Thanks Ken.
The Bosch Book says that Oxygen sensors (or Lambda sensors) are used to measure the mixture of fuel and oxygen fed to the engine to achieve the correct ratio of 14.7:1 at operating temperature, and a little richer when starting from cold. They communicate this information to the ECU which, after taking engine temperature, throttle position and other variables into account, decides how long to open the fuel injector nozzles;
Oxygen
Sensors are used to detect
the amount of excess oxygen in the exhaust gas after combustion to indicate the
relative richness or leanness of mixture composition.
The oxygen sensor contains two porous
platinum electrodes with a ceramic electrolyte between them. It compares
exhaust gas oxygen levels to atmospheric oxygen and produces a voltage in
relation to this.
The voltage produced by the oxygen sensor
will be typically as small as 100 mV [lean] up to a maximum of 900 mV [rich].
An active oxygen sensor would cycle between these two points as the engine
management system drives the mixture rich and lean to achieve an average sensor
voltage of ~465mV. This would represent the mixture ratio of 14.7:1.
This type of operation is normal for a
“narrow band” style of sensor; these are used for the majority of standard
vehicle applications.
The O2 sender that I removed from my car back in 2002 is a genuine VW unit P/N 025 906 265 B, date-stamped 04/1991, made by Bosch, and it appears that the Bosch P/N is 0 258 003 031. It has 3 wires 60cm long - two white wires to a dual plug, and one black wire to a single plug.
It appears that Bosch no longer manufactures the part. Googling the number merely returns a heap of generic sensors that are made to suit multiple vehicles, usually without the correct wiring connections. Feedback from other sites is that they don't work very well.
Many purveyors of cheap and nasty equiptment have tried to foist 2-wire senders on to the market, or 3-wire senders with incorrect connectors, or no connectors at all, which have to be spliced to the factory wiring.
A well-known local purveyor is advertising what looks (in the photo) like a generic 2-wire unit with 3-wire connectors at a ridiculously high price, so beware. It is advertised under the Bosch part number - check if it is genuine before you buy it.
The correct Bosch unit can be found under the VW P/N or the Bosch P/N 13931. Bosch also makes a generic part 1393 which will work at a pinch, but without the correct wiring.
The Bosch 025 906 265 B units are available at VanCafe and GoWesty, and the latter has a good technical rave on the subject. The ones I have used work well and have the correct connections, but the wires are twice as long as the original. This is not a problem.
The same sensors were used on some other VW models, Audi, Saab, Volvo and Ferrari in the '80's and '90's.
I will call Tooley's on Monday to see if they carry the Bosch units. Everyone needs to carry a spare under the seat, along with a 22mm spanner (there may not be enough room to use a shifter).
The faulty item that caused the problem on my van was a German-made Lowe 0 258 003 957. It looked the goods, but in practise it created a stumble when taking off from cold, but appeared to work well when warm, until the over-fuelling problem occurred after it had been fitted for several thousand k's.
The original item that replaced it is running reasonably well.
Bentley tells us how to check them with a multi-meter. That's the next job ....
Are their any electronic gurus out there who can make a 465 mV gizmo to plug in to the connecting wires to fool the ECU?