93 GMC Sierra 5.7L - Sputter and backfiring under load

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jt25
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Joined: Wed Oct 15, 2014 1:59 pm

93 GMC Sierra 5.7L - Sputter and backfiring under load

Unread post by jt25 » Wed Oct 15, 2014 2:04 pm

I have a 93 Sierra 1500 with the 5.7L V8 and it runs and idles find in neutral or park and when it is cold. When in gear under any load on the engine and once it gets barely warmed up the engine will start to sputter and backfire intermittently. The RPM's jump up and down by a couple hundred as you accelerate and then sometimes the engine drops out completely and even back fires before immediately regaining power. Sometimes it just seems as if it is starving for fuel.

But as soon as you put it in Nuetral the engine revs just fine!

I have no clue where to start. I can replace parts but diagnostics are not my thing.Please HELP!

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carriedi
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Joined: Wed Apr 10, 2013 12:42 pm

Re: 93 GMC Sierra 5.7L - Sputter and backfiring under load

Unread post by carriedi » Fri Oct 17, 2014 12:06 am

you should scan the engine and see what the fuel is doing and look at your coolant temp sensor too. With a scanner on the truck and watching your data stream you can see if you're running lean or rich. Or if the fuel system looks good and maybe it's an ignition problem.
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hardonthagas
Posts: 2
Joined: Tue Jun 16, 2015 6:04 am

GMC new fuel pump still wont crank

Unread post by hardonthagas » Mon Jun 22, 2015 5:11 am

I have a 1998 GMC Z71 the truck was running good. Got to the lake and killed it and it never cranked again. Fuel pump would not run so I replaced fuel pump brand new. Put it in and the pump came on now I have plenty of fuel to the pressure release valve and still won't crank. I replaced throttle position Sensor the MAP Sensor and it still won't crank. Truck is getting fire and it will run if I pour gas in it at the throttle body and will stay running till it runs dry. Anyone have any idea what is wrong.

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carriedi
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Joined: Wed Apr 10, 2013 12:42 pm

Re: Z71 GMC new fuel pump still wont crank

Unread post by carriedi » Mon Jun 22, 2015 9:47 pm

ok, cranks means turns over. cranking, turnover, is when the starter engages the engine and rotates the crankshaft. What you have is: the engine cranks but doesn't start.

if the engine runs with adding fuel manually then you only have to look at fuel pressure and injector pulse. fuel pressure should be 60-66 psi. a new fuel pump should give you that if everything else is good. Now, what about your injector pulse? That's the computer sending the signal for the injector to open and let fuel into the engine. to check the injector pulse is a little tough since your injector are not easy to get at. you will have to check the wiring that goes to the injectors. All the injectors have a battery voltage power wire that is pink. That battery voltage comes from the ECM1 fuse (20A) in the underhood fuse box. If you have battery voltage from the pink wire then that's good. T fire the injectors the computer grounds the circuit.
Injector1 black
Injector2 light green/black
Injector3 pink/black
Injector4 light blue/black
Injector5 black/white
Injector6 yellow/black
Injector7 red/black
Injector8 dark blue/black

you need to know how to check for the computer ground signal. A logic probe or an LED test light, some kind of lighting device that will use less current that the injector, in series with the injector being tested.

No injector pulse but the battery power side stays up at battery voltage, you may have a passlock problem where the antitheft turns off the injectors. What is your security light doing? Would have to scan the passlock for codes and/or look at the resistance/voltage coming back from the key.

No injector pulse but the battery voltage side goes down below battery voltage by a few volts or more, you could have a shorted injector that will rob the power from all the other injectors.
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hardonthagas
Posts: 2
Joined: Tue Jun 16, 2015 6:04 am

Re: Z71 GMC new fuel pump still wont crank

Unread post by hardonthagas » Tue Jun 23, 2015 2:39 pm

Ok will try that. Boy that sounds like a job and the security light goes off after about 3 seconds and truck has no codes at all.

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