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From: John De Armond
Subject: Re: 98 Cruisemaster mystery
Date: Tue, 02 Nov 1999 23:38:16 EST
Newsgroups: rec.outdoors.rv-travel

Steve Wolf wrote:

> Can any of you drug dazed engineers explain to me why I would want to put a
> fuel pump inside a fuel tank.  I'm sure there's a reason but I must not use
> the right drugs to allow me to fully open to the logic.  Is there a reason
> they didn't put a code in when it failed?  I believe the computer watches
> fuel pressure.

Well, I'm an engineer, I'm occasionally dazed and confused and I
just had a handful of aspirin so I guess I qualify to comment...

The reasons are cost, NVH, safety and logistics.  Cost and logistics
go together.  It is cheaper and easier for the OEM to subcontract
out a "fuel management assembly" that is supplied to them ready to
insert into the tank.  It saves a mounting bracket, a length of fuel
line, perhaps a vibration damper and assorted knickknacks. Plus
several assembly steps and the attendant risk of errors. From a
safety perspective, the pump is well protected in the tank.  One
need not be concerned that the pump will rip loose in a wreck and
leak gas.  For NVH (noise, vibration, harshness - interpret as
pleasantness factor), the pump is buffered in a large tank of gas
which dampens vibration and noise.  Even with an empty tank, the
tank walls act as an effective noise barrier.

The reason a pump failure doesn't directly generate a code is that
there is no easy way to detect failure.  Monitoring the current draw
won't work because the load on the pump varies with the gasoline
composition, the position of the throttle and the temperature. 
Simple fuel pressure measurement won't work because the pressure is
regulated to a set pressure referenced to manifold pressure.  The
gauge pressure varies with load.  A differential pressure sensor
would do the trick but at significant cost and with little return. 
It would also be an additional source of failure.  

Fuel pump failures are relatively rare, particularly the kind you
suffered.  Just not worth the money to try and diagnose and log such
rare failures.


From: John De Armond
Subject: Re: 98 Cruisemaster mystery
Date: Wed, 03 Nov 1999 12:57:37 EST
Newsgroups: rec.outdoors.rv-travel

Steve Wolf wrote:
> OK ... New question, same thread.
> Can I, at the expiration of my warranty, drop my tank, take out the fuel pump
> that has again failed, beat the living sh*t out of it with a hammer, install
> or modify the dip tube and install my own fuel pump just outside the tank?  I
> would have to find a pump that applies the same pressure, right?  Bolt it
> down and go, no?  No sender implications, right?  I'm a real flag waver when
> it comes to emissions.  Any emissions problems?

Sure.  The Bosch-style roller pump was widely used by all the
european makes and many of the japanese cars up through the early
90s.  Should be easily available from the junkyard.  Typical price
from the auto parts store is about $100, twice that from the OEM. 
These pumps are stone-cold reliable - I've never replaced one - but
are noisy.  If you get one from the junk yard, also get the mounting
bracket and the pulsation damper.  If you try to wing the mounting
or not using the pulsation damper, you'll get to enjoy the pump
growl throughout the vehicle.  You should be able to find a fuel
pickup/gauge module for your tank.  There is no emissions
consideration.  The pump's pressure capability is far in excess of
what is needed because the actual pressure is regulated by a
backpressure regulator on the engine.  The bosch pump has an
internal relief that lifts at about 125 psi.  The rail pressure is
typically 32 psi above manifold pressure.

The major consideration is whether the pump has the necessary
capacity.  Remember that with an EFI engine, the pump has to supply
the instantaneous volume of fuel required - no float bowl to buffer
things.  The Bosch-style pump will typically handle an un-modified
big block.  These pumps can be paralleled for more capacity.  I used
to have to do that for highly turbocharged engines.  I started the
second pump from manifold pressure so that the second pump didn't
have to run all the time.

There is another major consideration you need to be aware of.  Many
modern ECUs vary the speed of the pump according to engine load to
minimize noise, fuel heating and energy loss (matters for CAFE
considerations).  You would need to make sure your replacement pump
draws the same or less current than the one you want to replace. 
In-tank pumps typically draw more current than the Bosch-style pumps
but you have to make sure or else you could smoke the ECU.

This seems like a lot of work with dubious returns to me.  I know
that GM has had several recalls for defective in-tank pumps.  You
might check to see if there are any service bulletins on your model.


