98 5.7 fuel pump intermittent no prime / dies while driving

[dboyfrmfoez]

My buddy went through like 4 fuel pumps, I think the last time he put a new connector on the harness and it's been fine so far. You might try inspecting the harness for breaks and looseness. If your oil pressure switch only has one wire going to it, it's gauge only.

More than likey you have a three prong oil pressor sender switch/interrupter. When you are lower rpms your oil pressure sensor is interrupting your connection to the fuel pump which is then not allowing your pump to prime. This is a safety feature that works in numerous ways. One if the car were in an accident or if the oil pressure is too low from low oil it will interrupt the connection to the fuel pump and stall the engine. Also if the fuel pump relay goes bad while driving thr fuel pump will still be powered by the oil pressure sender.

Which harness is that? The one for the fuel pump itself?

So if it only has the one wire, it means that the truck should cut off with the relay removed because the oil pressure switch will be doing nothing to counteract that, correct?

My buddy went through like 4 fuel pumps, I think the last time he put a new connector on the harness and it's been fine so far. You might try inspecting the harness for breaks and looseness. If your oil pressure switch only has one wire going to it, it's gauge only.

More than likey you have a three prong oil pressor sender switch/interrupter. When you are lower rpms your oil pressure sensor is interrupting your connection to the fuel pump which is then not allowing your pump to prime. This is a safety feature that works in numerous ways. One if the car were in an accident or if the oil pressure is too low from low oil it will interrupt the connection to the fuel pump and stall the engine. Also if the fuel pump relay goes bad while driving thr fuel pump will still be powered by the oil pressure sender.
If your truck doesnt dies when you pull your fuel pump relay. Your oil pressure sensor switch for the gauge is bad...
Does your oil pressure guage go down as the motor stalls?
Does your truck not start after stall?
does your truck not register oil pressure while long cranking after stall?
If you want to rule out relay
Cut a 3 inch wire crimp the ends with the proper blades that can be inserted in prongs 30 and 87 for the fuel pump relay. You should hear the fuel pump. Start the truck. If oil pressure is registered on gauge break the connection by pulling your jumper wire. If the truck dies the oil pressure sender /gauge is bad. Relay is good if test does the same with relay.

 

[Schurkey]

More than likey you have a three prong oil pressor sender switch/interrupter. When you are lower rpms your oil pressure sensor is interrupting your connection to the fuel pump which is then not allowing your pump to prime. This is a safety feature that works in numerous ways. One if the car were in an accident or if the oil pressure is too low from low oil it will interrupt the connection to the fuel pump and stall the engine.

NO. The oil pressure switch CANNOT turn the fuel pump "off" if the fuel pump relay is turned (or stuck) on.

Also if the fuel pump relay goes bad while driving thr fuel pump will still be powered by the oil pressure sender.

YES. Typically results in long cranking time before the engine starts--engine has to build oil pressure before the oil pressure switch powers the fuel pump.

The fuel pump relay can turn the pump on.
The oil pressure switch can bypass the fuel pump relay to turn the pump on.

Neither one can turn the pump OFF, if the other is causing the pump to run.

If the fuel pump relay--or it's wire harness is faulty, or a fault in the ECM that causes the fuel pump relay to not activate, then the oil pressure switch that bypasses the relay could shut off the pump only if the oil pressure dropped below whatever it takes to close the contacts of that oil pressure switch--maybe 4--5 psi.

If your truck doesnt dies when you pull your fuel pump relay. Your oil pressure sensor switch for the gauge is bad...

If the engine does not die (fuel pump continues to run) when you remove the fuel pump relay, the two-wire oil pressure switch is WORKING to bypass the fuel pump relay and keep the pump running.

Has nothing at all to do with the one-wire sending unit for the dashboard gauge. (Newer trucks like the OP's '98 use a 3-wire combination switch and sending unit--one wire for the dash gauge, two wires for the fuel pump relay bypass.)

 

[nightowl3130]

Op sender, 1 wire, =gauge. 3 wires

Might be worth reading this.

