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Fuel pump failures...
Our present Audis 90 and 100 and the 100 we had before the present one all
needed replacement fuel pumps not long after purchase (all cars were bought
secondhand, so some may not have been on their original pump). The 90 pump
is external to the fuel tank, the 100 (5000?) pumps are internal. The
latest to require replacement (ie. buzzing becomes intolerable rather than
just irritating !) was that of the 100, at about 80,000 miles. I get the
impression that this is not uncommon.
The above impression was reinforced when I discovered on seeking to replace
the last pump that this part was modified by Audi in about 1991, at least
it was here in the UK. Up until then, a Bosch pump was fitted, whilst the
modified pump is of Pierburg manufacture. It is narrower in diameter than
the Bosch and therefore needs a matching modified pump housing/filter, in
which it sits in the bottom of the tank (along with a new union and washer
for the fuel pipe connection to the pump). The electrical supply wires on
the Pierburg are moulded into the pump whereas on the Bosch they go to
threaded terminals. I can only assume that this is Audi's attempt to reduce
the high failure rates of the Bosch pumps. Fortunately, the Pierburg was
available from a UK aftermarket parts supplier (German and Swedish) at a
very reasonable price, but the new housing from Audi cost about GPB 45.
Even so, the package was much cheaper than the usual outrageous price of a
'genuine' (ie., VW Audi logo and part number added) replacement Bosch from
Audi, and one presumes (touch wood many times !) will be more durable.
I don't know what 'oxygenated fuel' is - we don't have it in UK. You
mention that this fuel is incompatible with rubber etc. in pre-1990 fuel
systems, but, although no expert, I would doubt whether rubber features in
the construction of these fuel pumps. I have sometimes speculated whether
the change to unleaded petrol here in the UK and Europe has deprived the
internal mechanism of these pumps of lubrication effects, and would be
curious to hear other, no doubt more authoritative, opinions.