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Diesel or petrol? Decide now!

29/10/2014 09:31 |  Comments: 

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Former car restorer, damper designer, rotary-engine guru and also an automotive engineer, but generally doesn’t talk much about his former activities. András is our mag’s Leatherman tool: when there’s a project no-one would poke with a stick, he’s the one usually assigned to carry it through. When he’s in Hungary, he works 16 hours daily, then every once in a while he disappears from the horizon. Last time he’s been seen in Auckland… Has a huge garage, lives with a girlfriend.

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Since fuel prices have been creeping up steadily and the engineers have figured out how to build beefy diesel engines, cars with the letter “D” have got a different status. People keep on arguing, which is better, diesel or petrol, but there is no universal wisdom. However, there are some typical cases, where careful weighing-up and a bit of math can help to make the right decision.

Every week we get a huge pile of readers' letters and every fourth or fifth prospective used car buyer asks the D-question: diesel or petrol? Many times there are missing pieces in the puzzle-game, e.g. where and how much they are using their car or what is important for them, and to be perfectly honest, sometimes there is no clean-cut answer. But if there are so many of them up the creek, I have decided to put down at least the most important decision factors and numbers, which can provide some help. I hope, at the end of the article at least some of you will say: “Phew, now I know why I should own a petrol car”. Or a diesel.

Before we start off, I must warn you that naturally, every model and engine is different, so there are myriads of exceptions. Such a compilation can't possibly deal with those, so every statement can be false in a special case. I will put down bold wisdom which is true in let's say 80% of cases. Unfortunately, nobody will tell you about a given car which you probably want to buy, unfortunately when it's going to break down. The best you can do is to assess the risks. If you want more, consult your authorised dealership or tell the VIN to your fortune-teller.

Some math

Let's cut the shit and focus on raw figures. Every Totalcar Magazine reader should know that buying a diesel car means lower fuel consumption in exchange for higher maintenance costs (if you're crying out now, go back to the previous paragraph). In a typical, mid-size car you can save about 2 litres of fuel per 100 kilometres, which adds up to 3-4000 Euros per 100 000 kilometres. Not bad, eh? The main question, however, comes to this eventually: how much you'll have to give to the local mechanic from your savings. This is what you won't know before you try, but you can calculate some risks.

What are your preferences?

In my opinion the most important thing is that you like your car. Because you enjoy spending time in it, because it's huge, because it has a great sound, because it consumes almost nothing or because it's green – it's all the same. If the owner feels that their car makes their life better, all the rational arguments will fail, and you shouldn't think too long about that. No matter how often you have to repair it, how much you spend on petrol or what the neighbours think about it, if you have an emotional attachment to your car, just don't give a shit. That is, if you can afford it, because there are many who hate their car because of all the money they have to spend on it.

That's the easiest way of deciding the diesel versus petrol question. If you can't stand the smell of diesel, if you don't like filling up at the same place as the tractors do, or if you detest the harsh sound of diesels, you don't need more reasons to drive petrol. You shouldn't justify your decision by making up rational arguments.

On the other side of the wall, you have to admit that there are no good sayings against petrol cars. Even if I wrote that you should buy a diesel car in case it hurts you to look at the bill at the petrol station at each fill-up, one could list dozens of petrol cars with low consumption. There are also good (turbo) petrol engines with a similar grunt to diesels, so that's no exclusive advantage, either. But there are some typical attributes which can make a diesel engine very attractive: the low consumption – especially in bigger cars –, the low-end torque and the deep sound, so you don't have to be ashamed of being a diesel-fan. Anyway, if everybody listened only to rational reasons, the one and only car of our age would be the Toyota Prius.

User profile

If you use dirt roads, don't buy a Ferrarri. Almost as simple as this is the fact, that for extremely short trips – that is 2-3 kilometres – you shouldn't use an up-to-date diesel equipped with a particle filter. It won't be long before the DPF gets clogged, and even if it's a new or very young engine, it will be struggling to regenerate itself during normal use. The clogged DPF will need forced regeneration or a special cleaning treatment in the workshop – it's a hassle and a waste of money.

This problem happens even with brand new cars - used ones with a high mileage almost certainly will experience the DPF disease. With a user profile like this, it makes no sense at all to buy a modern diesel (i.e. from the last 5-10 years), because while running cold, they won't have a low fuel consumption, anyway. If you happen to use a diesel with DPF for very short distances, the only solution is to burn it free from time to time on the motorway, where it can do the regeneration procedure. You see, it isn't a green or money-saving habit.

