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Fuel injection questions
My careful rationalisation of the MB's method of operation is lying in ashes
this morning, so I'm appealing for ideas.
Brief recap - the MB engine's "warm-up regulator" is exactly that. There is no
vacuum connection for fuel enrichment. Question one - is this device (a warm-
up regulator with no provision for vacuum control) unique to the MB?
The MB also doesn't have a system pressure regulator inside the metering head.
Instead, it has a "pressure regulator" valve mounted on a bracket behind
the fuel metering head. I suspected mine was getting old (138k) and so decided
to change it. In the absence of a new part (eight weeks lead time from the
Fatherland) I fitted one from a 60k car. Then I conducted some experiments:
a) I used the on-board diagnostics to run the fuel pump and warm-up valve.
Then I applied vacuum to the pressure regulator. The fuel pump noise
rose in pitch, quite noticably.
b) I got the engine idling to check the system and control pressures. The warm-
up regulator does its job properly - starting at 2.something bar and drifting
to 3.something bar over two minutes or so. But the final control pressure is
now 3.6 bar - it was 3.4 bar before I changed the pressure regulator. So I
believe my first hypothesis stands - in a steady-state warm MB engine it's the
"pressure regulator" that is in charge of control pressure and not the
The surprise came when I applied vacuum to the pressure regulator. As I
expected, 0.5 bar of vacuum reduced control pressure - though by only 0.2 bar,
much less than I expected. Question two - in cars with vacuum enrichment via
the warm-up regulator, how much does control pressure drop for 0.5 bar of
A bigger surprise came when I measured system pressure. I had hypothesised
that the two were brought under one valve's control on the MB to allow system
pressure to be increased as control pressure is decreased as a result of the
vacuum input. WRONG! System pressure drops almost 1.0 bar with 0.5 bar of
Question three: is this correct? I can't see how it can be. I have a brand
new pressure regulator on order from Bosch (six weeks to go - $160) and I have
a workshop session booked with Martin at BR Motorsport a week on Wednesday.
Question four (addendum to Question one): do any other cars (not necessarily
Audis) use a valve like this in a K-type Bosch injection system?
Bye-the-bye: I wanted to set the idle "properly" - 430ma through the ISV at
800rpm and 0.75% CO - so I went out and bought a "Digital Automotive Analyser"
made by Draper. 200mV to 600V DC, 10A, 200ohm to 20Mohm, diode, and dwell &
tach for 3 to 8 cylinder engines - GBP26 ($40). It comes with crocodile and
probe leads, and is encased in a heavy rubbery housing, eminently suitable for
concrete workshop floors. How they do it for the money ...
Committee Member, UK Audi [ur-]quattro Owners Club