Fuel pump relay connections

Fuel pump relay is sitting in the terminals loose. This is causing an intermittent connection gap. When it’s hot I have trouble starting the truck. How can I fix this? Is it possible to pull up the fuse panel and bend the terminals back? Is there an easier way?

www.gmt400.com

i’ve seen a lot of posts that have given me good ideas of where to start, but i have not seen anything exactly like what i’m dealing with. i hope i’m not typing too much, just want to give y’all every relevant detail

so i have a ‘98 ecsb k2500 5.7 farm truck that i bought with ~202k miles and drove it ~2 hours back to my house. the next day when i went to drive it, it was sputtering and stalling but only at lower rpms. it was driving fine when i test drove it, and during the drive down, but i think the drive home was the straw that broke the camels back. long story short, by the advice of a couple of my more mechanically inclined “friends” i replaced the IAC, TPS, MAF, IAT, spark plugs, etc, all to find out using a $2 bottle of C&C cleaner that it was just the o-ring around the spider injector assembly.

we replaced the upper gaskets and cleaned the throttle body while we were in there, and the truck was running great until recently, when it just suddenly died on my way to work. the rpms went from ~2k, down to zero, back up to 2k, then back down to zero and it coasted to a stop all in the span of about 4 seconds. luckily i had just passed my girlfriend’s uncle, who is an actual mechanic (and the one who helped me discover and fix the aforementioned o-ring problem). he told me to try to start it again and said he couldn’t hear the fuel pump, which i thought was odd because i had a new fuel pump installed in march (this happened in mid august). we towed it to a nearby parking lot, he gave me a ride to work, and when i got off we went back to tow it. when we got it home, i turned the key to show them what it was doing and it fired right up, and ran fine for about five minutes until i cut it off. super weird, but i didn’t think too much of it, just chalked it up to bad gas as moved on.

then about a month or so later, i drove from work to the store (like a 3 minute drive), went in for about 5 minutes, then when i came out, it wouldn’t start. thought it might be the relay, so i had a friend that was nearby pick one up for me, to no avail. i noticed the ground on the frame right next to the tank was looking pretty rusty so i cleaned it up, and nothing. i did notice that the wiring harness was pinched between the tank and the frame of my flatbed, so i finally knew what the problem was. about that time, a buddy of mine who i hadn’t thought to call (the “friends” from earlier didn’t bother to pick up) pulled up and offered to tow me home. he said to put it and neutral so we could roll it back, and i said here, let me just start it and we laughed. then it actually started (after sitting for ~1 hour) and i drove home without a problem. i replaced the harness and all was well again, up until a couple weeks ago, when it crank, no started at work, and the fuel pump would not prime no matter what i tried. then after sitting for a couple hours, it worked just fine again.

finally, a few days ago when my dad and i were headed into town, it died (this time was just a slow decline to 0 rpms) and no finagling with the wires or fuses or relays would get it to start. then after waiting about 30 minutes it started fine.

as i sit in the truck right now, it’s priming and starting great, and i tested something i read on another post about removing the fuel pump relay while it’s running and how it should continue to run due to the oil pressure switch i think. when i unplugged the relay, the truck immediately lost power and turned off after about 2 seconds. so i’m wondering if that is a surefire way of knowing that switch is bad and then knowing if that could be why the truck is having this problem

Update: thank you all for the advice and insight, some of this stuff i never would have thought of. found information regarding the underhood fuse box here, so i’m about to dig into that now, and will post pictures. also i’m going to start the truck and let it build up oil pressure for longer to get a better “test” of the OPS

Update #2: Texas decided to storm all day so between that and other errands I didn’t get to do much besides watch some Youtube videos and read more posts here. I’ll get with my mechanic tomorrow and see what he thinks regarding everything; the relay and all connections relating to it, the crank position sensor, and the oil pressure switch

My 96 5.7 oil sender has 3 WIRES. There's a circuit that upon 5 lbs of oil pres, creats a power feed to fuel pump in case the relay fails. The after mkt's op senders had a prob switching that circuit off even after turning off key. The pump would remain on after engine stopped. Especially in colder climates, until eng warmed up. I thought this circuit was a shut off if you lost op until I studied the schematic. I can take relay out and crank over good & eng will start after 5lbs of op. Also poor connections at relay. Hope you see this & it helps..1 wire sender=gauge.

 

[TheTruckOfTheseus]

I appreciate it. Luckily I figured it out, just a plain bad fuel pump. I’m glad for the newfound knowledge, though I hope to not need it in the future. Thanks again

 

[nightowl3130]

I appreciate it. Luckily I figured it out, just a plain bad fuel pump. I’m glad for the newfound knowledge, though I hope to not need it in the future. Thanks again

Been there !! I maticously measured my tank FP location, cut me an opening in bed under tool box, made cutout to fasten back over cut out hole. Now if this happens on the road away from home, I can change pump in 30 min. Travel lots between TN, & Fl. No more dropping tank.