Ask the Mechanic

WHAT CAUSES FUEL TO MIX WITH ENGINE OIL?

Hello Paul, I have a 2001 Toyota RunX with a 3ZZ-FE engine. Of late, it overheats a lot and an engine overhaul has not solved this problem. Now, there is a problem of fuel mixing with oil. What could be the problem and how can it be fixed? 

Tdoó

Hello Tdoó, your Toyota RunX’s engine oil dilution with petrol is a serious problem. After the said engine overhaul, there seems to be a delivery of excess petrol in the combustion chamber and this unburnt fuel seeps past the engine piston rings and cylinder bores to the engine oil pan. When engine oil accidentally mixes with petrol, its protective and performance enhancing roles reduce. Fuel contamination of oil alters the oil viscosity or thickness, a critical aspect of engine protection and performance.

The oil’s thin film should be thick enough to prevent metal to metal contact and friction and sheer damage or seizure of fast moving important engine components such as pistons, cylinder boxes, crankshaft and valves, among others. Fuel contamination of the engine oil can lead to severe damage of the above components that were replaced during the engine overhaul.

The likely causes of this situation include fuel injector failure, spark plug wear, VVTi oil control valve break down or the engine computer failure. Fuel injector failure allows the pressurised fuel from the Direct Four (D4) fuel rail to flood the combustion chamber and seep down to the engine oil pan (sump) below. Spark plug wear can lead to accumulation of uncombusted fuel, which finds its way to the engine oil.

A failure of the variable valve timing oil control valve can prevent timely engine air intake valve lift which distorts the air fuel ratio. This would make the car ‘run rich’ with more fuel than air, a situation that leads to delivering excessive amounts of fuel in the combustion chamber. A breakdown of the engine computer (ECM) may lead to excess fuel delivered for combustion in your Toyota engine. The fuel injectors and VVTi system mentioned above are also controlled by the engine computer. A good diagnostic technician should be able to inspect and pinpoint the cause of this fuel infiltration of the lubrication system.

WHY DOES MY CAR FAIL TO START?

Hello Paul, I have a Subaru Legacy B4 model 2004, 2.0 with k25 power. Sometimes, after just a few minutes of the car being parked, the car refuses to start right away. It will, however, start after at second try. What could be the problem?

Martin

Hello Martin, your Subaru’s erratic start failure may be a result of an electrical fault. The range of possibilities is wide and definitely requires inspection or a computer diagnosis rather than a remote diagnosis. The symptoms you describe will require a good car repair technician to check power supply from the battery, including the terminals.

The starter system and fuel supply, its connections and relay should be inspected for loose connections which cause the intermittent failure to crank.

IS MY CAR’S TEMPERATURE GAUGE FAULTY?

Hello Paul, I have a 2001 RAV4 that has done 200,000kms and is reasonably maintained. At high speeds on the highway after 10 minutes or more (70kmh and above), the temperature gauge slowly starts dropping that it is almost cold. At slower speeds, traffic jam or going over humps it goes back up to medium (the optimal temperature). It does not go above optimum. Is this a normal situation or something for me to worry about?

Trevor

Hello Trevor, you may need a good mechanic or car electrician to investigate the performance of your RAV4 engine’s ECT sensor (coolant temperature switch or sensor). The ECT sensor is a thermistor switch designed to send different electrical impulses with changing temperature. This will be relayed to the engine computer, which in turn relays it to the temperature gauge and the engine cooling fan system.

As you drive, the temperature of the engine rises to the normal operating temperature, which is about 80 degrees centigrade. When the temperature exceeds 80 degrees, the fan speed usually increases as the thermostat opens to allow cooler water to flow into the engine and bring the temperature back to 80 degrees or just below. So, the temperature gauge should drop or rise consistently and not erratically as it is doing.

Once the ECT sensor is checked and found faulty, it should be replaced.  Should it be okay, then investigate the circuit between the ECT sensor and engine computer or the engine temperature gauge. A computer diagnostic tool will shorten the investigation time.

MY CAR’S COOLING FAN IS LOUDER THAN IT SHOULD BE

Hello Paul, I drive a BMW X6 3.5 petrol which has developed a strange fault lately. When I start the car, the engine fan starts to run fast and makes a loud noise. What should be checked?

Andrew

Hello Andrew, the engine blower fan is controlled by an automated system actuated by a control module which is linked to the main engine control module. This engine blower fan performance is regulated by changing engine temperature and coolant flow conditions. There are engine temperature and coolant pressure sensors which keep updating the engine control module (ECM) with changing conditions. The ECM will determine the blower fan speeds according to the changing values communicated by the coolant temperature and pressure sensors.

The performance of the blower fan is, therefore, affected by the operation of the thermostat and engine coolant water pump. In the event that one or both of these components fail, the engine computer will activate emergency running. This is designed to keep the engine extra cooled under the assumption that engine coolant is not flowing as it should.

Get a qualified BMW technician to run a computer diagnosis which should confirm my hunch that the thermostat or coolant pump or both have failed.

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