Volkswagen Fox 1.2
Probing the Simos 9.1 management system.
Volkswagen’s three-door Fox was intended to be cost-effective to buy and run, but how does it shape up diagnostically? Kim Henson and Dean Birch investigate the 1.2-litre three-cylinder version.
Over the years, many vehicles have been given the name ‘Fox’, including an NSU motorcycle from the late 1940s. The name became available to Volkswagen after the company acquired NSU two decades later.
Based on the platform of the contemporary Polo, but introduced as a deliberately inexpensive entry-level car, the Brazilian-built, three-door Fox arrived in Britain in the summer of 2006, replacing the Lupo as the range starter for Volkswagen. Two engine options were offered initially: a 1.2-litre three-cylinder petrol and a 1.4-litre four-cylinder petrol. A more powerful 60bhp 1.2-litre Fox was sold from 2010 to 2012, before the model was replaced by the new VW Up!, which went on to win the World Car of the Year award for that year.
Our guinea pig Fox for this month’s diagnostic feature is a three-cylinder, six-valve 1.2-litre petrol version with a BMD engine code, dating from 2008. The 1198cc engine in this model produces 54bhp and 108Nm (80lb ft) of torque, with typical real world fuel consumption of between 45-50mpg.
Our guide to this VW’S engine and its Simos 9.1 management system is Dean Birch of Onboard-diagnostics (info@onboard-diagnostics.co.uk).
Preparation
Underbonnet access on this Fox is excellent, with plenty of room around the power unit.
Before you commence any diagnostic operations, establish the past maintenance history of the vehicle. If you are unsure of when it was last serviced, carry out a full service on the vehicle at the outset. One important aspect that’s often overlooked is regular renewal of the fuel filter, as advised by Volkswagen; the filter is located under the rear of the vehicle on the right-hand side (see Photo No 3). Use only the correct type of filter, at least to Volkswagen original specifications.
Also ensure that the oil and air filters are high quality items – again, at least to VW OE specifications. The same applies to the spark plugs, which need to be clean and have their gaps set correctly. Closely inspect the breather hoses, ensuring that there are no splits or dodgy connections – if their condition is in any doubt, renew them.
Fault 1:
Mechanical problems within engine
Symptoms of our first fault will vary in severity depending on the depth and progression of the problem. They include an unstable tickover, misfiring, ‘popping’ in the exhaust, illumination of the management warning lamp (which may flash to indicate a misfire), and diagnostic interrogation revealing stored fault codes with ‘P0300’ designations, relating to cylinder problems. Random misfires would be suggested by a ‘P0300’
code, but 'P0301', 'P0302', 'P0303', etc, indicate specific problems with one or more individual cylinder.
In our experience, very often the operator or repairer of the vehicle will have treated such fault codes as being applicable to ignition or fuel-related difficulties. In such cases, the car may be fitted with new coils, fuel injectors, etc, which have made no difference to the engine's poor running.
If fault codes indicating cylinder misfires are stored, you should carry out a compression test. This will help to identify mechanical ailments within the engine, including poorly-sealing exhaust valves, to which this engine is prone, or piston/ring/cylinder problems. Thereõs no point renewing ignition components to try to cure an engine which is low on compression.
Ideally, you should see compression pressure figures of above 150psi, and the readings should be very similar across all cylinders. Low figures require investigation. If the compression figures are healthy, spark and fuel-related diagnostic work will be required.
In the early stages of such mechanical problems, you may notice little or nothing wrong, yet the management system can flag up misfires by the storing of fault codes. This is because the computer determines misfire events by calculating engine speed from the crankshaft sensor, analysing each cylinder's contribution to the crankshaft speed, and determining which cylinder is not pulling its weight.
Some of the more sophisticated
diagnostic tools enable the operator to display live data and, from available parameters, look at misfire counts, indicating which cylinders are in trouble.
Fault 2:
Oxygen/o2/lambda sensor wiring
Our second problem on this Fox can result in illumination of the engine management system warning lamp, although in terms of driveability there may be nothing amiss. Fault codes may be stored, indicating difficulties with ‘Upstream/downstream Lambda circuit’, ‘Lambda heater circuit malfunction’ or ‘Lambda sensor signal implausible’, etc.
The sensors are positioned in pre- and post-catalytic converter exhaust system locations, and these should be checked. However, there have been instances of ailments with the sensors’ wiring rather than the units themselves. Difficulties often arise at the point where the sensor wiring joins the main loom in the vicinity of the gearbox. Here, the cables and connections are exposed to the elements and, over time, can suffer from corrosion and/or breakage, usually close to the harness plug.
To carry out a detailed inspection, first raise and securely support the front of the car, then closely scrutinise the wiring from underneath the vehicle.
Do check the wiring and connections before considering renewal of the oxygen/ O2/lambda sensors (see Photo No 7).
Fault 3:
Throttle body ailments
Symptoms for the third fault include erratic idling, refusal of the engine to rev freely and illumination of the Electronic Power Control (EPC) and management system warning lamps. Effectively the engine has switched into ‘limp-home’ mode. Sometimes the engine will idle fine for a while, then a glitch occurs and the running speed will fluctuate. Fault codes may be stored relating to ‘Throttle body signal implausible’ or similar.
If the loom is gently waggled in the vicinity of the throttle body, it may be possible to produce the running symptoms described, indicating a wiring/connection problem.
The harness plug connections should be gold-plated (see Photo No 9), and it is important to ensure they have not been damaged by over-zealous probing, with spread pins that do not make full contact.
VW dealers can supply the relevant cables with already-crimped connections, the plastic shells for the connector assemblies and the weather seals.
It is essential that sound, weatherproof connections are made in this area. With this Fox’s drive-bywire throttle activation system, if the computer sees any irregularity in the signals received – even small voltage discrepancies – it is designed to play it safe and set engine operation to ‘failsafe’ or ‘limp-home’ mode.