Please help and thanks for the accept. GMC 3500 not staying running

[Checkov]

I have a 1999 GMC 3500 Diesel that is having a fuel issue and won’t start/rather won’t stay running. About 65,000 original miles.
At first I determined it was the oil pressure sensor/switch was the issue, even sure of it. I took it off after buying a 37 dollar new switch and 35 dollar special socket. I took it off and the thing was broken, almost in half, and I just knew I fixed it. That is Until I turned the key and it just cranked as usual with no run.

I determined the secondary fuel pump or lift was not getting power. I then got the vehicle started running well by running a hot wire to the positive wire at the fuel pump. It completed the circuit the motor kicked on and then the vehicle started and ran beautifully. I took the hot wire off and it ran for a few moments then died again.

Definitely the problem is no power to the fuel lift under the seat. But how do I fix from here? I’m not good with electrical issues.

Here is what I’ve tried/know:
1. I looked at all the fuses and relays I could find. Can’t really tell if the relay is bad—-it clicks. The fuses look OK.
2. I noticed a mouse nest under the upper air intake. I vacuumed out what I could see and can’t really see any gnawed wires but I can’t see much either.
3. I put on both new batteries which were definitely weak (and I recharged them twice in a few days, another 200 dollars) and made sure the cables were good.

Any suggestions? Is it worth ripping the intake manifold off and looking for gnawed wires? Are even any wires there?

if someone tells me to start looking for a short I’m probably going to need a guide and how to...again not an electrician.

My plan B is to wire the hot wire to a toggle switch and limp it to Jacks Auto Repair shop, I’m retired so my income is somewhat limited.

thanks.....just for laughs here is a pic of the oil pressure switch...still can’t believe that wasn’t it:eek:5920

[Robyn]

Check the circuit between the electric lift pump and the relay in the box under the hood.
The relay contacts could be shot......

Personally I would plug in a fresh relay....they are not expensive.

Check that out....
Keep us in the loop.....

Good luck

[DmaxMaverick]

Your broken OPS was probably still working, despite its appearance. It doesn't control the lift pump on OBD II trucks (1996+). The PCM will have a tantrum if it's missing, though.

I agree, the relay is a likely suspect. It should have the diagram and function actually printed on it. Check that you have Batt+ power at both Batt+ slots (in the relay socket) at key-on. I don't recall, but the 99 should power the lift pump circuit for at least a few seconds after key-on.

Rodents are serious business. If they're nesting, they're chewing. I live in the sticks, and have had many tens of thousands dollars damage over the years, including a car that burned down 20' from my front door, and an RV that's likely to be totaled. Mice and rats are relatively easy to deter, but ground squirrels are relentless and don't respond to normal methods. If I find evidence of any of them in or around the vehicles, I mount a full-on assault until they are either exterminated or stop coming around.

[DmaxMaverick]

Welcome aboard!

[Checkov]

Will do thanks.

[Checkov]

I hate mice’s to pieces as well.
my dad bought a brand new Nissan Stanza back I guess in the 90s. Parked it in the barn for some odd reason and the next morning the thing would not start. Drove us crazy until we took the air filter housing off. I kid you not in one summer night they had packed the air intake tube with grass and straw. Must have been a constant procession.
thanks.

[Checkov]

Thanks.

[richp]

Years ago, I parked my 6.5 at a remote Utah desert location for a few weeks while on a river trip. Came back, and for some reason was under the hood. Saw pink on the firewall, and discovered tufts of insulation sticking out in various places. Poked around, opened up the air cleaner and found a nice comfy pink nest.

Fast forward about ten years. Had my big fifth wheel at that same location for two nights. When I went to leave, I had no tail lights. Eventually located the severed wires up inside the underbelly, along with a nest that contained a granola bar pilfered from one of the cabinets -- wrapper intact. He was saving it up for a midnight snack...

[Checkov]

Dear Dmax or anyone...

I used my multimeter and measured for voltage with the key turned on.

I was standing on the drivers side engine well looking in and let’s use a dollar symbol as the points as it will likely cost me:

radiator——-engine———front screen——rest of truck

$ $
$ $ $

I am only getting voltage shown when the bottom left contact is involved (first $ 2nd row). It doesn’t matter which combination I use, if that contact is ignored there is no voltage change (dial stays at zero).

and RichP that is an amazing story.