From: John De Armond
Subject: Re: Fuel Pump Location Question
Date: Wed, 17 Nov 1999 03:32:26 EST
Newsgroups: rec.outdoors.rv-travel wrote:

>      Ford is going to warranty the bad pump, so I'm replacing it
> with another of their units.  However, I'm going to look at aftermarket
> racing FI fuel pumps tomorrow, and will be modifying the tank to take
> whatever I decide on, as a backup pump.  Warranty or not, this is the
> last time I pull this miserable pump out.  At the moment, I'm assuming
> the mods will be nothing more than a pipe thread fitting welded to the
> back of the tank, to plumb into.  Other option I'm considering is moving
> the mounting of the pump to the rear of the tank.  It's currently in the
> center, which makes it impossible to reach and service.  Placed at the
> rear of the tank, there's just enough room to squeeze in and work, and
> also enough clearance above the tank to pull the pump (I hope).

Couple of things you need to be aware of.  First, the in-tank pump
is a positive displacement pump which means you can't just idle it
and pump through it like you could a centrifugal or diaphragm pump. 
YOu'll have to bypass it.  Second, feeding an external pump for an
EFI system is a bit more involved than just taking suction from a
fitting in the tank.  An EFI system requires a "cornering well". 
The reason is there is no float or other buffering system.  If the
pump loses suction for even a second, such as when cornering with an
almost empty tank, the engine hesitates until the pump gets solid
suction again.  This corning well, which traps a bit of fuel in a
well that is immune from sloshing, is typically either a hunk of
pipe welded to the bottom of the tank or else a baffled depression.
The return line from the fuel pressure regulator is aimed to
discharge into this well when the tank is almost empty.  This well
holds enough gas to ride through a cornering transient.  

If you want to remove the pump from the tank, it can be done as I
describe in another post.  But you must take care to preserve the
integrity of the existing pickup.  Most easily done by either
finding a pumpless gas pickup if available or else removing the pump
and extending the pickup to where the pump's old pickup was.


From: John De Armond
Subject: Re: Fuel Pump Location Question
Date: Wed, 17 Nov 1999 23:20:58 EST
Newsgroups: rec.outdoors.rv-travel

Steve Wolf wrote:
> Neon John <> wrote in message
> > fitting in the tank.  An EFI system requires a "cornering well".
> (and comments about the computer controlling the existing pump)
> I've obviously never done this as Chevy has been kind enough to warranty all
> the pumps so far.
> Let me "brainstorm" in search of an opinion and wonder if what I would do
> would work.
> First, could the old pump be left in but brutally ported to effectively
> remove it from the system?  Fuel would pass straight through it regardless of
> it's operating condition.  The computer would then tell this existing pump
> what to do and think it was doing it.  It would also then use the same
> cornering well as the original.  The sender would still be stock.
> To power the new external pump, I would provide a new source of 12 volts
> which operated on a low draw relay powered by the power wires from the
> existing pump.  When the computer powered the existing pump, the relay would
> put power to the new pump.  The relay current draw could be as low as 11 mA
> but 150 mA would make things easier.  I am unsure what current the pump
> operates at but I suspect the pump and computer wouldn't notice the 150 mA
> relay.

I've only fooled with a few in-tank pumps, usually in the process of
trashing 'em to put a larger external pump in place to feed a high
performance engine.  The in-tank pumps tend to draw MORE power than
the Bosch pattern pumps.  Typical in-tank pump draw is 6-8 amps
while the Bosch pattern pump typically draws 2-3 amps.

So. I'd first do some measurements and research.  If your ECU does
not vary the speed of the pump, then replacement should be trivially
easy.  Even if it does vary the speed, if the current draw is
similar, the it should be a direct drop-in.  Some experimenting
should show you what kind of load it takes to keep the ECU's
diagnostics happy.  Since the only reason to vary the speed is to
reduce the heat input to the gasoline and since the Bosch pattern
pump dissipates much less power, the speed control becomes a

The most important test is to verify that the pump supplies
sufficient volume of fuel under all conditions.  The easiest way is
to hook a differential pressure gauge between the fuel rail and the
intake manifold and drive the vehicle while monitoring the
pressure.  The fuel pressure regulator is referenced to the intake
manifold so that the injector always has the same pressure across
it.  If the pump will maintain the pressure at max RPM and wide open
throttle, then the new pump is large enough.  If it won't, a second
pump can be paralleled.  On my turbocharged engines, I switched the
second pump from manifold pressure.  That is viable in a normally
aspirated engine using a vacuum switch of the type that used to
illuminate that "economy" light on old cars.  A switch on the
throttle would also do it.

I personally would not leave the old pump in place.  Just something
else to go wrong.


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