Ask the Mechanic

Thursday September 16 2021

A computor diagnosis at the garage should help determine what is faulty and needs to be replaced. PHOTO/internet

By Paul Kaganzi


Hi Paul, I drive a 2008 Mercedes ML 350 and its been a good vehicle for three years with no major issues. Recently, it started to self-brake while I drove fast or slow. The braking is intermittent and random. The ABS light and cruise control warning are displayed. What could be the cause of this problem? Please advise.

Anita, Kabuye.

Hello Anita, you may have a faulty ABS wheel speed sensor or damaged Yaw rate sensor. A computer diagnosis at the garage should help determine what is faulty and needs to be replaced.

ABS (Anti-Lock brake system) speed sensors convert pulses generated by their contact with magnetic rings on the moving wheel bearings.

These pulses are converted into speed signals by the ABS computer which shares this information with different controller modules in the engine, electronic stability programme and transmission. The Yaw rate sensor measures vehicle angular velocity about the vertical axis.


This information is shared with the above control modules which monitor vehicle stability or orientation during extreme cornering or maneuvering to help prevent roll over.

When either of these sensors malfunctions, the control modules mentioned will brake the car or reduce performance autonomously, hence the self-braking.



I kindly request you to differentiate between Shell V-Power and Total Excellium and which one is better for a motorcycle engine? Secondly, is it okay to be changing fuel between V-power and Excellium because I have been using V-power ever since I bought my 125cc TVS motorcycle, I just fear to alternate with Total Excellium. Thanks.

Quraish Golooba

Hello Quraish, there are two kinds of gasoline (petrol) unleaded fuels in Uganda, the higher octane 95 premium Shell V-Power and the lower octane 93 regular unleaded petrol fuels.

Both kinds of gasoline fuels are approved for use on your TVS 125cc motorcycle but work differently because of the science in their composition.

The major difference between Shell V-Power and other regular unleaded gasoline fuels is a higher octane number. Higher octane number improves engine efficiency and performance by reducing knock.  

Octane number is a standard measure of how long a fuel in an internal combustion engine, can withstand compression without detonation. Gasoline engines can experience cylinder knock, during detonation after ignition of fuel. This reduces the engine performance and efficiency, especially when the engine management system detunes to prevent engine damage.

 Knock is caused when fuel and air mixture ignite abnormally resulting in engine power loss and less efficiency.

Shell V-Power unleaded improves engine performance by reducing friction using a friction-reducing molecule that continues to perform across varying conditions.

This friction modifier gets to work where it is needed most in the heart of the engine. Shell V-Power Unleaded lays a thicker coat of lubrication against the cylinder walls. This helps the piston assembly, in the heart of your engine, to turn more freely to help reduce energy wasted as heat and friction.

Shell V-Power (SVP) Unleaded has a superior engine cleaning formula. Over time dirt can build up on air intake valves and block fuel injector nozzles which are as thin as human hair.

This causes poor fuel economy and reduced engine performance. SVP Unleaded’s dirt bursting molecule works three times harder than cleaning molecules in regular gasoline fuels, to remove dirt from vital engine fuel system components. This improves the air fuel mixture which restores or maintains excellent engine efficiency and performance.

Comparing the two kinds of gasoline fuels premium and regular is like comparing a cup of tea blended with herbs and spices with another cup blended with plain tea leaves.



Hello, I drive a Toyota Premio 2004 model that has recently started to show the ‘check engine’ light occasionally.  I took it for diagnosis and I was told that it has a rich fuel mixture, that is causing black soot on the spark plugs, and thus the ‘check engine’ light.  Spark plugs, mass air flow sensor and the clogged fuel filter/pump were all changed and the check engine light did not appear for about one week of driving but then it returned eventually.  Kindly advise on what I can do to fix this. Best regards.

Daniel Agaba

Hello Daniel, your mechanic or service provider ought to consider other factors that affect the ratios in the fuel air mixture.

The ratio of fuel and air is crucial for total combustion during engine running. Ultimately this affects efficiency and performance of your car’s engine.

A persistent rich mixture of fuel and air can damage engine intake valves and pistons by causing excessive build-up of deposits from crusting of un-burnt fuel.  When the ratio of fuel to air is lean you need to look for factors curtailing fuel pressure, whereas a rich fuel air mixture is one where there is more fuel in the mixture causing a distortion of recommended ratio.

In the case of your Toyota inspect the hoses in the vacuum system which may develop hairline cracks or may have a loose connection. Look out for faulty fuel injector(s) which can deliver unregulated amounts of fuel.

A dirty air cleaner will reduce the amount of air in the mixture while a bad oxygen sensor will give wrong values to the engine computer and lead to delivery of excessive amounts of fuel.

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