Car maintenance: Common fuel issues - Honest John answers your questions

Honest John has been the resident “agony uncle” at The Daily Telegraph for more than 20 years, responding to readers’ car-related queries with his trademark blend of experience, authority and, sometimes, bluntness.

He remains undeterred by the volume of correspondence he receives every week, from car-buying advice to the sort of complex legal cases from which he has gained a solid reputation as a champion of consumer rights.

Despite receiving many hundreds of queries each week from Telegraph readers and via his own website, Honest continues to deliver his no-nonsense - and, always, highly trusted - advice.

For ease of use, we’ve broken batches of Honest John queries into sections relating to the nature of the problem, such as tyres, odd noises, diesel issues, automatic gearboxes and the like.

If your car has developed a fault, or for consumer advice, turn to Honest John by emailing honestadvice@telegraph.co.uk

And if you’re one of the many readers who simply enjoys the amusing headlines (credit goes to Simon Arron for these), then go to our selection from the extensive Honest John archive.

Buying a small diesel car

I run a 1.6 petrol Vauxhall Zafira for 120 motorway miles a day. I’m thinking of buying a small, cheap diesel to do the run instead - maybe an Astra. What do you think? LL

The best result I ever recorded was with a 1.3 Fiat Multijet-powered Corsa, which regularly returned more than 70mpg. But there can be problems with the particulate filters. It might work out cheaper overall to get an old Toyota Yaris 1.3, because these were generally very reliable and that made up for the average 40-45mpg fuel economy.

Should I buy a car with a petrol or diesel engine?

I am considering a VW Tiguan 4x4. Is this sensible and would you suggest petrol or diesel? JG

This is now quite a nice vehicle. Autos have the DQ500 seven-speed wet clutch DSG. Go for a petrol TSI engine rather than a TDI, because of all the problems diesel engines are encountering with emissions control systems.

Should I buy a Ford Fiesta?

My wife and I have owned our Ford Focus for 12 years and 144,000 miles. We are due for a change and fancy a Fiesta 1.0 Ecoboost. I would like an automatic but am concerned about fuel consumption. What do you recommend? BA

Do not buy a Fiesta Powershift. Happily, with the latest Fiesta, Ford has replaced the Powershift with a reliable six-speed torque converter automatic. Alternatively, look at the Mazda2 1.5 auto, or a Suzuki Swift or Baleno 1.0T Boosterjet auto.

The best cars to tow a caravan

I am looking to replace my 2012 Ford Mondeo 2.2 TDCi auto, which is used for towing a caravan. With all the negativity about diesel engines and my low mileage (circa 10,000 a year), what would you recommend?JL

There are several options, including Mondeo 2.0 Ecoboost, Mazda6 2.0 Skyactiv G, VW Tiguan 2.0TSI, Skoda Karoq 2.0 TSI and Skoda Superb 2.0 TSI.

Free oil changes

I have had to have an early oil change on my Ranger Rover Evoque. This was done under warranty at no cost, so the proposed long-service interval has resulted in me getting a free interim oil and filter change as a result of contamination. I believe this situation is not confined to Land Rovers. MW

Yes, many thanks. That's the bigger story - some owners have been getting a free oil change at 5,000 miles. Higher-mileage drivers have gone up to 16,000 miles before the oil service light comes on.

Should I replace my diesel car with a petrol model?

I own a three-year-old Audi SQ5 diesel. It is a bit daft – a 155mph four-wheel-drive SUV – but it is beautifully made and very well mannered. It is the best car I have ever owned. I also run a petrol VW Up for short journeys. With all this demonising of diesels (potential city bans, fall in second-hand values), do you think I should trade the SQ5 for a petrol model. It must be good for at least another seven years. In principle I’d be happy to keep it. DW

I'd take a test drive in a petrol-engined Porsche Macan (it shares a lot with the Q5), and also try a new model Q5, a Jaguar F-Pace and an Alfa Romeo Stelvio. If one of those blows your socks off, see what kind of deal you can do.

Regular v premium fuel

I own a Toyota Yaris hybrid. What improvement can I expect if I change from regular unleaded  to BP Ultimate? KM

You’ll get more torque at low revs, allowing the epicyclic gears to go to a higher ratio earlier and save a little fuel, but the main thing is that you’ll be spared fuel-system problems: no gunge to gum up valves or injectors.

Should I swap my Ford Focus for a Skoda Fabia?

I have a 2011 Ford Focus Titanium with 45,000 miles on the clock. I am 83 years old and am considering a Skoda Fabia as a replacement. What is your view? RHF

The current Fabia arrived a couple of years ago and is OK, but - unlike the Seat Ibiza and VW Polo - is not on the VW Group’s latest MQB platform. Whatever you decide, do not buy a diesel.

Shortage of petrol cars

I visited my VW garage to see about a scrappage deal for my 2009 Passat estate, with a view to switching to a petrol Tiguan. I was told petrol Tiguans have for the time being been withdrawn from the UK, because VW can’t make enough engines, and thus I was unable to place an order via the scrappage scheme. JG

Demand for petrol engines throughout Europe has suddenly rocketed. Although VW recently introduced a range of 1.4 TSI and 2.0 TSI engines for the Tiguan, it is also in the throes of switching production from the 1.4 TSI to the outstanding new 1.5 TSI Evo. That’s the reason for the temporary shortage.

