1993 1500 fuel pump issue?

Gmc9305 · April 5, 2020

1993 GMC 1500 dump truck. Installed new fuel tank in January due to pin hole leak. Installed new fuel pump as well since I had the tank dropped. Noticed that the pump stayed running instead of the initial prime and then shut off. Truck ran fine until a couple weeks ago when it just shut down on me. Could crank but not start. Let sit and hit bottom of tank and it started again. Ran fine for the next week. Went to start this morning and it just cranks. Cannot hear pump running. Pump purchased from Rock Auto. I’m guessing the pump needs replaced or maybe it’s the relay? Any help is appreciated, thanks in advance.

diyer2 · April 5, 2020

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.

Gmc9305 · April 5, 2020

Since replacing the tank and pump I’ve never let the tank go below half. Are you saying it could be a relay problem or do I need to replace the pump?

diyer2 · April 5, 2020

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.

Ronsoy · April 5, 2020

What is the fuel pressure?

Ronsoy · April 5, 2020

Are you getting 12v to the pump? Did you check the relay power to be sure your getting 12v at the relay?

Gmc9305 · April 5, 2020

I did not get to check pressure or power yet. I can’t do it until tomorrow. I hate to think that a brand new pump quit already but I don’t want to drop the tank if it’s not the problem.

Gmc9305 · April 6, 2020

Don’t have a gauge but I loosened the fuel line into the throttle body. The pump primes just fine. A lot of fuel blows out under pressure when cranking. Besides checking spark next, I’m going to see if fuel is being sprayed down the throttle.

Gmc9305 · April 6, 2020

After tightening the connection again I cranked it again and after one or two times it fire up and will stay running. Maybe a clog or coil issue? I’m going to let it sit a few hours and try to recreate the problem.

James Collier Jr. · April 6, 2020

***UPDATED***

I'm assuming your 1993 has throttle-body injection (lower pressure than Vortec), but I am including some Vortec info as well just in case. 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 or some other control function locking out the fuel pump, be it anti-theft, low oil pressure cut-out, or blown fusible link (common on C/K/Silverado/Cheyenne with fuel injection). The fusible links that blow are usually in the harness coming up the firewall behind the engine if memory serves. A few people have uploaded videos on this to YouTube.

Could also be something similar to what I experienced with a Safari van. Most vehicles with fuel injection are going to have 4-6 wires going into the sending unit. Generally, only two actually go to the pump, the others are for controlling prime-up vs maintaining pressure. A lot of GM trucks, particularly those with Vortec engines, had issues with the wiring harness from the relay. Vortecs are also quite picky about fuel pressure, especially the early CPI engines. Spec calls for 62 prime pressure, 58-60 running. A mechanic I once used told me they won't run right below 55, if they run at all. And it is highly recommended to replace the harness inside the tank to the pump, no matter how good it looks or what anyone tells you. Quite common for these to be bad and look fine.

You might want to undo the wiring loom on the fuel pump harness and trace it farther up to the relay, mine had a botched previous repair about 6-12 inches into the loom and tape wrap. Apparently the original wiring cooked or was damaged and the idiot that called themselves repairing it, not only left 1/8 inch of bare wire hanging out one or both ends of the butt splices they used, they also wrapped all four splices in tape together, rather than individually as they should have been. Actually melted a hole in the top of the sending unit, as it was plastic.

Most modern vehicles with fuel injection also have this circuit tied in with the oil pressure switch circuit, which is supposed to cut the fuel if oil pressure drops, though I've never seen one actually do it. Another potential trouble spot if the oil pressure switch is bad or clogged with sludge (I believe the gauge is on a different sender if so equipped).

I believe throttle-body-injected engines use the tach signal from the ignition control module, so if the ignition module is getting flaky, that could cause an intermittent stall / no-start condition. Vortecs have a cam position sensor in the distributor that could cause intermittent stall / no-start as well. Some may also have a crankshaft position sensor, though I believe this sensor only helps to fine-tune the fuel, as with 3800s. 1993 should be throttle-body-injected, I believe, so I do not believe that system would have either, which leaves the ICM tach signal to control the injector drivers.

I know someone with a TBI-equipped G-van that has dropped their tank several times because the fuel pump keeps quitting. I keep trying to tell them what I've outlined here, but they won't listen because it fires and runs after replacing the pump (AGAIN) only to quit after a little while (AGAIN).

