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Help diagnosing, truck stalls

Ok just got back home from working out of town, rented a fuel pressure tester and the first thing I am noticing is when I key on and prime the system it rises to 12 psi but slowly goes back down to 0 within about 30-45 seconds.

About to take it out on a drive and see if I can recreate the issue. Holds 12 psi steady while at idle
 
thats prime shot , it does that with key on position but not running correct
 
Yes key on it does it, running it holds 11-12 psi.

It die out on me while it was warming up in the driveway that’s the first time that has happened. When I went to crank it back up it showed no psi and did not start. On the second key cycle it started and ran fine, just drove it 10 miles with no issues now I am going to let it sit for about 15 minutes and see what happens.
 
sounds like either pump is going out or pressure regulator if it has one
 
Ok I got it to do it a couple times again. It’s not loosing fuel pressure and then cutting off it cuts off and then a second later the fuel pressure fades away.

Is this getting me any closer to a solution ?
 
Also once while cranking it cranked over for about 5 seconds before starting and while cranking I saw the fuel pressure go from 10 to almost zero then back up to 10 and it started.
 
Could be a corroded power wire that runs to the fuel pump or the connector where the harness plugs in near the tank & pump...often they get corroded and not enough voltage can get to the pump,it may have 12V at first but after a while the wire or poor connection at the plug heats up and the voltage drops to the point the pump cant get enough amps to work properly and the pressure drops or the pump just stops temporarily..

Using a test light or multi-meter to check voltage at the pump wire isn't enough,you need to use something that draws enough amps to see if the wire will heat up and drop the voltage to the pump--something like a sealed beam headlight bulb or a blower motor makes a good tester--leave it connected several minutes and watch to see if the light dims,or the motor slows down,sometimes after 2-3 minutes the power is lost completely,and the wire will magically "fix itself" when it cools off inside,and the bad spot with much corrosion or just a few strands left touch again and let power go to the pump again..

Many new pumps get installed and do not cure the problem due to this..also make sure the ground wires are good, and are tight.

Many new in tank fuel pumps come with a new harness connector and the warranty is void if you fail to use it and check the wire for voltage drop..often a new wire will need to be run all the way back to the fuse box where the original wire was...

Other things like a failing fuel pump relay or oil pressure switch can cause the pump to not get enough voltage,or intermittently also...the pump itself could be failing too,they don't seem to last that long ,especially the "bargain brands"...the hose connecting the pump to the fuel feed lines in the tank often fail from ethanol too,the hose needs to be a special type to not swell up and split..
 
Second the wiring. Double check all of it, including the connections at the relay on the firewall, and the oil pressure switch which I think on your rig might be at the top back of the engine? Look over every wire, connector and hose at the same time. Look for bare wires, cracked wiring, broken connectors, cracked hoses, disconnected items, etc.

Run 12V directly to the pump, watch fuel pressure, and take it for a drive. You can run a wire from battery + (or junction block on the firewall) to the red wire dangling off the fuel pump relay. Even if the engine dies the pump should still run IF its not the pump.

If you had no fuel pressure and it wouldn't start, either the pump circuit is messed up, OR the ECM is not getting a reference signal that the engine is cranking over/running (which comes from the distributor module IIRC). As I said earlier, it takes about 15 seconds after the key is off, for the ECM to trigger the relay to prime the fuel system. Engine off, wait ~15 seconds, key to *run*, should prime.

If you can get it to not start again, pull off a plug wire and check for spark. If you aren't getting spark, it's ignition obviously. If you are, could still be ignition, but less likely.
 
I have seen cases where the ignition module controls both the spark & firing of the fuel injectors have one or both functions "come & go" intermittently too..
A late friend had a 90's GMC that refused to fire up every morning unless it was given a snort of ether--once it fired up and ran,it ran flawlessly the rest of the day,or until it sat long enough to get completely cooled off..(like 5 pm after work! )...

He tried replacing a lot of things and finally tried another new ignition module ,despite the other being recently purchased and didn't seem to make a difference..the truck started right up and ran "right" finally and gave no grief until he passed away over 2 years later..

Another common issue I'm told on 2000 and newer trucks ,have a short wiring harness that has some sort of resistor that fails ,I think its attached to the gas pedal...and it makes the engine not want to fire at colder temps when it goes sour --a guy I know lived with his 2000 chevy truck with a 4.8 not starting just about every cold winter morning or having to crank the snot out of it and keep cycling the key ,if you were lucky it would fire up suddenly ..he too kept a can of ether in the truck for when it refused to fire on its own..

He replaced the fuel pump,sending unit,fuel pump relay,oil pressure switch,inspected all the wires,found no damage or broken connections--nothing helped..it never set a code either..he was getting angry enough to junk the truck after it acting up every morning or night after work..

One day at a dealer,he asked the parts guy "what can cause this ?"- parts guy told him "You probably need this harness,its $35",he said "let me have it,I have already blown a few hundred on parts and my labor and still cant trust it..whats another $35?"..
He installed the harness and that cured it..:ooo:
 
So here is my update.

Changed out the fuel pump and filter, seemed to make a slight difference but still stalled.

Rebuilt the TBI and replaced the IAC with a new one i had sitting around for my Grand National, this seems to have done the trick has not stalled on me yet but came close a few times when pulling up to a stop. I think the issue is the idle is set too low, its at 500 in gear at a stop with the ac on ,with the ac off its about 600. How do i adjust the idle speed ? I want it to be around 650 ac on in gear at a stoplight.
 
Theoretically you do not adjust idle speed. Make sure it's within the specs on the core support sticker. I believe that still shows idle RPM. Any fudging you try to do with the idle RPM, the ECM is going to fight, because it has a target RPM coded into it...if it isn't at that RPM, it will start adjusting to try and make it that RPM.

However... https://www.thirdgen.org/forums/tbi/605542-iac-reset-minimum-idle.html#post4841988

Simply adjusting the throttle stop is not the fix, follow the procedure outlined above.
 
Have you tried spraying carb cleaner around all your hoses and intake area to check for leaks? Idle will increase slightly if you spray a spot that’s bad.
 
Didnt spray but have visually checked. I will get the spray out soon.

Its only seeming to be a problem with the ac on, is the rmp supposed to step up a bit once you turn on the ac ?

I really do appreciate all the help you guys are giving me, seems like the most simple issues but they are giving me a run for my money lol
 
IMO, if the RPM stays the same with the AC on, it's probably doing it's job. With my 73 carb'd truck, the idle drops 100RPM with the AC on.

What happens to battery voltage when the AC comes on?
 
I know a guy that was having similar trouble on a trail run and turned out to be a loose battery cable that eventually shorted out on the frame. It was killing power to the truck intermittently. You'd know if this was going on because you'd lose all electrical.

Same guy had a random no start, don't remember now if it would stall on him, just remember for sure there was a no start issue. It ended up being 1 lose wire connection in the TBI harness. Took him forever tracking that down and kinda found it by accident.

If you get the truck running you could try moving around the wire harnesses under the hood and see if moving any of them cause the truck to stall. If you can get it to happen consistently you might be able to track down the culprit.
 
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