1993 1500 fuel pump issue?

James Collier Jr. · April 12, 2020

On 4/7/2020 at 7:51 PM, Gmc9305 said:

I have the air cleaner off during this problem. I took a chance and replaced the relay with the same results. It started today, I saw nice fuel cones, then the fuel just shut off.
How do I jump out the test lead on the fire wall to diagnose other electrical components?

How do I put a jumper in to supply constant power to the pump to rule out a bad pump?

That sounds suspiciously like what's going on with the G-van I mentioned previously.

The test lead on the firewall is easy, it will be a larger pink or black wire, usually near the relay on the firewall. It will appear to be a loan wire disconnected, but it is indeed the test lead. Jump 12V positive and the pump should run and continue to run. If it does, you have a relay or control circuit problem.

You could also try jumping direct from the 12V feed at the relay, but you'll need a test light to figure out which wire it is.

On 4/7/2020 at 8:17 PM, riverbanks said:

Starting to sound like the oil pressure sending unit,they fail,and stop power to pump

On 4/7/2020 at 8:39 PM, Gmc9305 said:

How can I test that?

Most will have two sending units, one for the gauge (varying resistance for gauge reading), and one for the warning light / fuel cut-off (simple on/off switch). Some will be plumbed into the oil filter housing area, I have seen a few on the back of the block near the distributor.

The one for the gauge will likely have three wires, where the other will have only two. Unplug the connector with two, see if the problem persists. It's possible you will have to jump the pins at the connector to do this, but unlikely. If there is only one with three wires, check the schematic to see which one is the warning light trigger, that will be your fuel cut-off. I WOULD, however, verify that an oil pressure problem does not exist before doing this. Factory gauge accuracy has been known to be less than stellar, especially when you're checking for a flaky sending unit in the first place.

Gmc9305 · April 12, 2020

3 hours ago, James Collier Jr. said:

That sounds suspiciously like what's going on with the G-van I mentioned previously.

The test lead on the firewall is easy, it will be a larger pink or black wire, usually near the relay on the firewall. It will appear to be a loan wire disconnected, but it is indeed the test lead. Jump 12V positive and the pump should run and continue to run. If it does, you have a relay or control circuit problem.

You could also try jumping direct from the 12V feed at the relay, but you'll need a test light to figure out which wire it is.

Most will have two sending units, one for the gauge (varying resistance for gauge reading), and one for the warning light / fuel cut-off (simple on/off switch). Some will be plumbed into the oil filter housing area, I have seen a few on the back of the block near the distributor.

The one for the gauge will likely have three wires, where the other will have only two. Unplug the connector with two, see if the problem persists. It's possible you will have to jump the pins at the connector to do this, but unlikely. If there is only one with three wires, check the schematic to see which one is the warning light trigger, that will be your fuel cut-off. I WOULD, however, verify that an oil pressure problem does not exist before doing this. Factory gauge accuracy has been known to be less than stellar, especially when you're checking for a flaky sending unit in the first place.

Thank you. I tested the three lead connection by jumping it out. I’ve pretty much determined the problem was the Spectra fuel pump I got from rock auto.

During diagnosis, noticed a couple things: When the truck would run, the pump was noisy and seemed to “surge”. At one point I both loosened and disconnected the fuel line from the throttle body and no fuel came to that point even with jumping out the relay, I only heard minimal air being pushed during the times the pump intermittently ran.

I considered the fuel filter. This was removed and at one point fuel flowed out of the line. I blew compressed air through the other end to make sure it was clear and installed a new filter with no change.

Bought a Delphi pump from autozone and installed it. Can’t hear it run at all. Truck runs with relay and if I jump out the relay. I’ve let the truck sit to try to recreate my original problem at it starts each time. So hopefully it was just the pump.

James Collier Jr. · April 13, 2020

Good to hear, and high hopes you are correct about the pump. It certainly seems so. I can tell you that fuel pumps are the one thing you don't buy anything but OEM on, you're just asking for trouble. Even buying OEM at discount prices online can be a gamble, they are usually parts that work but do not meet OEM quality specs, and are auctioned off to resellers. Even though it may carry an OEM brand name and come in an OEM box, it will likely not perform as well, which can cause any manner of problems. Some may say 'parts is parts', but the old adage rings true -- You get what you pay for.

Food for thought -- when I went through four fuel pumps in a row on my Safari, every one that failed was an AirTex. Even after correcting the crappy harness repair. Finally spent the money for a proper Delphi and never had another minute's trouble. After that, as for me, I wouldn't stick anything but true-blue Motorcraft in Ford, Delphi or Delco in GM, or Mopar in Chrysler.

diyer2 · April 13, 2020

On 4/5/2020 at 7:35 AM, diyer2 said:

If it's not a GM pump, AC Delco may be the pump.

Never run the gas tank low on fuel because the gas cools and lubes the fuel pump.

You will have to check relay.

On 4/5/2020 at 7:46 AM, diyer2 said:

I'm saying you will have to do the testing to figure out what the problem is.

I can't tell you.

I will add hitting the bottom of the tank and it working points to fuel pump IMO.

Great you got it fixed.

Steven_R · April 24, 2020

I hope you got your truck running. I have a similar issue, so I will tell what I learned.

On 4/7/2020 at 8:17 PM, riverbanks said:

Starting to sound like the oil pressure sending unit,they fail,and stop power to pump

The oil pressure sending unit and the fuel pump relay are in parallel. Either one will power the pump. The oil pressure sender circuit is a backup in case the relay (or the control line to it) fails. So it cuts power only if the relay has failed also. I know this is the case for GM 5.7 and 7.4 engines up to about 2000. (I don't know about later systems)

On 4/6/2020 at 12:43 PM, James Collier Jr. said:

Most older GM trucks have a fuel pump test lead on the firewall, a single, large pink or red wire, usually near the relay. If you jump this out with 12V and the pump runs, you have a fried relay

James is correct. I have a 97 Express with a 5.7. It has a test lead that looks pink, but the GM manual's schematic says it is red. It faded. This goes to the NC terminal on the relay. So you need the (possibly bad) relay installed to test this way.

James Collier Jr. · April 25, 2020

On 4/24/2020 at 7:41 AM, Steven_R said:

I hope you got your truck running. I have a similar issue, so I will tell what I learned.

The oil pressure sending unit and the fuel pump relay are in parallel. Either one will power the pump. The oil pressure sender circuit is a backup in case the relay (or the control line to it) fails. So it cuts power only if the relay has failed also. I know this is the case for GM 5.7 and 7.4 engines up to about 2000. (I don't know about later systems)

Pretty much defeats the purpose of having a low oil-pressure cutout, doesn't it? That's about as bass-ackward as I've seen. If anything, the oil pressure sender should provide the ground to the relay, I would think. No oil pressure, kills the relay and it shuts down. The way you describe, as long as the relay is good the low-oil-pressure cutout is disabled. So what was the point? *smh*

Folks, this is what happens when people get engineering jobs after floating through college between binge highs and get hired at $100k a year to call themselves 'engineering' things. The university local to me, most of their stupids, er, students, couldn't chew bubble gum and tie their shoelaces without falling down.