I'd love to say that for short trips, petrol engines are unbeatable, but in reality, the cutting-edge, direct-injection, turbocharged ones tend to kill themselves, too, but in another way. It depends very much on the model, how often it happens, but there is a well-known problem called carbon build-up. The intake valves and the manifold can accumulate a thick black crust, when using the petrol engine mostly in the city. There are almost no new cars left with conventional port injection, albeit these, and the long extinct diesel engines with indirect injection are the ones that can cope with that kind of stress. You'd better switch to a scooter or a bike, or take public transport for short trips.

The other extreme is the marathon-run on the motorway. That's what diesels love. They can warm up to operating temperature, they'll work in the optimal rev range with considerable load - if you come across an engine with 5, 6 or 7 thousand kilometres on the clock, it will almost certainly be a motorway runner. If it gets the proper maintenance and if it's treated properly, you'll need to work long and hard to wear out a well-built diesel engine.

Obviously, there aren't many who use their private cars only for one purpose, but if you leave the city twice a year of if you travel 50 miles to work every day, you know which type you are.

How much can you spend?

If you can afford a new car, as long as the warranty lasts, you won't have to consider the most important argument against modern diesel engines: the risk of an unexpected, horribly expensive fuel system problem. But around here, there are ten, twenty or thirty times more used car buyers than new ones, so the vast majority is looking for a car for a very limited budget. Given a young, 2-3-year-old car with a proven history which promises about 100 thousand more problem-free kilometres, you probably don't have to be so afraid of a diesel. In most cases the repairs that cost 2-3-4000 Euros don't turn up before 200-250 thousand on the clock.

You may be sceptical about the mileage of any car, and you're right, as well as about the question when a diesel-engine will become risky, but you'll have to agree that somewhere between 200 and 300 thousand kilometres there is a good chance for a turbo failure, a fuel system problem or a broken dual-mass flywheel. These can cost a considerable amount of money, somewhere between a few hundred to a few thousand Euros, depending on the model and what kind of mechanic you find.

Dare I be so bold as to say that on the Eastern-Central European market there's a threshold of about 3000 Euros. If you want to buy a cheaper diesel, there's a huge risk, and I'm not exaggerating. Naturally, there is some spread, depending on size and brand, but if you buy an early common-rail diesel for about a 1000 Euros, you'll have to accept the risk of a fuel system overhaul that costs more than the car. You should know, this can happen any day.

There are not many diesel cars with a mileage lower than 200k for less than 3000 Euros. Anyone who is interested in these, should be well-educated in the technologies used in the given model, be it an injection pump, unit injectors or common-rail, so they know what to expect. The other extreme, the 20-30 year old cheap diesels which can be run on a budget are a true alternative for some, but I'm afraid those will rust away quite soon.

If you're a petrol fan and feel proud now, you'd better be careful. For the average motorist, there are less and less simple, naturally aspirated petrol engines available, which bear the least risks. Even in 10-15 year old cars there are EGR-valves, ignition coils and oxygen sensors, just to mention but a few petrol-specific problems. Those can cost a lot of money, too, although the risk and the repair bills are one magnitude lower, for sure.

However, as we come closer to the 2010s, we find more and more turbocharged and/or direct-injected petrol engines. You can say many things about those, except that they have a proven technology and are reliable. A GDI or TSI is, in my opinion, about as risky as an HDI or TDI. There are better and worse models, of course, and you can be almost sure that a petrol engine of the same age will have less kilometres on the clock. At least, that's good news.

I'm quite confident that a cheaper, old-school petrol car, especially if it has a low mileage, can financially beat an older diesel in the long run. On the other end of the scale, a newish diesel engine will be the better option for example in a people-carrier, if you compare it with a downsized petrol engine. To make it simple: more expensive, almost-new cars tend to be more economic with a diesel engine, but as you lower the price and the mileage rises, the scales will tip very soon towards the petrol engine.

How important is calculability?

For the ones who like to be on the safe side, I'd only recommend a naturally aspirated (NA) petrol engine, and one that has already proven its reliability. Japanese manufacturers are in most cases a good choice, they are the ones who still build relatively big petrol engines without turbos. Those are easy to inspect, they usually experience gradual wear, and the unexpected problems are normally caused by sensors or gaskets, which can cost some money, but it's nothing compared to a common-rail overhaul.

