Car Clinic: Impreza's warning light, faulty cruise control, dangers of pimping your car and squeaky door seals

The car clinic experts

TIM’LL FIX IT
Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motorists.

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car related accessory.

Q. Recently my Subaru Impreza’s engine management warning light came on. My normally excellent local garage fitted a new catalytic converter and the light went off, but it came back two weeks later. The garage then replaced an oxygen sensor, but the light quickly came back on. It now blames a “software problem” and wants to remove the warning light’s bulb. Is that wise?

KD, Barnsley, West Yorkshire

A. Removing the bulb will mask any future problems. More importantly, you have a problem now that must be fixed before it causes any serious damage. If the engine management warning light is on, the computer will have stored a fault code that says why it is on. A competent garage will read the fault code, carry out some logical fault diagnosis and then repair the problem. The solution should be quite straightforward with a fault-code reader.

The most likely explanation is that one of the many engine sensors is giving an abnormal reading. The sensors are there so that the computer can adjust the fuel/air mixture to give maximum efficiency for all possible conditions of load, outside temperature, throttle position, road speed and a host of other variables.

If a sensor fails, the computer substitutes an “average” value so that the engine will keep running and not leave you stranded, but the engine will not be running at peak performance. Whether the problem is, for example, a misfire (when one or more of the cylinders is not burning or firing properly) or a failed sensor, you will be using extra fuel and possibly causing further damage until the problem is cured.

TS

Q. My eight-year-old Ford Focus had a new cruise control switch fitted but it works only in fourth gear, not fifth. My dealership said nothing shows up on its diagnostic checks. Do you have any idea what could be wrong?

MB, Holywell, Flintshire

A. On a car with a manual gearbox such as yours the cruise control function is cancelled whenever the brake or clutch pedal is pressed. This works by way of a small switch attached to each pedal that detects when it is at the highest — or rest — position. As soon as the pedal is moved, the switch automatically disengages the cruise control.

It is most likely that after you use the clutch to change up to fifth gear, the pedal is not fully returning to the rest position. This problem can arise if the clutch pedal is slightly stiff, the return spring is weak, or the clutch hydraulics have a slight leak. Your dealer’s diagnostic system will not detect this as a fault because, electrically, everything is working normally.

There is a very simple way to check: next time you change up to fifth gear and find the cruise control will not work, try raising the clutch pedal with your foot. If the cruise control engages, you or your garage will need to investigate why the pedal is not returning to the rest position. Make sure you get it fixed — relying on raising the pedal with your foot is dangerous, as it makes you slow to respond to an emergency.

TS

Q. I’m thinking of adding some black stripes down the side of my new white Suzuki Splash. How should I go about this? I’m not very good at DIY.

JR, Braunton, Devon

A. In some cases the subtle use of stripes can improve the general appearance of a vehicle  — for example, along the side to break up plain door panels. But be aware that too much customisation can make a car difficult to sell. Graphics mimicking a souped-up rally car tend to look out of place on a basic hatchback, for example.

You can create narrow stripes — known as pinstripes — very easily using what looks like sticky tape. Halfords sells a roll of 12mm-wide black adhesive stripe for £5.99 (Halfords.com).

Applying it yourself is not as difficult as you might think, although it would help to have an assistant to confirm when the stripes are level. Clean the bodywork and then dampen it with a solution of warm water and a few drops of washing-up liquid. Cut the stripe to the approximate length and apply. Use a microfibre cloth to press the stripe in place and squeeze out the water, leaving the graphic neatly affixed. Cut off any excess with a sharp knife.

If anything goes wrong, a stripe can be removed quite easily with a little help from a hairdryer to soften the adhesive. If you remain wary, most garages will do the job for you, though expect to pay for about an hour’s labour for a stripe down both sides.

DP

Q. The door seals squeak on my four-year-old BMW 325i. My dealer told me to apply water to the seals, but this works for only a short time before the squeaking returns. Any suggestions?

JH, Bramcote, Nottinghamshire

A. Are you driving the cabriolet version? Squeaking rubber seals are particularly common on drop-tops because seals can be damaged with the repeated raising and lowering of the roof.

To silence your car’s squeak, start by cleaning the seals thoroughly with warm water and a little washing-up liquid. Use this as an opportunity to check their condition (if they are seriously damaged, you might want to consider having them replaced).

BMW used to sell a very effective product called Gummi-Pflege (“rubber care”) for exactly this sort of problem. Frustratingly the item then went out of production, but has recently been rebadged as a BMW own-brand accessory called Rubber Care Protect, which will cost £8.30 a bottle from your dealer. Apply it as directed to the clean seals and hopefully it should leave you squeakless.

DP

Got a car problem?

Email your question to carclinic@sunday-times.co.uk, 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.