, the mixture may be weak and therefore hotter because of the increased level of oxygen present, resulting in it detonating under pressure. Second, if the ignition timing is too far advanced, detonation will be premature, causing the pinking.
A third reason, common on higher-mileage engines, is carbon build-up in the combustion chambers. The carbon glows with the heat, and so acts as a source of premature ignition. Have a full emissions test done, to point you in the right direction.
The electronic control unit (ECU) on a fuel-injected car will be controlling the fuel/air mix using information supplied by the Lambda sensor fitted in the exhaust downpipe. If this sensor is faulty, it will supply incorrect data to the ECU, and that could explain the trouble. A workshop can quickly check the ignition timing.
Owners of older cars are advised to join the relevant single-marque club, in this case the Volvo Owners’ Club (02392 381 494, www.volvoclub.org.uk). For a relatively small fee you have access to technical help and often cheaper parts.
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Q My BMW 325 Convertible (first registered in June 2001) has just turned 35,000 miles. It is running perfectly, but I wonder whether it’s time to renew it. Would I be better off selling the car now, before the replacement model is launched next spring, or should I wait? And should I sell privately then, or trade it in against the replacement car from a BMW dealer? — DC from Wokingham
A You will generally get a better price for your car if you sell privately rather than part-exchanging it. Your BMW 325 has depreciated by about £900 in the past six months, according to the trade guide CAP. If that continues through to next spring you will be looking at further depreciation of about £600.
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The introduction of a new model can have one of two effects on second-hand values. Most commonly the new model makes the old one look less desirable and values weaken, but sometimes the introduction of a new model reignites interest in the old one, particularly if the styling of the cars is similar.
The new BMW 3-series Coupé Cabriolet is evolution, not revolution, in styling terms, and as your car has lost the bulk of its value already and will be worth about half the price of the new model, its value is unlikely to be affected. If you are happy with your current car, keep it until spring then sell it privately.
Q At night I park my Porsche Boxster in a basement car park and every morning I notice muddy paw marks and cat fur on the convertible hood, which is difficult to clean. How can I deter the offending cat without causing it harm? — CA from London
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A Not only is your hood more snooze-worthy for cats than a metal bonnet, but the mid-engined layout of the Boxster means that it will retain some of the rising heat from the engine.
To get rid of hair on the hood, gently apply Sellotape, sticky-side down. Don’t use a harsh clothes brush, which can cut through the hood’s waterproofing.
Also try one of the two-part (cleaner and reproofer) hood-cleaning kits, such as the Autoglym Cabriolet Fabric Hood Cleaning Kit or the Turtle Wax Soft Top Roof Cleaner and Conditioner Kit, at £19.99 and £16.99 respectively from Halfords (0845 762 6625, www.halfords.com).
Our experience of ultrasonic devices that are claimed to deter animals has so far been negative and they could conceivably interfere with car alarms in the proximity. Some pet shops sell sprays designed to repel cats, but it’s best not to spray the hood; rather around the garage floor and maybe the undersides of the bumpers.
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Q My 52-plate Golf Anniversary is the 150bhp diesel and in the main has been fine. Recently, though, despite starting fine in the morning, I have noticed that if I do a short journey to the paper shop, upon my return to the car it won’t restart unless I leave it for 30 minutes. A diagnostic check has shown no faults, and under advice I replaced relay 109. Local garages remain stumped. — CT from Middlesbrough
A The fuel relay you replaced is often the cause of such troubles on VW diesels and it’s unusual for a replacement not to effect a cure. Intermittent faults can be annoying as nobody can solve a fault that isn’t present. So leave the car overnight with either a dealer or a diesel specialist (check under “diesel fuel injection” in Yellow Pages) to give them a chance to experience the problem first hand.
They will need to check the fuel pressure when cranking, which could provide clues. Insufficient pressure may suggest a faulty injection pump, while no pressure points to a cut-off valve problem or maybe a fault with the engine immobiliser.
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Q I have bought a left-hand-drive 2006 Ford Mustang V6 in the United States and it arrives here in December. The SVA (Single Vehicle Approval) test requires certain modifications to its headlamps, rear indicators and rear fog lamp. Is there a converter kit that can be attached to the lamps or is this a job for a mechanic? — MS from Kent
A All vehicles on UK roads must meet the current strict European safety regulations, and are required to pass through the SVA test, the fee for which is £150. Many items are checked (see www.direct.gov.uk for a full list) and until the car has passed the SVA you can neither register nor tax it.
You cannot simply put beam converters over the headlamps, as these are intended for temporary use only. Import specialist Mildenhall Auto Centre (01638 713 962, www.mildenhallautocentre.ltd.uk), confirms that the car conforms with most current regulations with the exception of its lighting. At the front, the headlamps have to be realigned (to dip left rather than right), the side lamps have to be changed to white from the original amber, and a permanently fixed side-repeater indicator light must be added to each side of the vehicle.
At the rear, the indicator lamps must be changed to show amber rather than red, and the cheapest way is to deploy the existing reversing lamps (after some rewiring) and swap to amber bulbs.
Mildenhall usually fits an aftermarket fog lamp under the bumper, linking into either the front fog-lamp switch or adding a separate switch. The cost for the lighting modifications is about £560. Don’t get involved with the electrics as the wiring is complicated and the ECU easily damaged.
E-mail your questions for our experts to carclinic@ sunday-times.co.uk or write to Car Clinic, InGear, The Sunday Times, 1 Pennington Street, London E98 1ST. Please give a daytime telephone number and as much detail about your car as you can. We cannot send personal replies or deal with every letter. Please do not send original documents or SAEs. Advice is offered without legal responsibility.