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Fuel Pumps, Easy Starts, Thanks
Thanks for the help on good sources for Audi fuel
pumps (Advance Auto Parts (guaranteed for life
pumps) and www.thepartsconnection.com).
After a trip to Pennsylvania I am more convinced
than ever that the cause of hard starting in my
1990 Audi 100 has been a lazy/weak fuel pump.
It starts great in hot or sub zero weather if the
fuel pump runs prior to cranking. This lazy fuel
pump problem I believe caused the original owner
to sell this car early. The Audi Dealer changed
wires, distributor, and other parts for the original
owner to no avail. I watched while the dealer
checked fuel pressures (operating and residual)
and found everything in spec. Of course once the
car is started and running, everything is in spec.,
making this problem difficult to diagnose. I'll
bet weak fuel pumps are plagueing a lot of Audi
owners. I'll summarize what I learned, which I
couldn't find in any of my Audi manuals:
* Most Audis energize the pump for about 3 seconds
each time the ignition switch is turned on. This
is smart design because it pressurizes the fuel
lines prior to cranking and allows for easy starts.
* My Audi has a lazy/weak fuel pump, and often
misses this 3 second prep run. In fact when voltage
is connected to the pump, it sometimes sits there
for 10 seconds before deciding to run.
* Skinny feed wires to the pump from the fuel pump
relay probably don't help this pump work properly.
I doubt if much more than 11 volts actually makes
it back to the pump which draws 8 amps. However,
as my brother in law pointed out, if I put a relay
under the rear seat to give the pump more voltage,
it may work great for a year or two more, then
fail all at once on the road. In other words this
lazy pump may be an early failure warning.
* The lazy pump problem will be worse in hot weather
than cold. In hot weather the pump windings will
be hotter, causing the pump to draw less current,
giving it less torque to start.
I hope this helps solve some hard starting, and
hard hot starting problems with Audis out
there. If it does, you'll be doing better than
some Audi dealers who seem unaware of the role
the fuel pump plays in easier starts.
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