Filling your car with the wrong fuel

I put six litres of petrol in my Nissan Qashqai diesel to top up the tank. What can I do? AS

If it was only six litres you should be OK, as long as you quickly add a lubricity additive (such as Wynn’s), run the car on high-lubricity superdiesel fuel and do not let the tank go below half full for at least 2,000 miles. With luck, the petrol will float on top, then gradually disperse into the diesel without damaging the high-pressure diesel injection pump. If you allow the tank to get low, then petrol could find its way into the pump. The only alternative is to empty the tank and dispose responsibly of the contaminated diesel.

Filling your car with premium fuel

I’ve been using BP Ultimate in my Kia Sportage 1.6 GDi and have been getting 40.2mpg. Previously I used supermarket 95 fuel and got 40.1mpg. Apart from spending 10p a litre too much, what else am I doing wrong? CW

You are not using the extra torque from the higher cetane to change up earlier and, consequently, are using just as much fuel.

Is diesel supplied by supermarkets poor quality?

I was surprised to find that cheap fuel supplied by supermarkets is of inferior quality. Does this also apply to diesel? MM

For 15 years or more I have advised readers to use only higher-grade petrol or diesel. This give more torque at low rpm so you can change up earlier, saving fuel. And it keeps the engine fuel system clean. Win-win, in my book.

Should I buy a diesel or petrol SUV?

I’m buying a new SUV (VW Tiguan, Audi Q5 or Land Rover Discovery Sport). I drive about 10,000 miles a year and have a 14-mile morning commute that doesn’t involve any motorways. Should I buy diesel or petrol? Is diesel still best for larger SUVs? SC

For smaller SUVs, petrol makes the most sense. If you're a Tiguan fan, VW now offers a range of 1.4- and 2.0-litre petrol engines. Land Rover produces 240 and 290bhp 2.0 Ingenium petrol engines for the Discovery Sport. For bigger SUVs used for towing, diesel remains best.

Buying a hybrid car

I am considering a one-year-old Mercedes C350e hybrid, but am confused by the published fuel consumption figures. How can the manufacturer claim a figure of more than 130mpg, yet independent reviews state that one is lucky to get much above 40mpg? JH

That 130mpg is the NEDC 11-kilometre lab test figure that massively complements plug-in hybrids, which generally have an electric range of 20-30 miles. You will probably get 40mpg in normal day-to-day driving.

UK v US gallon sizes

Why do the Americans think 50-55mpg to the gallon in a fourth-generation Toyota Prius is so good? I have one and have no problem getting 60-70mpg even on a long run. Do they use a different type of petrol, or what? MW

Different size of gallon. US gallons are smaller than imperial gallons (3.79 litres rather than 4.55, so 55mpg in the US equals 66 in the UK).

Alternatives to petrol and diesel

Why is liquefied petroleum gas (LPG) not the obvious alternative to electricity as a replacement for petrol and diesel? It's in plentiful supply and cheap. MD

It's still a by-product of a fossil fuel that has to be burned.

Why should I buy a petrol car?

I had a 2003 VW Golf 1.9 TDI 130 estate and at 65-70mph on steady motorway run it gave us 63mpg at 2,000rpm. Monthly tax was £12.68. Now I have a 2007/57 VW Touran 1.6 petrol and at the same speed get 43mpg at 3,000rpm. Monthly tax is £23.62. What is the point in changing an older diesel for a newer petrol, when it costs more to run and tax? GM

If the petrol vehicle has an EU4 emissions rating it won't get banned from city centres in 2019, unlike non-EU6 diesels. You have also changed from a small estate to a brick-shaped MPV and are not comparing like with like.

Should I swap my paper driving license for a plastic photocard?

We are taking our car to Normandy and are aware of the many requirements to be fulfilled. At present, my husband and I have only the paper version of our British driving licences as we have had no reason to change them (and no desire to pay £20 for a plastic one for the sake of it). Do you think that we should get ourselves plastic versions in case we are stopped in France. GP

Yes. Your old paper licences remain legal but the idea is so archaic that it could cause problems. You can use your passport photos to apply for the Photocards online: www.gov.uk/exchange-paper-driving-licence.

Preventing drops in miles per gallon

I took my two-year-old Auris to a Toyota dealer for a number of reasons - one being that mpg had dropped from mid-50s to mid-40s. When I told them that I’d bought fuel from two major supermarkets they added Redex additive. It is now up to 52 and climbing. Is this the norm? IM

The Redex cleaned the deposits that cheap fuel left in the system. It’s best to use branded superunleaded petrol - that way, you won’t suffer the problems that the Redex cured.

Diesel cars in London

I have a seven-year old Saab, which I would like to keep, but I live in London and it is a diesel. What are my options? Could it be converted to petrol? MC

Yes, but it makes no sense. Diesels older than EU6 and petrol cars older than EU4 face prohibition from London, on pain of heavy daily penalties, from autumn 2019. Older cars could be converted at colossal expense, but would still be bound by their original EU emissions rating.

Fuel consumption

Can you advise on the consumption rate of AdBlue in mpg? I have a 2016 Ford Transit Custom 2.2-litre van with an AdBlue capacity of 4.6 gallons. The information would be useful for calculating topping-up intervals rather than waiting for low-level warning messages to appear in the information display. According to the vehicle manual the tank may either be filled at a “fuel station pump or from a portable container”. Have you ever seen such a pump? NG

AdBlue is normally consumed at between 4-8 per cent of the rate of diesel. You can buy it in 10-litre plastic containers for £12-£16, or refill directly from the AdBlue pump at the truck filling side of almost every service station. If you carry your own supply, be careful to secure it. It's nasty stuff with an unpleasant odour.