> I nursed it back home and decided to risk the "Alan Bowes patented
> burn-your-place-down test" -- I checked the fuel pump by pulling the outlet
> side loose and watching the squirt action while cranking the engine.
> Interestingly, it took about a minute of cranking before I saw any fuel (of
> course, it had just been exhibiting the symptoms) then when fuel finally
> arrived, the pulses of fuel were few and sometimes far between. There were a
> couple good blasts that shot out 3 feet, but for the most part, it was
> miserable. It was really sad to see it go 10 or 15 seconds without a squirt
> at all.
> I think I've found my problem.
> Steve " I survived-the-fuel-squirt-test" Johnston
Hi, Steve,
Well, I'm glad to hear that you didn't have an unscheduled BBQ in your driveway. The test
you performed has always been an effective one for narrowing the scope of an engine
problem. (Of course I can't RECOMMEND something so dangerous...)
Anyway, sounds like "la bomba" (the pump) probably needs surgery.
One more thing, while you've got the pump out, let gravity do its thing and see if the
fuel drains easily through the lines. This is a FAIRLY good test to see if the lines are
clear, although it does NOT check for soft/swollen/delaminated rubber lines that may only
create problems when they collapse under suction.
If a fuel line is delaminated and the inner layer collapses part way, it can severely
restrict or even block the flow. It will also draw a much higher vacuum than normal
between the pump and the point of restriction.
Paul Vandervort's comment about sucking in air through what I might term "overly porous"
rubber lines sounds like a symptom that can result from the increased suction from a
collapsed hose: Let's say you have a softened/damaged inner hose liner and a
weather-checked outer layer and the inner layer collapses part way, separating from the
outer layer. This separation can create a path for air to be sucked in through cracks in
the outer layer. These cracks might otherwise be covered or sealed by the inner liner when
not under suction. Hence the air could be sucked in when the pump is pumping, but you
might not spot a leak. Thanks, Paul.
Rubber fuel lines can leak in rather unpredictable and inconsistent ways, although
USUALLY, if the lines don't seem to be cracked or brittle on the outside, this is not too
likely. Still, it's no guarantee. Modern fuels and additives were not designed to be easy
on the various synthetic rubber compounds used in older vehicles. Occasionally the inside
of a hose is messed up, even though the outside looks OK.
If the rubber lines are at all cracked or brittle, replace them NOW whether they're
causing a problem or not. This will save you big headaches later on. As long as you're
taking the pump out, you might as well make sure there isn't a secondary problem that your
new fuel pump will find highly irritating.
If it were me, I would:
1) Replace the fuel pump. It's PROBABLY the culprit. I personally think pumps ought to be
replaced or rebuilt every five or six years anyway. That way, you're not always wondering
when the stupid thing's gonna conk out on you. It's more fun to replace a pump with the
vehicle parked in your driveway than it is alongside the freeway in a foot of snow.
2) Replace the rubber fuel lines with NEW lines. This is really just an exercise to buld
even more character and charm. It also impresses people at parties when they hear about
your new fuel hoses.
As to the fuel pump: I'd suggest a new or factory-rebuilt fuel pump. Don't buy a pump
that's been on the shelf for twenty years. You want fresh rubber parts. You might be able
to find an overhaul kit, but there are a few "gotchas" relating to reassembling a
diaphragm-type fuel pump, such as properly staking the new check valves so they don't leak
or come loose while not warping them, correctly positioning the pump lever when you screw
the fuel pump body parts back together so that the diaphragm is in a "neutral" position
when it is clamped into place, and evenly torquing the screws to avoid crushing the
diaphragm while still sealing it tightly. These are just some generic things common to
many fuel pump rebuilds. The requirements will, of course, vary from pump to pump. They
get a little more complex if you have a combination fuel/vacuum pump, but not too much
more. Really, the hardest part is replacing the check valves in pumps where they are
staked in (I'm not sure about yours). In case you're wondering, staking involves deforming
(sort of "mashing down") the edges of the pump body around the check valves to hold them
in place. It's easy to do this wrong. To do it right, you should have a hydraulic or arbor
press and a staking tool made to fit. I don't recall how they're handled on the M38.
Anyway, keep us posted on your progress and don't play with matches.
Alan
(Salt Lake City, Utah)
"Don't try this at home" (I'll have to find out how to write this in Latin.)
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