Re: Dallas area service and fuel gauge question

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <36979935.1D2BC654@worldnet.att.net> Frank Walian writes:

> I am going to check if anybody I know has some fuel gauges. Where do I
> connect them to the fuel distributor and what pressure reading should I
> expect to see?

The procedure is pretty well described in a scruffy note that comes
with the JC Whitney gauge.  It's also in Charles Probst's book.

Basically, you disconnect the line between the warm-up regulator
and the top of the metering head.  This is one of the braided lines
that runs over the top of the cam cover.  The JC Whitney gauge has
about five brass fittings - it's pretty obvious from the threads
which one goes into the top of the metering head.  The disconnected
line is then screwed into another of the brass adaptors (again, only
one fits) and is then attached to the other free end of the gauge.

The whole contraption is T-shaped - the ends of the bar are attached to
the metering head and the line from the warm-up regulator, and the
gauge itself is on the end of the centre leg of the T.  There's a tap
in one of the legs, and this is the leg that has to be attached to the
line from the warm-up regulator.

There are several measurements.  On an 87 5000, you attach the gauge
and run the first stage of the onboard diagnostics (see Scott Mockry's
web site) to get the pump running.  With the tap open, you see 'control
pressure'.  With it shut, you see 'system pressure'.  On the 5000,
the fuel pump is wired in parallel with the heater in the warm-up
regulator - if the car is cold when you start, you'll see 'cold control
pressure', which should be around 1 bar.  As the warm-up regulator
heats up, this will rise.  On a fully hot engine, it should be around
3.4 bar.  If you close the tape, you'll see 'system pressure' which
should be around 5.7 bar.

When you switch the pump off, the pressure will dance for a fraction of
a second and then settle at about 3.2 bar.  It can decline to about 2.8
bar over about twenty minutes.  Any more than this, and you have a
leakdown problem - a leaky injector or fuel pump check valve.

Gotchas:  You should _ALWAYS_ replace any crush washers with new ones.
I've just restocked - I usually carry about fifty (just in case I have
to strip a WR/WX metering head out).  They cost peanuts and stop engine
bay fires.  When you're done and everything is back together, run the
diagnostics again (to run the pump) and check _rigorously_ for fuel
leaks.  Audi fuel injection systems use three sizes of crush washer -
N 013 811 5, N 013 807 6, N 013 812 8.  Over here, they cost the local
equivalent of 40c each.  You need at least two N 013 807 6 for this job.

The JC Whitney gauge is about $60 and (IMO) is just as usable as the
$$$$ Bosch version.  It _is_ sensitive to sharp taps when it is showing
a reading.

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
 (The contents of this post will _NOT_ appear in the UK Newsletter.)