5.7 Starting Issue

[Schurkey]

I thought it might be the relay, but everybody says relays either work, or they don't....no sometimes.

Of course they can be intermittent. Not common, though.

Or the contacts get corroded, so even though the relay engages, little current flows.

 

[Slime]

Well, I guess it's dead. Tried to start it tonight...nothing. Jumped the relay terminal too...

 

[Slime]

So, is one of the signs of a fuel pump going bad intermittent working? Because, once again...it started and ran. WTF Finally stopped raining, so maybe I can start checking other things..

 

[Schurkey]

So, is one of the signs of a fuel pump going bad intermittent working?

I spent eight+ hours driving 150 miles in my '93 Lumina because the fuel pump worked...didn't work...worked...didn't work...all the way across the Sierra Nevada mountains from east of Reno to Sacramento area.

When it did "work", my fuel pressure was about ten psi when it should be fifty.

A new fuel pump later, ($500 because I was 1800 miles from my garage and tools] and all was well.

 

[Slime]

I spent eight+ hours driving 150 miles in my '93 Lumina because the fuel pump worked...didn't work...worked...didn't work...all the way across the Sierra Nevada mountains from east of Reno to Sacramento area.

When it did "work", my fuel pressure was about ten psi when it should be fifty.

A new fuel pump later, ($500 because I was 1800 miles from my garage and tools] and all was well.

Thanks...I guess I should just bite the bullet. I don't really mind, just trying to make sure thats the issue .

 

[Ken K]

The issue with fuel pump or fuel pump control has to be properly diagnosed. Even the "Richy Cunninham" video scares me. First of all, the terminals are 1.5mm wide or 150 "Metri-Pac", so the next time you are at the bone yard, cut some off and pocket them. Remember, the male side is always the component, while the harness side is female. Go for a window regulator, seat connector or something easy. Purchase an in-line fuse holder and solder it in place with some shrink tubing. Now you have a fused jumper wire. I have not seen every GM truck ever made, but usually next to the fuel pump relay, on the table, there is a empty spot that says "Prime". Apply your fuse jumper wire to B+ and this by-passes the relay directly to the pump. You should hear it run. Having a valve on the fuel line for gauge access is nice, but many do not have this. It saves $0.33 per vehicle to leave this valve off the injector rail. Fine!

The 150 "Metri-Pac" fit allow you to test terminal fit. What? Yes, when inserting the terminal and pulling it out, the tension will tell you if they are tight or loose. Fix these or replace with new. Also, here is another place to use dielectric grease to keep corrosion out of the terminal, causing intermittent problems and all kinds of headaches.

Turing the key on should allow a voltage check at the relays socket to verify a VCM/PCM voltage signal for fuel request. Is the relay bad. Maybe...just swap it out for another BLK one, right. No. Unless the same brand with the same part number it, it may not pass the required amperage. Test it. How? Voltage drop. Using 4 jumper wires, with male on one end and female on the other, about 6" long, plug them into the relay and identify terminal 87, as 30 is already hot with key or VCM/PCM. Use your digital multi meter (Or dom) and place on the voltage setting and check for voltage drop with you meter on the relays wire, and the other to ground. When on, you should hear the relay click, but battery voltage. Repeat, note reading, repeat again. Yes, do this several times to "Catch-It" with a huge voltage drop, like 1.2 vdc or 2.8 vdc. A good reading is no voltage drop, but in reality, it will read 0.050 volts roughly. (Your will very) If good, reach under the bed, unplug the fuel pump connector, then plug in an old head light with jumper wires that fit. Turn key on or jumper it and the light should be bright. If it is and there are no signs of corrosion, then it's time for a pump.
Yes, pulling the bed is the easiest using some notched 2 X 6 framing lumbar in an "H" pattern and a cherry picker or engine hoist.
Clean the pump area, remove the retainer ring and remove the pump.
Inspect the pump, canister and sock type filters. If you have the plastic canister type, use a pocket screw driver and push the 3 tabs in and slide the canister off. Look inside to see if there is a build up of dust & dirt. Yep, the tank is dirty. After years of operation, how many miles are showing? What is you MPG? Do the math. This can come out to 1,000's of re-fueling events. Do you think that all the fuel you purchase is clean and free from dust or dirt...no! Is there a carbon canister malfunction or do you drive on gravel roads all of the times? Yes, while running, purge is on. Fresh dirty dusty air is pulled in thru the canister tank valve, and this air create an exothermic reaction inside the canister as it removes all of the gas fumes or hydrocarbons from the canister a little at a time. While the sloshing tank will create more fumes that needs to be removed. Some of this dust finds it way into the tank.
The tank had to be emptied, cleaned with hot water, rocked back and forth, then drained. (This is what radiator shops did for years)
With all of the dust and dirt out, install your new ACDelco / Delphi fuel pump, place dielectric grease in the connector, change the metal filter and if real bad...flush the injector rail and lines before you go too far. Now it will last for another 120k mile or more. Ever had one fail soon? Why?
Now you know! There is best practices and then there is the other way. Which one do your want in your truck?

