Advice on tuning your car for extra performance or fuel economy, or even both, is one of the most hotly discussed topics at Diesel Car Towers thanks to a steady flow of reader enquiries and questions. Here we ask Doctor Diesel to answer all of the most important questions that are on the tip of our reader’s tongues, and many more that they wish they hadn’t.

Anna P, Barnsley

You would have to be unlucky to get caught out Anna, but there’s a remote possibility of having a nasty accident, when your car might be impounded and inspected by the police. If the box was then spotted, you might well find yourself uninsured, at best losing the value of your car, at worst another party claiming against you for repairs, injuries, or worse. Shop around for a quote for the car before getting it tuned, get it covered properly, and you’ll sleep easier at night.

Doc D

I’m worried that the increased performance and torque of a tuning conversion will put extra strain on the gearbox of my car. Need I be concerned about this?
Mike F, Enfield

To be honest Mike, you could easily put as much strain on a standard car’s gearbox by daft driving as you’re likely to with your tuned car, if you’re sensible in how you drive it. If you start laying down the rubber to show off to your mates, you could easily do some damage; but if you feed in the torque gradually and don’t drop the clutch hard, you’ll be alright. It’s probably the dual mass flywheel that would go first anyway, not the gearbox, but that would still cost you quite a few hundred notes.
Doctor Diesel

My Skoda Octavia is only a month old, but I really want to get some extra performance, as the 1.6-litre TDI engine feels really slow. Is it wise to tune such a new car?
P.D. Harrogate

Sounds as if you didn’t take a decent test drive, or maybe you thought the 1.6 TDI test car went well enough? Your new engine has got a good way to go before it loosens up and delivers full performance, so take it out and give it some healthy exercise, get the engine spinning, and leave any decision about tuning until you’ve got at least 3,000 to 4,000 miles on the clock.
Doc

Which is the best type of tuning system, a remap, a software rewrite, or a tuning box, and what’s the difference between them?
Martin F, Cardiff

A remap or a rewrite are the same thing Martin, where the standard engine management system is modified, usually by plugging a laptop into the diagnostic socket, and then replacing the existing software. Tuning boxes are simply plugged into the engine wiring circuit of the ECU and the fuel injection system, to simply modify signals being fed to the ECU to make it inject more fuel, usually at a higher pressure. The cheapest boxes are crude and overpriced for what you get, although surprisingly many do work alright. But the better units offer quite complex modification of several injection parameters and are very good, if you want something easy to install and remove at will. As with most things, you get what you pay for, so always buy from a reputable source, that will provide excellent back-up if you ever need it.
Doctor Diesel

Are there any tuning systems where you can switch between tuned and standard tune pretty much instantly, so that you can maybe lend your son or daughter the car set back to standard tune, and take it in for service set that way?
V Cage, Perth

Yes indeed, some companies offer portable tuning systems that allow you to plug into the diagnostic port and switch between settings, which generally takes about three to five minutes. They send you the device, which you plug in to read the standard software, and send to them through broadband; they then check out the engine numbers and send you back the correct rewrite which you install in the device from your computer. You can then plug into the car and switch easily between standard and tuned settings, and the cost is comparable to a good quality remap.
Doc D

I’ve just had my engine tuned. Should I take any precautions, like running it in, or can I use all the extra performance straight away?
Raj S, Wembley

You ought to check out that your brakes and tyres are all in good shape Raj, and it might be worth having an oil service, if you’re going to work the engine a bit harder, using the best oil specified for your engine. Now might be a good time to find a garage that smiles when you tell them you’ve just had your engine tuned, and offers you some good advice on maintenance and servicing.
The Doc

I want to improve the fuel economy on my car by tuning it. What is the best way of doing this, and what kind of fuel economy gains can I expect?
Robert S, Coventry

If you don’t use the extra performance all the time, which is pretty much impossible on our roads, you’ll probably gain a few mpg with most good electronic tuning conversions. Some companies offer specific conversions for better fuel economy, where they aim at boosting mid-range torque, rather than high-end power. This lets you use higher gears more often, taking away the need for high engine speeds in most motoring. Expect anything from five to fifteen per cent economy gains.
Doctor D

Since my car was tuned, I am leaving clouds of black smoke in my wake. Why is this happening?
Ted A, Brighton

Sounds like you’ve possibly been sold a pup, Ted. The tuning set-up is wrong or something’s wrong with your engine. There’s too much fuel being injected and/or your engine is incapable of burning it properly with the air it is getting. If it was an add-on box you fitted on a DIY basis, then (a) check your air filter, and replace it if it looks dirty, and (b) try a double dose of Millers Diesel Power Ecomax in your fuel – your injectors may be clogged and Millers could clean them up. If the engine was tuned at a garage, or by a mobile fitter, this should have been obvious on a test run. Take the car back and demand your money back if it’s not sorted out, and the same goes for the add-on box, if you can’t get it running smoke-free.
Doctor Diesel

I had my car tuned a while ago, and recently when I put the throttle to the floor, the engine feels like it hits a brick wall and the engine management light comes on. What’s up?
David, Colchester

A bit surprising this didn’t happen before, but something is hitting a manufacturer preset limit – like turbo boost pressure, or fuel pressure, presuming that the engine is otherwise alright. The engine is dropping into “limp-home” mode and, as you’ve probably found, after an ignition on/off everything is alright again until you hit full throttle. Have you by any chance got an add-on box with variable settings? Maybe you have moved it to a setting that’s too high, when dropping it back one or two steps may sort your problems? If it’s a remap though, you need to go back to the tuning company.
Doc

