Your diesel engine needs to be overhauled after all the revving

Hello JM,

I am a car enthusiast and an ardent reader of your column.

A few days ago, I was driving a Toyota Hiace, powered by a 3 litre 5L engine of Toyota’s famous L Series. The clutch kit had been replaced a few weeks back and was running smoothly. I was going up a steep slope on fourth gear, until I needed more power to complete the slope, so I did the normal thing – eased off the gas pedal, stepped on the clutch and pushed the stick “towards gear 3”.

On releasing the clutch, the engine revved really hard, so hard I thought I was going to break everything, including my eardrums. I thought the revving would go away if I took the clutch back in but it didn’t and in  less than give seconds, I quickly turned off the engine. By then I had  realised I had not pushed the gear shift hard enough to 3 and had instead stopped at neutral, a very expensive mistake when moving uphill, but that I could handle. What I couldn’t is, when I ignited the engine, it resumed the mad revving! It revved so hard that I had to turn it off . This got me puzzled because I was in neutral, with my right foot actually on the brake pedal. Beginning to brace myself for a night out in the cold, I ignited the engine timidly; it picked up well, luckily, and the rest of my drive was normal.

What could be the reason for this?

D.K.

There is one thing we are not quite clear on. When the engine  was revving itself, was your foot on the accelerator pedal? If yes, then the answer is obvious: take your foot off the throttle. But I’m guessing the answer is no. This is what might have happened.

You might have damaged your engine slightly during that fateful gear change. My suspicion  that is when you tried to downshift into third, you did not stop at neutral as you suspect; you might have gone into first gear instead. The two gear positions are juxtaposed anyway, and with a wonky linkage, missing one for the other is not unheard of. Going into too low a gear for the prevailing road speed causes the engine to over-rev. The problem with over-revving a diesel engine is that it suffers from a phenomenon I have discussed here before called “hydraulicking”, whereby the engine feeds on itself.

Diesel engines are made of very heavy components that are designed to withstand the attendant high pressures and comparatively low engine speeds. So when the engine speed shoots up suddenly, the laws of physics governing inertia and momentum come into play and one of the many things that might  happen is that the valve seals or piston rings might get shattered, which means oil starts seeping into the cylinders in sufficient quantities.

Sufficient? Yes, sufficient. You see, diesel fuel is actually very similar to oil (and is sometimes referred to as diesel oil) and a diesel engine can actually run on engine oil. So when hot engine oil starts entering the cylinders, it is no longer lubrication, it is now fuel; and this begins a vicious circle that might quickly destroy your engine. The leaking oil, now acting as fuel, causes the engine to rev up - diesel engine speeds are controlled by metering fuel, not air like in petrol engines, so with a surfeit of “fuel” (oil), the engine revs rise to the maximum. The high engine speeds create a greater suction effect, which in turn pulls more oil into the cylinders, which cause the revs to rise even higher, thereby increasing the suction effect even more and.... you can see where this is going. Within seconds your engine will be roaring in a horrific final battle cry as it eats itself alive, inevitably ending in a massive seizure and the need for a new engine.

It seems in your case that the seal breach was not as bad as it could have been, seeing how the car behaved itself afterwards. Have that engine overhauled, though, if you don’t want a repeat of this incident.

The second time round could prove fatal for your vehicle.

Hi Baraza,

Thank you for the great insights you provide into motoring issues every week; I am a huge fan of your column.

I need some serious insight into a problem with my car.  I have a BMW 320i E46, M54. I have parked it for a month now because it stalled as I was driving on the highway after suddenly losing power, jerking and acting up and my frustrating efforts to stay on the accelerator to avoid the stalling were in vain. It just picked and lost momentum then ground to a halt.

I’ve had three different mechanics look into my problem and they have all given me different, scary responses. I have since serviced it, i.e. changed plugs, oil, fuel filter etc., washed my fuel tank, checked my fuel pump and fuel pressure but it still won’t start. I’ve heard all sorts of jua kali diagnoses (Boss, hapa inabidi ujitayarishe), from knocking, oil pressure, etc. You know what this means to my wallet and I will have to  take it seriously. However, the last mechanic said a diagnostic scan was needed to identify the problem, and after the scanning, I got the following error codes:

12  Signal, camshaft sensor, exhaust

D9  Signal CAN EGS

96 Oxygen-sensor before catalytic converter,bank1

37  Activation oxygen sensor heater before catalytic converter, bank2

67 VANOS inlet camshaft end position not reached

68  VANOS exhaust camshaft end position not reached.

He suggested that all we needed to do was change was the camshaft positioning sensor and I would be good to go but with all these errors, I feel I need to a lot more  information  before I let any spanner on my BMW.

Could you offer me any guidance since I am about to lose my mind because when it rains, I would really want to enjoy the comfort of my car.

 K. Karanja

The guidance here might not be what you want to hear but it will at least offer you peace of mind at the end of the day. Pay a visit to the local BMW dealer (Bavaria Motors). They  have the expertise to let you know whether all you need to do is replace the errant sensors, or if there are actually problems with the camshaft positions and emissions control systems that call for extensive repairs. Not to break your heart, but my money is on the latter.

Hi Baraza,

I have a Mitsubishi Gallant that needs serious repair. I need advise on possible cost-effective options that can help me achieve this.

Hello,

The best possible cost-effective option for a Galant that needs serious repair is to find a Mitsubishi mechanic who knows what he is doing. There is no other way out because selling the car might not really be an option if we are using terms like “cost effective”. Galants already suffer from very low pricing on the used-car market,  so selling one in a sad state of disrepair will be similar to pulling a hen’s teeth, unless you want to give it away for free which, as already stated, does not fall under “cost effective”.

Hello Baraza,

Looking at a 2005 Subaru Forester 2000cc and 2005 Caldina 1800cc, the Subaru is considered more macho but the Caldina wins on consumption. I’m told it does 14-15km/l while the Subaru does 9-10km/l.

Please compare the two.

Steve

I think you already have. What more did you need to know?

Hi Baraza,

I appreciate your column very much, keep up the good work.

Now, I want to get a used car but my budget is lean (between Sh300,000 and Sh350,000) but I am not sure about  the better car to buy between a Toyota 110, Nissan Note/Wingroad/B15. Which of the above would you recommend in terms of performance, spares can fuel efficiency?

B. Kamau

This will be as clichéd as you can imagine, but the answer is fairly obvious, and for obvious reasons. Buy the Toyota 110. It is more reliable than the Nissan trio, it looks better than the Nissan trio (I know this is subjective, but come on, you agree with me, don’t you?) and while it might not be as practical as the Wingroad, it is more practical than the Note.

Also, with between Sh300,000 and Sh350,000, any “newish” car you might get will more likely than not suffer from a litany of underlying issues, which will be a pain and a bother to put right.

Having car trouble? Write to [email protected] for free advice.