alexa CAR CLINIC: Single slip at the pump can mean death to your engine - Daily Nation

CAR CLINIC: Single slip at the pump can mean death to your engine

Wednesday July 25 2018

Don’t just give order and look away as fuel attendant fills your tank. Your diesel engine might just be fed with petrol or vice versa.

Don’t just give order and look away as fuel attendant fills your tank. Your diesel engine might just be fed with petrol or vice versa. PHOTO | FILE 

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They say you don’t seek professional advice until you are in trouble. I’m a regular reader of your column and I need advice. Recently, I was the victim of a careless fuel pump attendant. This attendant most likely was in a foul mood since she could tell my pickup has a diesel engine, but went ahead to fill it with petrol.

Luckily, I noticed this mistake about two litres in. I consulted the fuel station’s management who brought in their mechanic and we drained the tank. What would have happened had I not noticed this mistake? Please elaborate on mechanical and operational issues that would have risen.

And lastly, a caution to motorists: always be conscious while fuelling, shout ‘diesel’ if your engine runs on diesel. Or does anyone have a remedy to this kind of mistake?


A diesel-powered engine has to be bled when it
A diesel-powered engine has to be bled when it generates an airlock or is fed the wrong fuel. PHOTO | FOTOSEARCH

Hi Ben,


That was a close one, huh? I reckon yours is an older model vehicle or at least one with comparatively less complex underskin electrotrickery for the era. Given that diesel engines have to be bled when they generate an airlock or are fed the wrong fuel, having a complicated contemporary car will be a curse if you ever need to bleed the system.

Yes, it pays to be vigilant whenever someone is handling our vehicle. Had you tried to run your car on petrol, you may have damaged your injectors and possibly knocked your engine. Let's start with the first:

Diesel fuel is thick and viscous, so for it to be atomised to burn properly in the engine, it has to be pushed at very high pressure through the injector nozzles. This high pressure is generated by a tiny piston within the injector that operates with a great amount of force. It is designed to do so. So when petrol, which is much thinner and less viscous comes into the injector, the tiny piston meets less resistance and moves faster and further than it is designed to, and may thus bump against the nozzle aperture or lose its synchronicity.

There will also be dribble of liquid petrol through the nozzle into the cylinder, which washes away lubricating oil from the cylinder walls. Given the size of components we are talking about, this cannot be fixed by hand; you will just have to replace the injectors.

As for knock: petrol and diesel burn at different rates using different ignition systems. Petrol is highly volatile while diesel isn't. Their respective engines also run different timings, which is the relationship between when the fuel is injected into the cylinder and the moment it is ignited. Putting petrol where diesel is meant to go, and with the extremely high attendant compression ratios of a diesel engine, means hot air and petrol will mix at moments and in places where they are not meant to, such as outside the cylinder, which means knock at best and a wrecked engine on a bad day.

In layman's terms, this means that as your
In layman's terms, this means that as your tyres get progressively wider (and with lower profile), you need to put relatively less pressure. PHOTO | FOTOSEARCH


Learn physics of tyre pressure for better driving

Hi Baraza,

I am a fan of the car clinic articles. My car tyres size is 225/45R17. I recently took the car for wheel balancing and in the process, the mechanic noticed one of the rims had a slight bend. He said I might have hit a pothole, resulting in the bend.

The mechanic then asked how much pressure I usually inflate the tires with, which is 35 (as advised by the previous owner). He said for low-profile tires, the pressure should be at least 40 psi. Should I go with his advice or will I be over inflating hence creating new problems?

I have also noticed when getting over a small obstacle or when at standstill in an uneven surface, the tyre on the side of the car with more weight seems underinflated.

Anthony Mwangi


Hi Anthony,


Remember that difficult little subject in school called Physics? I have heard people snidely remark about the uselessness of studying it given that “we will have nowhere to apply those ridiculous formulae in real life". Well, well, well... Look whose chickens have come home to roost.