If you like to gamble, and repair bills of a few hundred or thousand Euros don't bother you too much, you can hop on the turbo petrol train. Those modern engines also have advantages, but to rule out the problematic models and even model years which have timing chain wear and/or carbon build-up, you have to be an expert. Sometimes the manufacturers changed some bits and pieces on the neurotic models during the lifecycle, and after a certain year or VIN the illness was cured. But beware: the worst petrol engines often have horrible problems even within the first 50 thousand kilometres.

The real adventurers buy modern diesels with a high mileage. If they're lucky and it doesn't break down while they own it, that can be a real bargain. There is a chance that within 10-20 thousand miles, before they resell it, nothing will fail. But the plan to fix everything and then it'll be problem-free for the next few years, can be expensive. After cleaning the DPF, probably you'll need to overhaul the fuel system, then the turbocharger and possibly the dual-mass flywheel, too. Soon you'll have spent more than you'll be able to save on fuel in the next five years. Don't believe it won't happen to you.

The internet forums are full of nightmare situations and wannabe or real gurus, so if you're really determined, with thorough research, you can find out quite a few things, but there are not many who could give you a list of problematic engines. If you need your car every day, if you have a nervous breakdown when the check engine light goes on, if you fear the situation of standing on the hard shoulder with an open bonnet, avoid the risks.

What size do you need?

Some time ago, there were lots of manufacturers who didn't even offer diesel engines in small cars – not without reason. The little runarounds usually don't do high mileage, plus there is no big difference in fuel consumption, so the more expensive diesel technology doesn't pay off quickly. Today, this has changed a bit, even the smallest cars became quite versatile, and there are many who drive them to the moon and back. But the fuel consumption difference is almost the same: you can easily find petrol cars which don't take more than 6 litres on 100 kilometres, while it's close to impossible to drive on less than 4 litres with any diesel engine.

The huge and heavy cars are the ones, where a petrol engine is a luxury. A family estate or a people carrier will be happy with 7-8 litres if it has a good diesel engine, while the same model with an adequate petrol engine will consume as much as 10-12 litres or more. Taken to the extreme, if you have to fill up 5 litres more on every 100 kilometres, within 25k you will have spent about 1500 Euros more at the petrol station. That's the price of a quite serious engine repair, and even the worst diesel engine doesn't break down every 25k kilometres. So the rule of thumb is: petrol in a small car, diesel in a big one.

Is the environment important for you?

The soot-spitting diesel is lung cancer on four wheels, according to a Chinese proverb. One could argue about this for weeks, but it's hard to prove the opposite. I'd be the happiest person on earth if I found an independent study which analyses the health effects of the exhaust gases of different engine designs. For now, I can only trust my feelings.

And my feelings say that petrol engines produce less toxic gases. Okay, petrol engines with direct injection can also emit soot, their tail-pipes are often black, which is not the case on a well-kept diesel with DPF, but around here, where disabled catalysts and chip-tuning is common, you'd die quite soon behind most diesel cars. For those who care about that, the newer and the more specifically petrol their car is, the clearer their conscience will be.

Here I have to mention again the Prius and the true hybrids, which not only perform well in terms of fuel consumption or ecological footprint, but they also emit less toxic gases than most of the normal cars.

How many kilometres do you drive?

Many say, for less than 20k kilometres a year it's not worth buying a diesel. That's not a bad guideline, but I'd also consider, how much I want to drive in total with that car. There are few people who keep their cars for 8 or 10 years, the majority sell them after 4 or 5. Within that period, usually something changes in our lives, we move, we get another job or there is a financial crisis going on in the world. It's worth a thought, how much the yearly mileage will be in the end.

You drive 10k from Christmas to Christmas and you know that in 3 years you'll be bored of your car? I wouldn't recommend a diesel with a high mileage. If there happens to be some trouble in those 30k miles, you won't be happy if you have to spend the value of the car on repairs. Adventurers welcome. Do you drive 40k a year and usually keep your car for at least 4-5 years? In your case, I'd consider a younger diesel. Even if some bad things do happen, and there is a 1-2000 Euro job, so many petrol bills will easily balance the expense. Just be careful to buy a robust car that doesn't collapse at the end of the 5th year.

Some more maths

I won't convey any more wisdoms, we now have the most important parameters. If you're curious how much you'll spend on your next car, you can take an excel-sheet and do your homework. I'm sure it'll help to make your decision, just be prepared for big numbers.

I'll leave you with the conclusion that in used, especially cheaper cars, a diesel engine is only recommended in special cases: i.e. if you need a big car, if you drive a lot and usually out of the city and if a bigger repair bill doesn't mess up your family life too much. I'd spend an hour or two, calculating all the possible costs of your next car, and then you probably won't have a nasty surprise.

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