91 GMC pickup with 350 engine, fuel injected throttle body.
I replaced the fuel pump with a used one, then another used one, and
finally bought a new one. They all do the same thing. It runs fine
for a few days, or runs rough like it's starving for gas. Then it
wont start at all. No fuel to the engine. Pouring gas in the
throttle body will make it run for a few seconds.
The new fuel pump was just installed a week ago. I removed the line
from the fuel filter at the pump end of filter. I only get a small
trickle of gas with the key turned on, and it stops after about 20
seconds. I got a half inch in a coffee cup.
A tester lights very dim on the middle wire on the fuel pump plug
(gets real bright across the battery).
#1. Which wire is the fuel pump itself? What color?
(The wires are black - Purple - Green (ground is white)
#2. Do these pumps quit pumping when a line is disconnected (I was
told yes by one guy, no by another).
#3. Where is the fuel pump relay located? (I cant find it)
Anyone have a schematic or book to help?
I was having the same problem with my s-10 Blazer.I thought it wasn't
getting fuel,when it really was.Caused me a lot of trouble.I purchased a
set of noid lights and found out it was the module in the distrubitor.It
wasn't sending a computer signal for the injectors to fire.Sounds lile
your fuel pump is working.I got a set of noid lights at Harbour
Freight.If you need any more info let me know.
Sounds like cavity "B" which is the wire to the fuel gauge.
There is no green wire to the in tank fuel pump module.
Wiring diagram shows a black ground, purple to the dash gauge and gray
to the fuel pump relay,the hot fuel module and the oil pressure switch.
No, they will pump as long as it is powered.
Under the black plastic cover underhood passenger side.
Under that cover you should also find a loose pigtail wire, it is called
the fuel pump test lead, you can power the pump there, test for voltage
Yes. Are you near Milwaukee?
Test for powers and grounds, do not waste time with a small test light,
use a 12 volt sealbeam headlamp at the pump end to simulate the load on
the circuit or simply voltage drop the circuits in question with
basic circuit description;
Fused 12 volts is fed to one fuel pump relay contact and the oil
pressure switch, the oil pressure switch is in parallel with the fuel
When the PCM detects reference pulses from the distributor during engine
cranking or engine running the PCM applies 12 volts the coil in the fuel
pump relay energizing it allowing 12 volts to the fuel pump.
When there is sufficient oil pressure, the oil pressure switch closes
also allowing 12 volts to the fuel pump. If no reference pulses are
received by the PCM, the fuel pump relay is energized for 2 seconds and
then shut off except if a hot fuel condition is present, then the hot
fuel module over rides the fuel pump relay and runs the fuel pump for an
additional 20 seconds to help prevent vapor lock.
When all of these fuel pumps were being shotgunned in, did anyone bother
to check the integrity of the connectors at the tank connections both
inside and outside of the tank?
You were right about the colors. The gray is what I thought was green
(looked kind of green with the dirt on it, but I wiped it clean).
No, I'm not near Milwaukee, but I did actually live in West Allis many
years ago. Now I'm 5 hours away.
I bypassed the oil pressure switch, then unplugged the relay and
jumped the orange wire on the plug to the gray one. The gas came out
like a flood, and filled a gallon jug in a half minute or less. I
suspect the relay is bad, but I'm going to see if I can trigger the
coil on it manually. The oil pressure switch has only one wire on it.
Now for another problem. The gas that came out of the tank is all
full of apparently water. In the jug, the top half looked like gas,
the bottom was all white. I think that seal around the fuel pump
leaks, and I cant see how to seal it better. I used a new gasket.
Maybe all I can do is glue some heavy plastic over it. I suppose I'll
have to run the pump into a gas can till it's empty. I cut a hole in
the bed to work on the pumps, but this now allows the water to pool on
the tank around the sending unit. I guess all I can do is get some
thick plastic of soem sort to fit over the top.
There's supposed to be a test wire for the fuel pump up around that
relay, but I sure cant find it. However, jumping those wires confirms
that the pump is not getting power.
Thanks for your help.
Then there is another oil pressure switch that controls the pump.
The two typical locations are; next to the distributor screwed into the
top of the block and near the oil filter.
There is no hard rule with GM whether the oil pressure switch is
combined with the dash unit (the single wire sender that you found) or
whether it is a separate stand alone switch.
I mention this because it appears that the oil pressure switch has
failed also... When you primed the throttle body and the engine ran for
a second or two, that should have been sufficient to close the oil
pressure switch and keep the pump running.
There should be a large O-ring to seal the sending unit to the tank.
That should be sufficient to keep water out of the tank but the plastic
shield would now be mandatory to keep water from pooling on top of the
tank and causing rust issues.
The test lead sometimes is a red wire, usually it's under the cover
where you found the relay, other times it just breaks out of the wire
harness wherever GM feels iike putting it...
The oil pressure gauge runs off the sender next to the oil filter, and the
one on top of the motor near the distrib is the one used by the ECM.
Like walker said, the o-ring rubber gasket is more then enough to seal the
top. Make sure you hammer those tabs as far as they will go. If your getting
water in there, might be a crack in one of the hard lines at the top.
I know its some money, but I like to replace the entire thing, not just the pump
itself. Also, its a good idea to coat the entire top of the fuel pump area
lines go in, and ring seal ring) with bearing grease. That will keep the ring
rusting itself to the tank.
I highly recommend that you drop the tank out no matter what, get rid of that
and check the tank for rust throughs. LMCTRUCK.COM sells good tanks
for those trucks (OEM tanks, not universals), at a really good price. They also
carry the pumps and sending units.
Proud Owner of a 1989 K2500
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