I am not accusing you of falsely laying bare the impertinence of physics in the day-to-day life, but what I can say with confidence is neither you nor your mechanic has a grasp of it. Dimensions, the first topic in the first year of university, explains the origins and definitions of SI units. Simple physics shows pressure is force per unit area. Let me explain:

The car exerts a force towards the ground because of its mass being acted upon by gravitational pull. This force is exerted over a certain area, which is the sum total of the contact patches of all tyres (usually four).

So, the force required to hold the car up comfortably (both literally and mechanically) is constant, since the vehicle mass is constant (more or less) and so is gravity. This leaves two dependent variables in the equation: air pressure inside the tyre and the surface area of the contact patch of the tyre tread. From

P = F/A,

we can correlate the two dependent variables thus,

A = F/P

which means that area and pressure are inversely proportional, with the constant being force. What does this mean?

This means that as area (size of the contact patch) increases, pressure reduces. In layman's terms, this means that as your tyres get progressively wider (and with lower profile), you need to put relatively less pressure. This explains why motorcycle riders put very high pressure in their tyres, up to 70 psi, while people with really wide, sporty tyres do only 25, or even less.

"Donuts" (space-saver spare) are very thin, so the recommended tyre pressure for them is 60 psi. I have 195-section tyres (don't ask!) into which I add pressure up to 40 psi, which is admittedly on the higher side, but I have my reasons. The common recommendation for this size of tyre is between 30 and 32. You say you have 225-section tyres, so you should be doing less than 30 psi to avoid overinflating your tyres.

(Note: the examples cited are for ordinary passenger cars. SUVs, commercial vehicles and trucks come with their own sets of numbers due to load bearing considerations. Note 2: when loading up a car, the relationship that varies is between F — force — and P — pressure — which is why the heavier the load in the vehicle, the more the pressure needed in the tyres).

Range Rover is majestic, but costly to maintain

Hi there. You one time lugubriously took me down when I told you I wanted a Range Rover. (The UN guy). Well, I bought a 2007 Range Rover Sport Hse from someone in Parklands. What a great experience it is.

But, recently, actually a week ago, I noticed my vehicle was abnormally close to the ground on the passenger’s side. The throttle stays open (and seemingly open a bit longer) for some time after I remove my foot from the accelerator pedal. It troubled me at slower speeds and during parallel parking.

When simple idling is expected, the vehicle seems to continue as if the accelerator is lightly pressed. What on this earth did I get myself into? This time around be more kind.


A Range Rover is painfully expensive to run.
A Range Rover is painfully expensive to run. PHOTO | FOTOSEARCH


Hahaa, look who's back!

Hello, Mr Okumu. Let's be clear, I didn't take you down, lugubriously or otherwise, I just presented facts and unpleasant truths. But, it is interesting to note that you chose to buy a Range Rover after all. Naturally, Range Rovers will offer a sublime experience, whether from a driver's or passenger's perspective — and this was just the Sport. Wait till you try the grandfather, the real thing, the Vogue. You will be blown away.

To be blown away along with you will be your money. I don't know why you people make me repeat myself, but the answer to your question "what did I get myself into" is this: "Exactly what I told you would happen".

Am I right or not? I'm not proud to be right, because I love Range Rovers, but they are painfully expensive to run and their increasing complexity model after model is not doing any favours to their questionable reputation on the reliability front. So, your Sport is sitting lower than usual on one side? These could be leaky air bags for the air suspension. They will need replacement and while at it, check the compressor pump just to make sure it is not about to retire at short notice. These are problems to be expected with a Sport.

The erratic throttle and basket-case idling may not be an expected problem, but it is a clear and present one. Sounds electronic if you ask me, so find someone with some software and RS232 links or something. A hammer and a socket wrench won't solve this one.

Did I say electronic? The parking brake is electronic as well and guess what? That is a problem area to expect. Tick tock...