 

[Schurkey]

I don't really mind, just trying to make sure thats the issue .

1. Fuel pressure test
2. Fuel pump voltage test--MUST be done as close to the fuel pump as practical--typically at the wire harness connector just before the wires go up over the top of the tank to the fuel tank connector (which would be inaccessable with the tank in place.) You'd also have to check the voltage on the ground wire, again as close to the pump as you can get.
3. Fuel pump amperage test. A multimeter should be able to test the ~7 amp draw that's typical. Most will go to 10 amps, some will go higher. The REAL test is to connect a low-amperage probe to an oscilloscope, so you can look at the amperage pattern.

Not many folks have a low-amps probe and an oscilloscope--but--I refuse to yank the tank without performing this test. I can see the amperage draw of each bar in the fuel pump armature, compare it to the others, calculate the motor RPM, and verify that the electric motor part of the fuel pump is still serviceable.

 

[Ken K]

Not many folks have a low-amps probe and an oscilloscope--but--I refuse to yank the tank without performing this test. I can see the amperage draw of each bar in the fuel pump armature, compare it to the others, calculate the motor RPM, and verify that the electric motor part of the fuel pump is still serviceable.

This is correct. In a shop setting, this equipment and procedure will diagnose correctly and sell more fuel pumps with proof. As stated, not the standard equipment found at home.
As the bush of a DC motor line up directly over the armatures commutator 's plate, current flow at it highest point. If this connection is askew due to worn brushes or armature, the current will drop if only a multi meter amperage feature is used. But a scope will show the actual electrical activity inside the motor. If you have the ability to print, this turns to gold and isolates the "Root" cause.
Shown below are worn armature plates, good current waveform and bad waveform for reference.

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[Eveready]

Same pattern today. went out Sunday and no start. Monday morning, because my car is dead i try to start the truck...same thing. So, i pull the relay and jump the #30 to the #87 pin. I hear the line pressurize . reinstall the relay and start the truck. Drove to work. Drove it home, even stopped at the store...still starts. I thought it might be the relay, but everybody says relays either work, or they don't....no sometimes.

Starter solenoids tend to behave that way when they are almost gone and solenoids and relays work in much the same way. I would replace the relay and see if that fixes it. Also check any grounds you haven't already worked over.

 

[Slime]

Same issue. Starts and runs fine... I was checking every other day. Then...no start. Two days later...starts. I swapped out the relay when it wouldn't start. Didn't help and it was a new AC relay. Today, I checked the fuel pressure since it's working..Question. After testing the fuel pressure I'm only showing 52 PSI at idle. I think 56-62 is the appropriate PSI...is that too low? I still have to raise the bed to test the electrical at the pump, but just wondering if 52 PSI is too far below the threshold ?