You can confirm if the engine is losing fuel or spark by spraying carb or brake cleaner in a vacuum port and cranking. The engine should at least fire and run a short time this way, which will eliminate spark and point to fuel. Also, if the inline filter hasn't been changed in a long time, dirt or internal collapse could be blocking fuel flow, which can also overload the pump.

Hope that helps!

Edited April 6, 2020 by James Collier Jr.

Gmc9305 · April 6, 2020

27 minutes ago, James Collier Jr. said:

***UPDATED***

I'm assuming your 1993 has throttle-body injection (lower pressure than Vortec), but I am including some Vortec info as well just in case. 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 or some other control function locking out the fuel pump, be it anti-theft, low oil pressure cut-out, or blown fusible link (common on C/K/Silverado/Cheyenne with fuel injection). The fusible links that blow are usually in the harness coming up the firewall behind the engine if memory serves. A few people have uploaded videos on this to YouTube.

Could also be something similar to what I experienced with a Safari van. Most vehicles with fuel injection are going to have 4-6 wires going into the sending unit. Generally, only two actually go to the pump, the others are for controlling prime-up vs maintaining pressure. A lot of GM trucks, particularly those with Vortec engines, had issues with the wiring harness from the relay. Vortecs are also quite picky about fuel pressure, especially the early CPI engines. Spec calls for 62 prime pressure, 58-60 running. A mechanic I once used told me they won't run right below 55, if they run at all. And it is highly recommended to replace the harness inside the tank to the pump, no matter how good it looks or what anyone tells you. Quite common for these to be bad and look fine.

You might want to undo the wiring loom on the fuel pump harness and trace it farther up to the relay, mine had a botched previous repair about 6-12 inches into the loom and tape wrap. Apparently the original wiring cooked or was damaged and the idiot that called themselves repairing it, not only left 1/8 inch of bare wire hanging out one or both ends of the butt splices they used, they also wrapped all four splices in tape together, rather than individually as they should have been. Actually melted a hole in the top of the sending unit, as it was plastic.

Most modern vehicles with fuel injection also have this circuit tied in with the oil pressure switch circuit, which is supposed to cut the fuel if oil pressure drops, though I've never seen one actually do it. Another potential trouble spot if the oil pressure switch is bad or clogged with sludge (I believe the gauge is on a different sender if so equipped).

I believe throttle-body-injected engines use the tach signal from the ignition control module, so if the ignition module is getting flaky, that could cause an intermittent stall / no-start condition. Vortecs have a cam position sensor in the distributor that could cause intermittent stall / no-start as well. Some may also have a crankshaft position sensor, though I believe this sensor only helps to fine-tune the fuel, as with 3800s. 1993 should be throttle-body-injected, I believe, so I do not believe that system would have either, which leaves the ICM tach signal to control the injector drivers.

I know someone with a TBI-equipped G-van that has dropped their tank several times because the fuel pump keeps quitting. I keep trying to tell them what I've outlined here, but they won't listen because it fires and runs after replacing the pump (AGAIN) only to quit after a little while (AGAIN).

You can confirm if the engine is losing fuel or spark by spraying carb or brake cleaner in a vacuum port and cranking. The engine should at least fire and run a short time this way, which will eliminate spark and point to fuel. Also, if the inline filter hasn't been changed in a long time, dirt or internal collapse could be blocking fuel flow, which can also overload the pump.

Hope that helps!

Thanks for the reply. I thought about testing the coil or if maybe the problem is the ignition control mondule.

James Collier Jr. · April 6, 2020

13 minutes ago, Gmc9305 said:

Thanks for the reply. I thought about testing the coil or if maybe the problem is the ignition control mondule.

UPDATED:

ICMs are quite common on GM HEI, especially if a previous replacement was a cheapie. I'd start with the simple stuff first and see if it runs on brake cleaner or carb cleaner. If it does, then try jumping out the test lead on the firewall. If it runs that way, you know it's a control issue, which could point to the ICM, oil pressure sensor / switch (some have both), or possibly some anti-theft system (equipped vehicles have a resistor chip in the key). Wiring problems are always a possibility, as they can be intermittent.

One other thing, on TBI engines, you can see the injectors spray by removing the breather lid. This is also an easier way to spray carb cleaner or brake cleaner in.

Edited April 6, 2020 by James Collier Jr.

Gmc9305 · April 7, 2020

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?

I’m going to check for a bad ground tomorrow and also try replacing the fuel filter. Again, all this intermittent stuff is while it’s just sitting level in the driveway.

Thanks again everyone for your help.