96 Tahoe 5.7 leaving me stumped.

[Montanaredneck406]

Hey everyone, my name is Arron & I’ve got a 1996 Tahoe LS w/the 5.7 vortec (4x4)
. I can’t seem to figure out what the issue is. Hopefully y’all can help.


So a few months ago I started to notice that when I would step on it to pass on the hwy it would downshift but not really accelerate until I backed off the throttle a bit. A day or two after it started not firing right away when I’d try to start it in the morning but if I turn the key & let the fuel pump cycle 2 or 3 times it’d fire up the rest of the day right away. Few days later I was hauling a load of firewood and went to get into the throttle and it sounds like something was clogged up. erratic throttle response, loss of power & sounded like backfiring in the intake manifold. Next day I have to prime w/ a shot of gas into throttle body to get it to start, it’ll run somewhat ok for the first half mile then it’ll lose almost all power, bogging down hard & maxing out at maybe 50mph afterwards if I stop and put into park it will idle but die when I shift into gear. ( then I have to prime it again before it will fire). Also if I turn the hvac On it starts running even worse. So far I’ve replaced distributor cap & rotor with high performance setup. Did plugs, wires, air filter, fuel filter, TPS sensor, map sensor, ignition control module new alternator, removed catalytic converter & muffler (ran straight pipes with glass oaksthat dump out right before back tires) none of these have worked. I just romved spider injectors to inspect all lines, none had any holes or cracks. No visible damage to unit or lines. Put intake manifold back on& now it won’t start unless I prime it and won’t stay running after for more than a few seconds. Can hear Fuel pump kick on and ive me got pressure in the line. If anyone can give me the slightest clue please let me know! Thanks in advance

-Arron

 

[strutaeng]

You need to hook up a scanner that reads live data and start looking at things like fuel trims, O2 waveform, sensors data, etc.

It could be that it's running rich or lean (I've seen lean AND rich at the same time from leaking FPR dumping fuel inside the intake plenum.) Also, hook up a fuel pressure gauge and see what the pressure is a prime, then note if the fuel pressure holds for 5-10 minutes. Injector balance test would also reveal if an injector is stuck open or stuck close (which injector.)

Without these test, it's going to be really difficult to fix the problem.

 

[B-train]

The first thing I'd look at is fuel pressure. The fact you were key cycling it 2 or 3 times which then degraded into giving it a shot of fuel to start leads me to believe you need to look there.
1. Check fuel pressure as stated by @strutaeng
2. Replace the fuel filter on the frame rail. Your scenario also could be some bad fuel that slowly plugged up the filter. This is a cheap and easy try before dropping the tank.
3. Last resort, drop the tank and inspect/replace fuel pump. These were known for eating pumps back in the day. Especially if they were over worked or continually run low in fuel and got hot.

 

[OR VietVet]

Welcome to the forum from Oregon.

I see you did replace the fuel filter but the existing fuel in the tank could be contaminated due to crud build up and clogged again. Yes, check the fuel pressure with cold engine, KOEO and then shut off and see if pressure bleeds off right away and then watch while cranking and again get pressure when idling. Watch the fuel pressure while driving. All your scenarios sound like a fuel delivery problem.

You removed the spider injectors and did not just replace while it was apart? There are updated spiders converting to MFI:

More Information for ACDELCO 2173245

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

 

[Eman85]

"Can hear Fuel pump kick on and ive me got pressure in the line. If anyone can give me the slightest clue please let me know! Thanks in advance"


You need a fuel pressure gauge! Get a gauge and take an actual fuel pressure reading. Right from the first clue of cycling the key the diagnostic Ouija board was heading toward fuel pump.

 

[DaveO9]

Yep, as soon as I started reading the first post I immediately started gravitating to fuel pump. Check pressure as others have said.

I also second @OR VietVet in replacing the spider with the upgraded one if you haven't already done so. The original (1996-2001) had the injectors centrally located in the "body" of the spider with poppet valves in each of the 8 legs. Very problematic. GM redesigned and 2001 on used a spider that eliminated the poppet valves and has the actual injectors in the legs. The redesigned unit was manufactured by Delphi and it is a direct replacement for the old design. I'm not sure if you can still get the OEM Delphi units new. There are tons of Chinese repops on Amazon/eBay, but I'd be wary of those. Many, many trucks with the old design were converted - I did some pick-n-pull hunting and found a couple trucks that had the upgrade. I had grandiose plans to convert my '91 TBI suburban to the upgraded spider system - had all the parts including one of those junkyard spiders, but then I sold the truck. (and sold all the stuff)

Motor Man | GM Vortec Spider Conversion CSFI to MFI | 4.3L 5.0L 5.7L Vortec

Motor Man Fuel Injection offers OE CSFI to MFI conversion Spider Injector systems for 1996 and up GM 4.3L 5.0L 5.7L Vortec engines

www.motormanfuelinjection.com

Come to think of it, the spider could be at least part of your problem. I know some of the symptoms you are experiencing can be caused by it.