Car clinic: How to reject a false charge, clearing out a diesel car's particulate filter and curing a boiling Fiesta

Your motoring problems solved

The Car Clinic experts

Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motorists.

Nick Freeman is a solicitor who runs a legal practice in Manchester and specialises in road traffic law.


Q. My friend has received a letter from the police stating that his car’s details were left on the windscreen of a damaged vehicle by a witness. My friend is adamant that he did not hit another car, and his car’s bodywork is unmarked. The police say he could be cautioned, or charged with leaving the scene of an accident and driving without due care and attention. How should he defend himself? And if he does receive a caution, will this affect his insurance premiums?

GH, Reading, Berkshire

A. Your friend needs to respond to the police letter by denying any suggestion that his vehicle was involved in the alleged incident. In addition, he should gather photographs of his car, with times and dates recorded, to demonstrate its undamaged condition. He should also make a note of where he was at the relevant time, and collect any evidence supporting this. (I would advise against disclosing such information to the police: it is for them to prove first that he was the driver of the vehicle and second that he was involved in the accident.)

If the police wish to pursue a case of driving without due care and attention and leaving the scene of an accident, I would expect them to ask to interview your friend under caution about the allegation. If they do so, he should obtain legal advice before attending.

Note that this is not the same as being cautioned, which is a more serious procedure akin to a conviction. A caution can be administered only if someone admits an offence; if your friend believes he is not guilty, he should admit nothing.

Occasionally a driver is involved in an accident unawares. This could result in a summons for driving without due care and attention, but would not lead to an additional one for failing to stop and failing to report the accident — the police have to prove the driver of the vehicle had knowledge of the accident to charge him with that.

Any admission of liability, whether reflected by a caution or your friend having to take a speed awareness course or driver improvement course, is likely to affect insurance premiums.



Q. In a previous Car Clinic answer you said that if a reader wanted to clear his diesel particulate filter (DPF) he would need to drive for 20 minutes with an engine speed of 2500rpm or more. In some cars that could mean hitting more than 90mph, or dropping down gears — a choice between breaking the law and wasting fuel. Is that really necessary? And how do taxis — most of which never leave the city — keep their DPFs clear?

AA, Cambridge

A. You do need to drop down a gear or two, and I’ll explain why. The soot that collects in the DPF must be burnt off from time to time to stop the filter getting blocked — a process known as “regeneration”. For this to take place the filter needs to be heated to at least 500C.

If a driver frequently does motorway runs of an hour or more, the soot will be burnt off as quickly as it accumulates (a process known as passive regeneration). If a vehicle generally does shorter journeys, some help is required.

When the engine management system detects that the filter is about 75% full, it automatically alters the fuel injection in a way that increases the temperature of the exhaust gas. This heats the DPF enough to burn off the accumulated soot in about 15 to 20 minutes (this is active regeneration). In both these cases no warning light will come on and no action by the driver is needed.

However, if the engine never stays hot enough for long enough for either process to be completed, the DPF warning light will illuminate when the filter is about 80% full. Only now does my previous advice about doing a 20-minute run at 2500rpm come in.

You are right that in top gear most modern cars will reach 70mph before the engine speed is high enough. Selecting one or two gears below top will increase the engine speed enough to make the process work (though, yes, you will use a bit more fuel).

Taxis driven all day in busy urban conditions will probably generate enough heat in the exhaust for active regeneration, although many do suffer DPF problems. Some aftermarket companies offer to remove the DPF altogether, an option considered illegal by the Department for Transport (although this has yet to be tested in court).



Q. My Ford Fiesta’s engine overheats when the car exceeds 60mph, to such an extent that we have to stop, let it cool down and then refill the radiator. Any advice?

RM, Otterton, Devon

A. To state the obvious, you have an engine cooling problem. At low speeds your car’s motor won’t get very hot and even an inefficient cooling system may cope, but as you drive faster, the unit works harder and gets hotter, and if there is not enough coolant (a mixture of water and antifreeze, which also raises water’s boiling point), or that coolant is not circulating around the system quickly enough, the engine will overheat.

The most likely problem is a leak, or, failing that, a blockage that is preventing the coolant from circulating properly. Ask your garage to carry out a pressure test on the system to check for leaks. If it cannot sustain pressure but there is no obvious leak, it could indicate an internal leak — typically through a damaged head gasket.

If there are no leaks, the next step is to check for blockages, particularly in the radiator, and to make sure the thermostat (which allows coolant into the radiator at a certain temperature) is opening properly. A worn water pump is a less common fault but can also stop coolant circulating efficiently or cause coolant loss.

Finally, your garage should check that the coolant reservoir cap is closing and opening when it should. It contains a valve designed to allow steam to escape from the system if the engine gets too hot and the coolant starts to boil (otherwise the pressure could build up and cause an explosion).

A faulty cap valve is a common source of regular coolant loss — because it escapes as steam, there is no obvious sign of leakage.

If your problem is caused by a blockage rather than a leak, it may be that the coolant loss is caused by the reservoir cap letting off steam as it should — a situation that will rectify itself once the blockage has been removed.




Got a car problem?

Email your question to, or write to Car Clinic, Driving, The Sunday Times, 3 Thomas More Square, London E98 1ST, with a daytime phone number, your address and as much detail about your car as possible. We can’t reply personally, so please don’t send original documents or SAEs. Advice is offered without legal responsibility.