A financial manager will tell you to now draft a new monthly budget to cater for the rising expenses, but this won't work. The reason Land Rover products are the subject of derision in 4x4 forums is because along their predictable issues (air suspension, turbos, gaskets etc.), there are unpredictable ones and these outnumber the bad weather in the forecast by two to one. There are parts of these new cars that will fail and you will not see it coming, but it will cost you dearly. Or not; sometimes they run beautifully year after year, offering little offence to your bank balance and lulling you into a false sense of security — and a possible argument with Car Clinic about trashing brand reputations and burning images (kuchoma picha). This is before a reality check comes through the mail to remind you that you are dabbling in the same automotive accessorising exercise as the Queen of England and “it may be time for the unwashed plebe to return to his roots, so will you please see him out the door, Jeeves?

How, m'lady?

Hit him with the repair invoice for an airmatic suspension whatchamacallit, Jeeves, and give him throttle problems that will not be easily solved... and give him a need for a new turbo too, while at it. Provide him with enough internets to write in to his fellow peasant at the newspaper, the one that acts like he invented cars. Let them play among each other, Jeeves; royalty will not tolerate intrusion of this kind”.



Same car, different horsepower ratings. Why?


Hi Baraza,

I never thought at I’ll need advice from a motoring expert, not because I lack interest, but because I consider myself a know-it-all car enthusiast. But I guess I'm wrong. I would like to know about the new BR9 wagon Legacy GT turbo (2009-2012 model) 5 speed automatic. I plan on buying one soon but I'm not sure how it compares to the new Forester XT SH9 turbo in terms of performance and acceleration. Is it a fast car or should I look elsewhere if I want a fast car? I really love fast cars. If you have driven one please share your experience too.

Secondly, the Legacy turbo is mostly rated at 265hp, but recently I discovered that Japanese models boast of 281hp while US models boast of 265hp. So, what is the horsepower rating that we get from the new Legacies sold at our local car yards?



BR9 wagon Legacy GT turbo is a quicker Subaru
BR9 wagon Legacy GT turbo is a quicker Subaru than the SH9 turbo. PHOTO | FOTOSEARCH

Hello Fredrick,

The new BR9 wagon in GT spec should be a generally quicker vehicle than the SH9 turbo, unless the SH9 is a tS (which means tuned by STi), in which case, well... yes, driver mettle comes to the fore. These cars don't drive themselves anyway; I know a lot of people who would be faster in a weaker vehicle compared to 99 per cent of drivers in more powerful stuff.

Then there is the Legacy of my dreams, the 2.0 Di-T, which stands for direct injection turbo, which in turn means a legitimate 300 horsepower count straight off the factory line. It is not the prettiest car (in fact, the BR is one of the more unsightly contemporary Subaru products of late), and the tech — direct injection and turbocharging — means reliability is a fine dance on the thin line separating if and when; but I don't really care. The 300hp factory tunes coupled to 6-speed manual transmissions and symmetrical AWD all add up to one thing: indefatigability and invincibility on tarmac.

Not all cars are built the same for all markets, despite unchanging outward appearances and model designations. Market peculiarities such as taste, legislation and fuel quality means what is sold in Japan may not be the same as what is sold in the US, which is not the same as what is sold in the UK. Generally — or rather, particularly with Subarus — Japan gets the hot version, with UK getting something a little cooled down. The US will get whatever their emissions regulations will allow. A good example is the GDB Subaru Impreza WRX STi, the so-called "blob-eye". It did the exact same figures per market that you quote for the Legacy GT: the Japanese car had 281hp, but what the UK got officially (via the Subaru dealer, and not Prodrive) had the number set at 265. This means that the horsepower rating of what we get from the new Legacy cars sold at our local yards will depend on where the car came from.

It will also depend on how well kept it is and what kind of fuel it is running on. Engines adapt to fuel type (RON-based, not petrol vs diesel) to save themselves from damage, so an engine running on bad fuel will cut power automatically. Then it will also come down to what altitude the engine is operating at. Yes, 265hp is not really 265hp when you are 2km above sea level. The difference in air pressure creates a difference in performance too, turbo or not. There is something called altitude compensation, but let's avoid techno-babbly rabbit holes for now.