[Checkov]

(I am speaking of the points under where I removed the relay labeled fuel system)

[DmaxMaverick]

If the engine isn't running, the lift pump relay won't stay on long after the key is turned to on. Someone will have to verify, but I think it's only 3-4 seconds. If it isn't getting power at the relay when it should, suspect the engine harness from the PCM. It doesn't sound like you're having enough other issues to suspect the ignition harness/switch. If your ONLY issue is lift pump operation, maybe consider a stand-alone lift pump controller. They aren't cheap ($145 @ www.kennedydiesel.com (https://www.kennedydiesel.com/categoryresults2.cfm?Category=1&SubCategory=158)), but may be much less expensive than chasing and repairing a yet unknown problem, especially considering labor costs if that labor isn't yours. The controller is a DIY install with minimal skill, or less than an hour at a good shop.

[Robyn]

Rodents are just an awful mess.
Had a friend with a 6.5 that was running terrible....WE started searching and found a big AZZ rat in the intake.

He had chewed through the air filter and marched right on until he got stopped by the Turbo.....So he set up housekeeping.

The nest clogged and stalled the turbo.

When we discovered the beasts operations HE WAS HOME....And not at all pleased at being evicted.

This bad boy was a big one too.....And quite ready to do battle with these two Hooooomans that had him cornered in the air pipe.

He was hissing and making other nasty remarks...

Not wanting to get bit.....we shot a few squirts of Ma Startly in the air pipe and waited till said Rodent was well asleep then dragged his ugly carcass out.

I believe the dog got him as a chew toy.....

Yessss...Rodents....as Maverick mentioned....Squirrels are a real menace..

Mice are a nuisance, rats are nasty and mean.....Squirrels are cute and destructive.....

Good luck on the issue at hand

[JohnC]

Squirrels are cute and destructive.....

Rats with bushy tails...

[Robyn]

Chipmunks are abundant here too.

Fun to watch them shinny up the bird feeder poll

Then I go and grease the poll....haaaaaaaaaaaa....way more fun to watch....

The Squirrels will then climb the deck and try to jump across to the bird feeders..

Usually good for lottssa laughs..
Nasty woodpecker on occasion will rip the squirrel a new one.....

[Yukon6.2]

Find an old clay pigeon thrower...
Mount it on your deck railing so it dosn't fall off,then with a piece of whatever mounted to the clay pigeon holder to make a 90,place your favorite squirrel bait.
Wait for a squirrel to find the bait and PULL
Film it for UTube it will get a lot of hits

[DmaxMaverick]

Skeet traps need to be mounted SOLID. Not just "screwed to the rail". It will rip the lumber right off in a hurry. That squirrel slinger idea is already on Youtube, somewhere. Saw that years ago. Funnier than any cat vid. Ever. A home-made catapult would probably be a better idea, and it can be rigged with a trigger so you don't have to watch it every second. Leave a baby monitor camera or cheap dash-cam on it. I have ZERO love for (most) rodents, if you can't tell. You can catch/release all you want. They're buzzard (condor) and coyote bait, by whatever means, around here.

[Checkov]

Update, went to my elderly parent to help them this weekend then my youngest grandkids (what a difference) and just now back to it. I bought a new Relay and installed it with no effect.

so if you recall I determined it was no power to lift pump and already put in a new OPS with no effect. So my plan is:
1. Follow the wire up from the lift pump as far as I can and look for damage...if nothing—>
2. Take off the intake manifold and look for damage near the mouse nest (why did GM engineers design a little perfect mouse cavern under the intake?????—ive already had it taken off once where they used it as a palace and chewed though a fuel line). If nothing—>
3. Wire up the $4 toggle switch I bought with the relay at NAPA from my battery to the lift pump and run it that way this summer then in the fall when I quite doing my projects take it to Jacks.

by the way who ever thought of using a skeet puller to launch rodents is genius.

[Checkov]

The more I look the more I think I’m skipping step 2 and going to 3. It looks like a job and I will have to probably order a new gasket etc. I’m thinking this is a winter job or shop. Got some projects to use the truck for and could take days for this......

