Fuel pump trouble

[Dariusz Salomon]

Try starting the truck, if it won't start, hit the bottom of the fuel tank with a rubber hammer, and re-try. The fuel pump is inline with the fuel filler door, so aim the hammer there.

The ground wire is on the frame rail in the same area.

EDIT: Just relized you have a Tahoe.... I don't know the postioning of the various components on the SUVs, I was thinking of my pickup, but if you look underneath you can probably get close.

I roughly know where it is-bit in the middle-but that's actually great idea. That's why I like to ask questions on this forum as someone always comes up with some awesome tips and solutions.

 

[Dariusz Salomon]

I'm sure the pump running all the time and the fuel pressure regulator being in the same position all the time (close to wide open) caused mine to stick and caused me earlier starting difficulties. That's why I have a switch on the dash now to switch the pump off when it's not needed.
An injection system should have a programmable option to switch the pump off when running on LPG.


If it's an injection system you won't be able to. Two giveaways - two banks of injectors feeding pipes into the manifold, the system changing from petrol to LPG automatically at a pre-set coolant temperature.
Or, a mixer system which can be programmed to always start on LPG (not advisable) and can be forced to do it by toggling the changeover switch. Ignition on, switch on LPG, switch it to petrol then back to LPG and if the yellow light shows continuously it will deliver LPG on cranking. Mine is set-up to feed (without being drawn) from the 'idle' screw to help this.

If you post a photo of your engine bay I'll tell you which system you have - mixer or injection.

Pre set coolant temp for sure-I was playing with it some time ago. It's Stag 400

 

[Dariusz Salomon]

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

I just noticed your post-I didn't see it earlier for some reasob-I will try to do that when I get a chance.

 

[Nad_Yvalhosert]

Imma stick to dumb-if they design the tank to go on the passenger side of the chassis then there would be more weight balance,isn't it?

There's another reason that the underside is designed that way. The cayalytic converter, and the associated heat generated by said converter would be under the drivers feet.
This way the entire fuel system, wiring harness, and Evap are on one side, the hot exhaust is on the other. And the exhaust exits as far from the driver as possible, keeping carbon monoxide away from the driver.

Yes they would probably have to design the transfer box differently but this could've been done at the beginning of design process. NM that.

You should probably check the square body trucks for that exact design. Driver side exhaust, dual saddle tanks, passenger side drop t-case and axles.

 

[Pinger]

Pre set coolant temp for sure-I was playing with it some time ago. It's Stag 400

You have an injection system. It's a more sophisticated and better system than mine but lacks the simplicity that allows mine to be easier manipulated when needed.
As your system may (or may not) have the facility to switch the fuel pump off bear that in mind. It is unlikely that its ECU would screw that up but if it intercepts pump wiring then there's always a possibility of a fault there.
If you can find the specs (online) for the Stag system it'd be worth knowing if it can or cannot switch the pump - and how (if it can). If you can't find that out but suspect it is a problem, there's a forum I can ask on.

 

[Dariusz Salomon]

You have an injection system. It's a more sophisticated and better system than mine but lacks the simplicity that allows mine to be easier manipulated when needed.
As your system may (or may not) have the facility to switch the fuel pump off bear that in mind. It is unlikely that its ECU would screw that up but if it intercepts pump wiring then there's always a possibility of a fault there.
If you can find the specs (online) for the Stag system it'd be worth knowing if it can or cannot switch the pump - and how (if it can). If you can't find that out but suspect it is a problem, there's a forum I can ask on.

Thanks-I'll talk to the guy who put the system in first-he will know this. But if I get nowhere I'll let you know.

 

[Pinger]

Download of Stag 400 manual here >> https://fdocuments.net/download/stag-400-dpi-manual

I've skimmed it and didn't see any mention of switching off fuel pump.
Are you seeing any fuel at all when you depress the Schrader type valve in the fuel line?

 

[Dariusz Salomon]

Download of Stag 400 manual here >> https://fdocuments.net/download/stag-400-dpi-manual

I've skimmed it and didn't see any mention of switching off fuel pump.
Are you seeing any fuel at all when you depress the Schrader type valve in the fuel line?

I didn't look yet-I'm very busy at work. Had no chance to test anything. I'll have more time to dig this weekend.

 

[Pinger]

What I found with mine is that when the pump wasn't working there was no fuel at the Schrader at all.
When the pump was working but the FPR stuck open a small spurt for about a second which could be repeated by re-priming the pump.
A properly working pump will create a huge spurt which will end up over the distributor. I wouldn't dare do that with a hot or running engine.