RE: 1987 740, no fuel to injectors.

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Jamie - 05 Apr 2007 01:26 GMT

Following up to my posts about my 1987 740 with the replaced sending
unit and a no start:

1- I am getting power to the main pump. Pre-pump, still unknown power.
With fuel line to main pump off, no fuel comes out of line while
cranking.
2- I disconnected the in-tank wiring, and reconnected the fuel line to
the pump. I then disconnected the main fuel line at the rail and tried
to see if the main pump would suck fuel out of the tank. No fuel came
out.
3- I disconnected the return fuel line from the rail, thinking there
may be a vacuum. Still no fuel coming out.
4- Pump is wired Purple to Positive and Black to negative.

Question: Even if pre-pump has failed, why would main pump not be able
to suck gas out of the tank? I can blow into the fuel return line,
pressurize the tank and have gas come out that way.

Thanks!
Jamie

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James Sweet - 05 Apr 2007 06:07 GMT

> Following up to my posts about my 1987 740 with the replaced sending
> unit and a no start:
[quoted text clipped - 16 lines]
> Thanks!
> Jamie

The main pump should be able to suck fuel, it might take 20-30 seconds
of the pump running in order to prime though. Didn't you say fuel was
coming out the hose from the pre pump?

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Jamie - 05 Apr 2007 14:55 GMT

> > Following up to my posts about my 1987 740 with the replaced sending
> > unit and a no start:
[quoted text clipped - 20 lines]
> of the pump running in order to prime though. Didn't you say fuel was
> coming out the hose from the pre pump?

What I was able to do was disconnect the hose to the main pump from
the pre-pump. I would then go under the hood, disconnect the fuel
return line, blow hard into it, and this would force gas out of the
tank, through the main fuel line to the pump, an into a bucket. I
could never get the main pump to suck fuel from the tank, holding the
key 30 seconds or more, 2-3 times.

I even disconnected the pre-pump wiring in case it was doing bad
things.

Now, I was about to put the old main pump back on the car, because I
think it's still good. But the car ran dry with the old pump on -- so
something happened when I installed the replacement sending unit that
shut off fuel supply.

-- Reused the O-ring. Maybe not seated right? Sucking air? But I put
that unit on/off the car 3 times. Bad seal 3 times?
-- Cracks in sending unit? Maybe where tubes go in lid, air is getting
in?

The sending unit is the only common factor. I am currently looking
hard for a 1987 740 in-tank sending unit. I think if I swap this, all
will be well.

Jamie

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James Sweet - 05 Apr 2007 19:45 GMT

>>>Following up to my posts about my 1987 740 with the replaced sending
>>>unit and a no start:
[quoted text clipped - 46 lines]
>
> Jamie

I'm still fuzzy on the most important troubleshooting detail here, if
you disconnect the input hose from the main pump, which is connected
directly to the output of the pre-pump, and get the fuel pumps to run,
does fuel squirt out this hose or does it not? You have to know this
*first* because it will tell you if the problem is with the prepump/fuel
tank, or with the main pump. If fuel squirts out this hose, then
everything in the tank is fine. If it doesn't, then your prepump is bad
or incorrectly installed. It's super easy to do this part and only takes
a few minutes.

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