Fuel pump trouble

[Dariusz Salomon]

Hi all-99 Tahoe Z71-350 vortec. It happens randomly-but today it happened multiple times. Turn the key in ignition and can't hear the pump-and of course it cranks but doesn't start( well I don't even try when I can't hear pump priming). Security light doesn't stay on. I try few times-it suddenly works. Then works no problem. I'm pretty sure if I try tomorrow it will happen again.
Before anyone suggests-I replaced relay some time ago.
Do you think pump is on the way out? Or where would you look? Looking for ideas.

 

[stutaeng]

What is your fuel pressure?

I would have a fuel pressure gauge and a DVOM handy for when it acts up. If no pressure, check for voltage at the pump. Or even do an amp draw test.

 

[Dariusz Salomon]

What is your fuel pressure?

I would have a fuel pressure gauge and a DVOM handy for when it acts up. If no pressure, check for voltage at the pump. Or even do an amp draw test.

I don't have a gauge-but the issue here is that it doesn't even prime. Checkin voltage at the pump not easy as I'd have to drop the tank and it's nearly full-I never let it get too low as when I first got her she would cut out shortly after start sometimes-and full tank cured it. But maybe it's time to use up that fuel.

 

[Schurkey]

The relay system can be faulty, if you crank the engine long enough to build oil pressure, the oil pressure switch should bypass the relay system to run the fuel pump.

That would narrow down the problem to the relay system, versus the fuel pump and pump wiring up to the oil pressure switch.

The relay system would include the ECM, the fuel pump relay, and the wire harness connecting them.

 

[Pinger]

I had my pump failing to pump intermittently but as it had had a new pump recently (previous owner) I suspected the problem was elsewhere and wasn't in a hurry to drop an even half full tank.
What I found was a bad ground connection in the wiring. From underneath, ahead of the rear axle I got to the wiring as it passes over the tank and found the fault in the connection between the pump pigtail and wiring running forward. Done badly (crimps) when the pump was replaced I think. No matter, a proper soldered repair and it's been trouble free since.
Guessing your Tahoe has the rear mounted tank like the Suburban - if so, worth checking the above.

 

[Dariusz Salomon]

I had my pump failing to pump intermittently but as it had had a new pump recently (previous owner) I suspected the problem was elsewhere and wasn't in a hurry to drop an even half full tank.
What I found was a bad ground connection in the wiring. From underneath, ahead of the rear axle I got to the wiring as it passes over the tank and found the fault in the connection between the pump pigtail and wiring running forward. Done badly (crimps) when the pump was replaced I think. No matter, a proper soldered repair and it's been trouble free since.
Guessing your Tahoe has the rear mounted tank like the Suburban - if so, worth checking the above.

Well it hasn't-the tank is sideways on driver side-runing under the driver and passenger seats which imo opinion is dumb as hell-most of the time you have 100kg driver with over 100 liter tank on one side of the vehicle. I can't find any info on grounds location on tahoes-apart from obvious ones.

 

[stutaeng]

I've seen guys on more advance diagnostics YT videos hook up a scope to the fuel pump and look at the electrical waveform. You can look at the condition of the motor brushes or commutator(?).

A basic scope is on my things to buy sometime. They are pretty expensive even for the a basic model.

 

[HotWheelsBurban]

Well it hasn't-the tank is sideways on driver side-runing under the driver and passenger seats which imo opinion is dumb as hell-most of the time you have 100kg driver with over 100 liter tank on one side of the vehicle. I can't find any info on grounds location on tahoes-apart from obvious ones.

4 door Tahoe has a different tank than the 2door ones, and the 2doors use one like the Suburban. Part of this is because of the spare tire location.

 

[Dariusz Salomon]

Found this online-I must say that my relay can get quite hot

 

[Schurkey]

I've seen guys on more advance diagnostics YT videos hook up a scope to the fuel pump and look at the electrical waveform. You can look at the condition of the motor brushes or commutator(?).

A basic scope is on my things to buy sometime. They are pretty expensive even for the a basic model.

ABSOLUTELY.

You'll need a digital 'scope that can "freeze" the image, and a low-current (amperage) probe.

Set up the 'scope so that it's reading the signal from the low-amps probe, with the probe connected to the power-supply wire of the fuel pump. Adjust the settings as needed to get a clear view.

You'll get a series of pulses on the 'scope screen that relate to the amperage draw of each individual fuel pump motor armature bar. Each one is like a fingerprint--they look exactly alike until you look close enough, and then each one is distinguishable from the others. Freeze the image, pick one of the pulses to study, then count the number of pulses until the pattern repeats. That gives you the number of armature bars in the fuel pump electric motor (usually 8, sometimes 12, but any number is possible depending on the design of the motor.)

Once you see the pattern repeat, you can use the cursor functions of the 'scope to determine the milliseconds between repeating patterns. And then mathematics can reduce that to the RPM of the electric motor. If there are gross differences in the amount of current pulled by one or more bars, the pump motor is probably junk. If the whole motor draws excess current, the motor is probably junk (or the fuel filter is plugged.) If the motor doesn't draw enough current, there may be an internal leak, or low fuel pressure due to a failed pressure regulator.

The secret to all this is to test multiple GOOD pumps, so when you suspect a bad one, you'll recognize something out-of-the-ordinary. The same principles apply when testing HVAC blower motors, power antenna motors, power seat motors...

Not a video, but I posted a batch of photos of all this on another site.
(Ad blockers will disable the photos. Once I switched-off my ad blockers, the photos reappeared.)

TBI--Fuel Pump Upgrade w/Photos - Third Generation F-Body Message Boards

TBI - TBI--Fuel Pump Upgrade w/Photos - This seems to be the finest web site in existence for TBI information. While I recognize that this is primarily a Camaro/Firebird site, much of the information applies to GM TBI vehicles in general. In my case, I have an '88 Chevy K1500 work truck with the...

www.thirdgen.org