1990 GMC fuel pump problems

Davstur

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#1
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MAKE:GMC
MODEL:Suburban
YEAR:1990
MILES:100,000 +
ENGINE:5.7
DESCRIBE ISSUE....Fuel Pump Installed 3 Pump in three years. Vehicle sits for long periods without being started. Checked voltage at wire harness connector near fuel tank only receiving about 5.6 volt when ignition is turned on. installed new relay on fire wall and new 10 amp fuse for ECM. Fuse was blown when checking current at relay harness.
 
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#2
with KOEO the fuel pump will only show 12v for @3 seconds - too fast to turn key and get back under vehicle and ck wire - 2 man job.
I believe There is a reference voltage to fuel module @5.5v that is likely what you probed.
Telling us the wire colors will be a help id the circuits

Use only a AC Delco pump - aftermarket units are problematic.
The ground side of the pump circuit is a known problem, be sure to check -

Hopefully one of the regulars can add a diagram of this ground point. And second the OEM only pump recommendation.
 
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billr

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#3
What are the pumps being replaced for? Low fuel pressure, I'm guessing; but how low? Some trouble-shooting is in order to keep from throwing pumps at it.
 

Davstur

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#4
Thank you Kev2 and billr for replying. Here is the history of this nightmare. Approx. 4 years ago I took on this job for an elderly man with a sentimental 1990 GMC Suburban. He does not drive it much because he has newer vehicles. However I fix what is wrong with it and it sits for most of the year. About 4 years ago it would not start so I isolated the problem to fuel pump I found the short rubber hose connect the pump soft and recirculating gas in the tank. I then installed new Oreily brand pump in tank with lifetime warranty. Worked good when I gave it back to owner. Year later would not start no fuel to fuel filter check pump and found it was not pumping so. took it back got refunded and installed new Delphi (Delco) pump. Gave it back to owner and it performed well. a year later well That is where I am at right now loosened fitting at the inlet of fuel filter and found no gas when I broke the connection and when turn key on still no fuel. Call my brother to turn ingnition switch while I was under vehicle reading volt meter. preformed that test 3 time within 30 seconds to minute intervals just reading 5.6 volts. As for as ground are you talking about ground on the chasis of the vehicle pump assembly? Does the ECM have any function in this? Can I supply 12 volts to the connector at fuel tank safely to see if pump will supply fuel to filter? I believe the supply wire is green.
 
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#6
The PCM function is only to control (ground) the FP relay. So yes you can supply 12v to pump- I remember there is a test or prime connector next to FP relay for this type of testing.... I'll try to get a picture of that....


I will get the diagrams -- a 1990 GMC Suburban "1500" , engine is a 5.7L and this has TBI*..... Pump is just a 2 wire connector, am I correct? 1990 has been awhile:)


* this is a relatively low pressure system @13PSI.
 
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#7
I will get a better diagram later - computer issues right now, maybe a colleague can attach a better diagram....
 

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#8
How much gas does customer keep in tank on regular basis?

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nickb2

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#10
And Yes Kev, low pressure system. Spec is 9-13psi. Of note, oil pressure switch and fuel module if equipped on heavy duty models. Those also have to be considered.

Also of note, No oil pressure, no fuel pump signal.
 
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#11
Same fuel pump used in our 92 DeVille as that 93 Bravada, former operated at 35 psi and drew 4 amps, later that 55 psi, drew 8 amps. And if the voltage was much lower than 13.5V to the pump, could not reach operating speed, that 8 Amps would skyrocket. While the shop manual showed the oil pressure switch in series, on this thing was bypassed. GM had a couple of liability suits where if that switch opened, would stall on the interstate causing a pile up.

Kid got this thing used, could tell the pump was changed, rather than removing those three line, cut them and spliced with hoses that leaked, out came the tubing tools. And 55 really means 55 on these things, any lower and the thing would not start. Pump was hanging on a rubber hose, replaced that with soldered copper, that could be unsoldered. Was a major job, other guys were saying with the engine off, luck to hold pressure for 20 minutes. When you hit the key to start, just a two second pulse, if that didn't hit 55, would not start. Replaces the wire with heavier gauge, not only the relay contacts, but the ignition switch as well, polished that to where I could see my smile, and the fuel pressure held indefinitely. Thing had 80K miles on it, kid put on over 200K, as usual around here, became a pile of rust, never had fuel pump problems after this, but was quite the job.

Recall the job replacing the fuel filter on this thing, mounted on top of the frame before the body was placed on, could barely get a wrench on it.