[DmaxMaverick]

That sounds like a workable plan. If it comes down to the switched power option, even if temporary, please use a relay through the OPS to power the switch. The relay doesn't have to be fancy or expensive. A $10 Ford or garden tractor starter relay/solenoid will do just fine. Power the switch to the relay through the OPS, and Batt+ through the solenoid to the lift pump. The reason for this is, if for any reason the engine stops running (such as in a crash), the lift pump will stop pumping. The alternative is to pump fuel on a potential vehicle fire. You may not be able to hit the switch, for any number of reasons. This doesn't have to be a temporary solution, if you don't find the root cause.

[Checkov]

Thank you for the help!!!! But the plot thickens....as the saying goes “hold the papers and stop the presses!”.

I think I have a bad electric fuel pump again and probably a short to boot. So bear with me and I will try to itemize what happened:

1. I Wired a toggle switch to a terminal near the battery and tested and was getting a good 12 volts or so.

2. Just like I did last week, ran that positive wire from the toggle switch to the fuel pump that got the vehicle running so well last week....SILENCE. No fuel no start.

3. So I reasoned it has a short so bad the electric charge is finding a way back to ground and refusing to go through the motor. So I CUT the old factory positive wire going to the pump and wired the hot wire directly to the pump...still SILENCE.

4. OK. It has to be a bad ground wire? So I grind a fresh spot around a bolt hole in the frame and run a good wire to that and bolt it to ground. I then test the gap between the good hot wire from the battery and the new ground wire...shows a strong 12 or better voltage. So I have a new good ground wire and new good hot wire.

5. I even cut the ground wire to the motor so I can eliminate any possibility of...whatever...and directly wire the known good ground and hot wire to the motor...SILENCE.

So it has to be a failing motor correct? I’m guessing it was intermittently working and I had bad luck the other day when it ran (throwing me off the true issue). So I suppose the next step is a new fuel lift pump? I mean it has to be that correct? I think it’s possible I also have a mouse chewed wire because I can’t find voltage in the old factory wires that feed to the motor. However even to my very un-Tesla understanding of electricity if a DC motor is getting proper charge and ground it has no choice but to work unless it’s broke.

Dmax please let me know your opinion (or anyone else with some confidence) and I’m going to wait before I buy the new motor.

[Checkov]

Also just read the story Robyn on the rat..wow. Live rat in a turbo, got to be a first.

[DmaxMaverick]

Stranger things have happened. If you have good ground and good Batt+ voltage to the pump, and it doesn't pump, it's a bad pump. They are simple, meaning, apply power and they just work, or they don't. No magic involved. A failing/failed pump can also overload the circuit during the process, so recheck the relay and fuses. Before dropping the change on a new pump, check it again just as before, but this time, check the voltage on the circuit WHILE it should be pumping. If the voltage drops, the circuit won't carry the load, or the pump is internally shorted. If you have any indication of either, proceed accordingly.

[Checkov]

Issue resolved ������.

It WAS the fuel lift pump. I put a new one in and didn’t even need the wiring work. If anyone comes across this for future references here are a few points to consider:
1. The pump does not have to completely and immediately die, mine worked intermittently then finally failed. This put quite a bit of error in my problem solving as since it ran on one test I assumed it was good then went to the wiring as suspect. So I would test it multiple times and for an extended run.
2. On testing the charge from the battery I skipped over and neglected a small point made by Dmax. The fuel pump will only run for 4-5 seconds then shuts off power. I tested the wires from the engine and found no charge so assumed I had a short. In reality the key was on and it had been well over 5 seconds so the system shut off charge to it. That didn’t cost me much cash (yet) but it did send me off on a wrong tangent costing a few hours of work and reading fruitlessly.

speaking of cash here is what it cost:
new relay $20
new fuel filter $35
new toggle switch and assorted wire and clips $5
new pump $60

A total about 120 all done, some of which was unnecessary due to my errors listed. All in all I feel pleased.

Thanks for the help and I will probably be back eventually, hopefully not soon.

[DmaxMaverick]

That's fantastic! We'll leave the light on for ya.

[ronniejoe]

Your broken OPS was probably still working, despite its appearance. It doesn't control the lift pump on OBD II trucks (1996+). The PCM will have a tantrum if it's missing, though.

The oil pressure switch does control the lift pump on OBD2 trucks. The circuit diagrams are very similar from OBD1 to OBD2. In short, both the fuel pump relay and the oil pressure switch must be in working order for the lift pump system to function properly.

If you need the circuit diagram, a subscription to AlldataDIY.com is only about $15 for a year. They have the same information that the GM service manuals have.