[s-cars] Can you teach an old fuel pump new tricks ???

Mark Strangways Strangconst at rogers.com
Tue Jul 27 08:07:41 EDT 2004

Maybe find someone with a wideband O2 sensor and do some testes at WOT.
Maybe you don't have fuel issues at all.

Look to the IC, is it clean inside and out.
If it has some dirt clogging the passages that would account (extra heat) for the ping.

Mark S
  ----- Original Message ----- 
  From: Dave Forgie 
  To: s-cars 
  Sent: Tuesday, July 27, 2004 3:24 AM
  Subject: [s-cars] Can you teach an old fuel pump new tricks ???

  S-gruppees:   I just want to bounce this off you and see what you
  think.  Sean Douglas and I are now quite convinced that, based on his
  fuel pressure readings, his (and my) "new" fuel pump(s) are not putting
  out enough pressure to the fuel rail at full RS2 (plus) boost levels,
  e.g. 22 to 28 psi.  We have already noted that when the car is running
  (and the alternator is running), the maximum voltage at the fuel pump is
  only 12.5 V and it goes down as low as 11.8 V as the car gets warmer and
  more boost is requested (by our heavy right feetses).

  So, we have a brain wave:  what if we double relay the fuel pump?  Right
  now, power goes from the battery/alternator to the central relay panel
  position 6 under the drivers side dash.  The ignition switch energizes
  the relay and three contacts are closed.  As far as I can tell by
  examining the relay (I opened it up) and Bentley, two contacts seem to
  be for the fuel pump and one seems to be for the O2 sensor heater. .
  >From there the fuel pump power goes through Fuse No. 17 at the drivers
  side end of the dash and then it runs ALL the way back to the fuel pump
  which is located in the fuel tank which sits at the back of the trunk,
  over the rear diff assembly.  In going  this round the world route, the
  voltage drops in the order of 1.2 to 1.9 V depending on the load.  I
  just bet this doesn't help the fuel delivery and pressure situation.

  Now as it happens, the fuel pump is only about 3.5 feet from the
  battery, through the passenger seat bulkhead.  So I am thinking, what if
  we use the normal voltage from the existing relay to trigger a second
  new relay located near the battery and then run a nice thick gauge hot
  wire from the battery to this new relay and then to the pump.  The
  intent would be similar to adding relays to the headlights - cut out the
  line losses and get more juice to the driven object, in this case, the
  OEM fuel pump. Done properly, this should (hopefully) give us 13.7 ish
  Volts at the fuel pump with the engine running.   This has to make the
  fuel pump spin a bit faster and, therefore, I suspect we may be able to
  over come the existing fuel pressure deficits that Sean has estimated
  and, therefore, get enough fuel to the engine under full boost that we
  can start saying "Yeeeee HHhaaaaaah!!!" again (instead of tuning our
  ears to the dreaded pinging death rattle).

  Anysink wronk mit dis plan?  Any fatal flaws in ze logic??

  The other alternative, the Audi Sport fuel pump, is still suggested to
  a) not have enough pressure and b) too much flow so this double relay
  trick seems to be a potential good (and cheap)  solution.  The other is
  too buy a case of OEM pumps and test each of them and pick the best two
  of the litter.  Unfortunately, I don't have that much time or money to
  play that game.


  Dave F.

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