How do I check the fuel pump voltage? The junction block shows over 14 volts while running at all connections.
TBI 454 Misfire
- Thread starter Rock Hard Concrete
- Start date
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Lets, for a moment, assume the fuel pump voltage and pressure are spot on.
What would be the next step after that?
What would be the next step after that?
Connect the scan tool.
A fuel pressure test at prime, and under load is easier, and provides the actual information we want to know compared to a voltage or amperage check--which is more-often done when the pump fails a pressure/volume test.
Fuel pump voltage is measured at the rearmost (closest to the pump) fuel-pump wire connector accessible, with the pump RUNNING. And that still won't catch problems in the harness that goes up 'n' over the fuel tank, or the in-tank wiring. Ideally, you'd have reasonably-high voltage. GM has a nasty habit of using undersized wire; there's always some voltage drop (VD) in the circuit. So if the alternator is pushing 14.2 volts, I'd want to see at least 12.2 volts at the rear by the fuel pump, and I'd hope for more than that.
Next you'd be checking for voltage on the ground wire, again as close to the pump as practical. Again, GM uses undersized wire and a steel frame, so there's some resistance--but anything over 1 volt is too much and I'd hope for less.
A fuel pressure test at prime, and under load is easier, and provides the actual information we want to know compared to a voltage or amperage check--which is more-often done when the pump fails a pressure/volume test.
Fuel pump voltage is measured at the rearmost (closest to the pump) fuel-pump wire connector accessible, with the pump RUNNING. And that still won't catch problems in the harness that goes up 'n' over the fuel tank, or the in-tank wiring. Ideally, you'd have reasonably-high voltage. GM has a nasty habit of using undersized wire; there's always some voltage drop (VD) in the circuit. So if the alternator is pushing 14.2 volts, I'd want to see at least 12.2 volts at the rear by the fuel pump, and I'd hope for more than that.
Next you'd be checking for voltage on the ground wire, again as close to the pump as practical. Again, GM uses undersized wire and a steel frame, so there's some resistance--but anything over 1 volt is too much and I'd hope for less.
I am not a mechanic I do not know the location/color of the wires.
What wire do I check from the harness going to the pump? Where is the ground?
How far into the sheathing do I have to stab the probe?
I am very mechanically inclined, but about completely out of my element with electrical stuff.
What wire do I check from the harness going to the pump? Where is the ground?
How far into the sheathing do I have to stab the probe?
I am very mechanically inclined, but about completely out of my element with electrical stuff.
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Download the service manual set for your vehicle from the links in the Sticky section of the Engine forum, right here.
I don't know on Vortec vehicles. I suppose I'll find out next summer when I will--likely--be dicking with the fuel pump on my '97 K2500.
On my '88, it looks like this, and it's probably "about" the same on the newer ones.
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At the bottom of the photo, you see a 3-wire connector from the fuel tank, plugged into a 3-wire connector with red wires. The red wires and connectors are a splice I made--male on one end, female on the other, all done in Weatherpac connectors and pins--to add about a foot of "sacrificial" wire that I could poke with a probe and not destroy the actual vehicle wires.
Two of the three wires are power and ground for the sending unit to drive the dash fuel gauge. One wire is the power supply for the pump, with the pump ground being the single-wire attached to the framerail.
DO NOT STAB VEHICLE WIRING, ever. The last thing you need is pierced insulation letting-in road splash to corrode the copper wire inside.
Pop the connector apart, install a splice of some sort. Or--and for me this is a distinctly second choice--buy some "backprobe" pins that you can slide underneath the connector weather seals to reach the conductive wire end without damaging the integrity of the weather protection seals.
I bought these fairly recently, but have not actually used 'em. There's dozens of competing products.
www.amazon.com/gp/product/B00RGNFAMC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Fairly common. Electrical "stuff" is often difficult for folks that don't have a "Math" background, NOT because of the actual mathematics of electrical circuitry (of which there is plenty--but also mostly not applicable to a system that's already been engineered and in-use) but because both math and electrical diagnosis (vehicle diagnosis in general) are heavily dependent on logic, and logical thinking is not common in our society.
I do not have a "Math" background. And some electrical "stuff" leaves me baffled. That's when I break-out the harsh language.
That is more work than I am willing to do. The fuel pump is new, I am going to assume the fuel pressure, at least to the throttle body is not the issue.
Assuming the fuel pressure is not causing my problem, what is the next most likely cause? Could my ecu be fried causing intermittent issues?
Assuming the fuel pressure is not causing my problem, what is the next most likely cause? Could my ecu be fried causing intermittent issues?
There's a dozen things it "could" be.
If you're not willing to do proper diagnostic work, and want to guess your way through this, I'll bow out.
If you're not willing to do proper diagnostic work, and want to guess your way through this, I'll bow out.
check fuel pressure, you're going to need to buy an adapter as TBI doesn't have a Schraeder valve like modern EFI. this is a must with a TBI vehicle. you want 13 psi, you can modify your stock regulator to obtain this.
did you fabricate a block off plate for EGR?
do you know 100% the distributor has been replaced? i went through 3 reman distributors on a GMT400 i did work on a few years ago. i finally gave up and rebuilt the original and it finally ran right.
a long long time ago i saw a "good" alternator (14V changing) with bad diodes cause the ignition to break up but only at higher RPMs. long shot but might be worth swapping alternators if you have access to another one.
did you fabricate a block off plate for EGR?
do you know 100% the distributor has been replaced? i went through 3 reman distributors on a GMT400 i did work on a few years ago. i finally gave up and rebuilt the original and it finally ran right.
a long long time ago i saw a "good" alternator (14V changing) with bad diodes cause the ignition to break up but only at higher RPMs. long shot but might be worth swapping alternators if you have access to another one.
Took truck for freeway drive. Low power, backfired out of the throttle body a few times.
Bumped up pressure at the regulator.
Truck pulled the same 5% grade it needed wot to maintain 60 mph, with half throttle, overdrive with the converter locked, and accelerating.
Still not perfect, noticed the truck has more power at 4/5ths throttle than at full.
I am going to bump the pressure a little more tomorrow.
Bumped up pressure at the regulator.
Truck pulled the same 5% grade it needed wot to maintain 60 mph, with half throttle, overdrive with the converter locked, and accelerating.
Still not perfect, noticed the truck has more power at 4/5ths throttle than at full.
I am going to bump the pressure a little more tomorrow.
No block off plate yet.
I only have the word of the first mechanic that the distributor itself was replaced. It did look pretty new when I was under the cap. Also, the stalling issue that I was originally having is gone, apparently from the dist. being replaced.
If bumping the pressure a bit more doesn't fix the issue I'll rent a fuel test kit.
