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Many of you will have been involved with scores of breakdowns caused by contaminated petrol.
When the contaminated fuel issue first hit the news, the suspected contaminant was excessive quantities of ethanol presumably, because the affected site already supplied petrol containing some ethanol. This theory was demolished fairly quickly, but I was intrigued why the experts then suspected silicon. This became clearer when we found out that the damaged oxygen sensors were coated in a grey/white ash-like deposit, with a pink tinge.
This is similar to the damage seen when the head gasket fails and lots of antifreeze ends up in the exhaust. The silicate concentration in most antifreezes has been substantially reduced, although BMW still favour a high silicone antifreeze. [It’s the silicones which react when you mix the wrong types of antifreeze and precipitate out as a nice silicate goo].
Contamination of the sensor stops it ‘seeing’ oxygen so it stops switching and produces a constant signal. The ECU thinks the mixture is too rich and keeps weakening the mixture until the engine loses power and eventually cuts out – which is what happened in the South East when fuel from the Vopak terminal on the Thames Estuary became contaminated with high levels of silicone.
The silicone was apparently found in four finishing tanks in which fuel is mixed with the additive package. One of the normal additives is toluene and it was this which contained the silicone.
Lambda sensors work by producing a voltage signal, based on the amount of unburned oxygen in the exhaust.
For the conventional (i.e. not broadband) lambda sensor, manufacturers either use zirconium or titanium in the tip of the sensor. The two metals make them behave in quite different ways.
ZirconiumWhen the sensor reaches its operating temperature of around 250ºC, it compares the amount of oxygen in the exhaust gases with that in the surrounding air. The zirconium dioxide uses this difference to produce a voltage – the greater the difference, the higher the sensor’s output voltage.
At 0.2 volts, the mixture is lean. 0.8 volts is rich. Stoichiometric fuel mixture of 14.7 parts air to 1 part fuel gives a voltage of around 0.45 volts.
Oxygen sensors don’t hold the same voltage – the voltages rise and fall from rich to lean and back again. This voltage reversal from high to low or low to high is called a ‘cross count’.
An oxygen sensor in good condition should alternate from rich to lean around once every second. Most lambdas cycle from rich to lean in somewhere between 50 and 100 milliseconds. The change from lean to rich takes 75 to 150 milliseconds and this is called the ‘transition’ time. If these times become much bigger than once every second it’s time for a new lambda sensor. Sluggish oxygen sensors can cause hesitation during sudden acceleration. An oscilloscope pattern is a good way of looking at this.
Heated Oxygen Sensors (HEGO)
A cold oxygen sensor isn’t a lot of use – they need to be hot to work properly. If you fit a heating element, they can reach 500ºC in around eight seconds. When the oxygen sensor is hot, the engine management system can work properly in closed loop mode – so emissions are better and so is fuel economy. HEGOs can also be fitted further down the exhaust pipe. You may have noticed chronic space depletion under the bonnet of the majority of modern cars.
Titanium
Not all manufacturers use zirconia – some, like Vauxhall, prefer titanium – which doesn’t work anything like zirconia. It has an identity problem – thinks it’s a coolant temperature sensor and behaves like one. A titanium lambda changes resistance as the air/fuel ratio goes from rich to lean. It doesn’t do this gradually, it switches very, very quickly from a low resistance of less than 1000 ohms when the mixture is rich to high resistance of over 20 000 ohms when the mixture is lean.
The engine management ECU supplies a base reference voltage of 1 volt to the titanium sensor and then reads the voltage flowing through the sensor to monitor the air/fuel ratio. When the mixture is rich the resistance is low and the sensor’s voltage signal is high. A lean fuel mixture means resistance rises really quickly and the voltage signal drops.
What Goes Wrong?
Sensors can fail if they get sooted up, covered in lead from leaded petrol or silicone ash from antifreeze or contaminated fuel.
They also become sluggish with age and eventually either produce either an unchanging signal – or no signal at all. This is likely to trigger the MIL and the car won’t drive very well because the mixture is too rich. Symptoms are poor fuel economy, higher carbon monoxide and hydrocarbon emissions, lousy idle and/or hesitation during acceleration.
If a lambda’s average voltage is high – that’s over half a volt, that means the mixture is rich. This could be caused by a faulty MAP, air flow. coolant temperature sensor, fuel pressure regulator – or possibly a leaky injector.
Low voltage readings (less than 0.45 volts) means a weak mixture so look out for an air leak, blocked injectors faulty fuel pressure regulator or a poorly sensor.
As mentioned earlier, in the contamination bit, if the sensor keeps reading rich, the ECU will compensate by weakening the mixture until the engine loses power and eventually cuts out. Lots of cars did just that and needed new oxygen sensors.
Other symptoms included lean misfire, hesitation, poor idle and high hydrocarbon emissions caused by the misfire.
If the lambda continually reads low – that’s lean - the ECU enriches the mixture by increasing the injector pulse width – fuel consumption rises and carbon monoxide emissions go up. The constantly rich mixture causes the catalyst to overheat and it may be damaged.
If a heated sensor has a faulty heating circuit or element the sensor can cool down at idle and so the system will go into open loop. This usually results in a fixed, rich fuel mixture with high emissions.
Enquiries from members (in relation to garage repairs) suggest a disappointing number of oxygen sensors are changed without any further investigation. An air leak can still cause weak mixtures, so can a dodgy spark plug.
A lambda sensor is just that – a sensor. It doesn’t have a brain, it measures oxygen. It doesn’t wander back up the exhaust pipe and find out where the oxygen is coming from - it just tells the ECU it’s got lots.
Lambda sensors fitted in the exhaust, earth through the exhaust manifold. Like anywhere else rust on this earthing point can create a high resistance and might upset the sensor’s output. If you use your multimeter to eliminate this possibility check for volt drop between the sensor casing and the engine block. More than 0.1V, suggests a problem.
Testing Oxygen Sensors
Early sensors used a single live wire to send a signal to the ECU and they earthed on the exhaust pipe. Heated sensors, which reach operating temperature much earlier have four wires:
- sensor’s heating element supply wire
- earth wire
- sensor’s signal wire
- HEGOs also use the exhaust as an earth, but the latest ones tend to use an independent sensor wire to improve reliability.
To test the sensor, set the voltmeter to DC and, while all the sensor wires are still connected, connect the positive probe of the voltmeter to the sensor’s signal wire. The negative meter probe needs to go to the battery’s negative connection and then start the engine. We saw earlier that lambda sensors at working temperature, send signals which fluctuate between 0.2V for a lean mixture and 0.8V when the mixture is rich – set the multimeter to record the highest and lowest readings and then the average reading this should be 0.45 volts indicating correct combustion. If the reading is lower then we have a weak mixture. higher a rich mixture so these are the readings you are looking for in a correctly working oxygen sensor.
© Vanessa Guyll, June 08 | 