I’m thinking of changing my car, and am looking at the Jaguar XF 2.2 Diesel. Would I be better to buy the 161bhp edition and have it tuned, or shell out the extra for the 197bhp version. The price difference is £1,000 and would welcome some good advice.
Ben T, Luton

In your position, I would probably go for the 197bhp standard car Ben. You could get 197bhp from the 161bhp car by spending a lot less than £1,000, but the trade-in value of the standard car will be better, it will be easier to sell, and you’ll never have any warranty worries. But it’s a close decision and, if you could insure the tuned 161bhp car without suffering any premium loading, it sits a few groups lower (depending on trim level) than the 197bhp car.
Doc Diesel

I’ve seen some tuning boxes for sale on eBay quite cheaply. Are they worth it, or would you suggest giving them a wide berth?
Danny W, Burntwood

Like I often say Danny, you usually get what you pay for. With all due respect to eBay, you’re buying very blind there and I would always want to feel more secure than that buying something that could put you off the road if the electronics went wrong. If you fancy a tune, start by looking at well-established companies that advertise in Diesel Car and you’ll not go far wrong.
The Doc

I bought a tuning box to help when towing my caravan, and the extra torque has certainly helped. Would you advise taking it off the car when not towing?
Ali K, Bradford

No reason to take it off Ali. You’ll probably find you will use less fuel (as I’m sure you find is the case when towing) all year, and get a more enjoyable drive, with more in-gear flexibility.
Doctor Diesel

Earlier this year, I had a tuning box fitted by a reputable tuning company, but I can’t seem to get the kind of economy figures that the company claimed. Where do I stand and what would you advise that I do?
Stuart D, Norwich

Could just be the way you’re driving on those flat traffic-free Norfolk roads Stuart! If you are using all that extra performance, then there’s going to be a penalty at the fuel pumps. But it all depends on what they claimed, which should be up to ten per cent better mpg. If they quoted some higher figures, that was unwise, and unrealistic. Go back and see what they say, but first check out your car. Has it been serviced lately, are the tyres and brakes alright?
Doc D

I had my car remapped earlier this year, but with the annual service fast approaching, I’m worried about taking it to the dealer and it invalidating my warranty. What should I do? Restore it to factory settings, or take it in and hope for the best?
Chris W, Dartford

I think the best way to approach this is to call in before the service and have a discrete word with the service department. Say that you’re thinking of having the car tuned, and ask what would be their attitude. Some franchised dealers are actually agents for respectable tuning companies and they are not necessarily anti-tuning, or obliged to do anything if they spot that your car has been modified, which is fairly unlikely anyway. But there is the small risk that a software update might wipe out your remap, so it’s best to let them know that your car is tuned, and ask them to call you before they do any ECU software updates.
Doctor Diesel

I have my car for sale in the local paper, and I’m worried about the tuning box that I’ve had fitted to it for the past three years. Should I include it with the sale, or remove it first and not tell the buyer anything about it? I’m concerned that any potential purchaser might be put off buying if they think that the tuning box might have harmed the engine.
Scott Y, Worcester

Not everyone knows what a careful and responsible driver you are Scott, and how carefully you’ve treated the car. “Never raced or rallied” nudge nudge! I would get it taken off and not mention it, because the upside is minimal and the downside is significant for someone who doesn’t understand how tuning works. Sell the box separately and gain yourself a few extra quid!
The Doc

I have an ancient Renault 19 1.9 Diesel that suffers with sluggish performance. I have looked on the websites of companies advertising in Diesel Car, but none list my car. They do have the turbo diesel version, but not the non-turbo. Why is that? Is it because my car is so old?
Tom M, Oxford

I’m afraid so Tom! To be honest, your 19 is getting a bit long in the tooth, and tuning it would be a bit like supplying Viagra to a 90-year old bachelor with a dolly bird. You might have a bit of fun for a short while, but it might all be a bit much for your engine’s ticker. To be honest, there’s very little potential to tune a non-turbocharged diesel, and you’d be best just changing or uprating the air filter and giving the engine a good double dose of Millers Diesel Power Ecomax.
Doctor D

I am a total novice when it comes to cars. Most of the tuning company websites say that their boxes are do-it-yourself units and very easy to fit. Can you talk me through what to expect when installing a tuning box to my 2007 Ford Focus.
Alex T, Swindon

It would be best ask the tuning company exactly what is involved for your car, as each installation differs from model to model. If they can’t or won’t tell you, or send you printed details, then go elsewhere to someone that can. Usually the problem is getting to a plug located on the engine block, which naturally can be hot if you’ve had a recent run, and awkward if you have big meaty hands. If that’s the case, better sweet talk a lady friend who fancies getting her hands dirty.
Doctor Diesel

I’ve had a tuning box fitted that I bought from a company with a good history and I’m very pleased with the performance. But the computer says that the fuel consumption is a lot better too, but this doesn’t agree with my mpg calculations from fill-ups and mileage, and I’m actually using a bit more fuel than before. Can you throw any light on this?
Barry Brown, Croydon

This is something that happens with some engines Barry. The tuning box basically fools the injection system into pushing out more fuel than with the standard software, whilst the car’s computer carries on calculating the fuel as it would at the standard settings. So you get false mpg figures and this is why some people claim ridiculous gains in fuel economy that are simply not credible.
Doc D