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Discussion Starter #1
I've got a 1996 GMC with a vortec 383, I was driving down the road last night at about 60 then it just started back firing and popping out the exhaust. All the guages are good and there is no engine light on. I stopped and looked at everything and all looks good and when it ws idling it sounded almost like normal. I limped it about a half mile from home and it stalled and would not start back up. When I as limping it home I could not get it over about 20 and it sounded like popcorn popping the whole time and would barely make it up the hills. It strange because all looks good and no engine light. I was going to pull some plugs today to see what they look like but what else can I check??

Thanks for any advice
 

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Does your check engine light work? You may have codes but the bulb is burned out. It should come on when you turn the key to On without starting the engine. It's hard to imagine those symptoms and no code.
 

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Yeah the light works, it comes on when I have the key 'on'. I know its very strange it seems like an electrical/sensor/ignition problem but there is no code. I'm realy not too sure where to start looking. I've been gone all weekend and will hopefully have some time this week to get into it.
 

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JUGS Racing
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Check for fuel and spark. For fuel you could spray some brake clean down the throttle body and crank it. If it fires then its fuel/crank position sensor related.
 

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A bad crank position sensor wont allow for spark

Dump a tad of fuel down the t-body and if it fires it points towards a bad pump.

Fuel pressure needs to be over 55 psi, most Vortecs run at 60-66 psi with Key On Engine Off(KOEO). If your fuel system bleeds down, be sure to check fuel pressure whilst the pump is running,(only runs for 2 seconds once the key is hit). If you measure fuel pressure at Key On Engine Running (KOER) the FPR will bring down fuel pressure to around 55-58 psi or so, and a quick stab of the throttle will spike the fuel pressure.

You wont necessarily get a SES light with a bad pump.
Fuel
spark
Air

peace
Hog
 

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Discussion Starter #6
Thanks for the advice guys. I put a fuel pump in it a few years ago, I could not hear it when I turned the key on. I can hear the pump now when I turn the key on but maybe there is not enough pressure. Do I have to buy a fuel pressure gauge to check pressure or is there a way to do it with a oil pressure guage tha I have laying around?
 

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You may be able to get a free loaner from a parts store.

Harbor Freight sells a good one for $20. It doesn't come with adapters to connect it to GM TBI systems...I doubt yours is TBI but I was trying to look up your fuel pressure specification (sorry, couldn't find it) and TBI was listed. I thought TBI was gone by 1996.
 

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TBI was gone for 1996 in FS GM trucks, it will be CSFI and the spec again is 60-66 psi.

peace
Hog
 

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Sounds like you broke or skipped your timing chain. Had a similar thing happen on an 87 350 TBI. Check your air, fuel, and spark first though.

Do you have cats? Ive also heard of similar issues when a cat collapsed and completely blocked the exhaust on a Wrangler.
 

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Discussion Starter #10
Well like I said I can hear the pump come on when I turn the key on. Should it turn on everytime I cycle the key?? I didn't get a fuel pressure gauge but I did push down the 'bleeder' on the fuel rail and it doesn't seem like it has enough pressure. Shouldn't it really spray out of there with some force?? It really just kind of flows or bubbles out. With the key on when I am pushing the 'bleeder' down shouldn't the pump kick back on to keep pressure in the system?

Also when trying to start it up it never really fires but just pops and backfires out of the exhaust.

Thanks
 

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Yes every time you turn the key from off to on it should prime the system. If you turned the key off before you checked the pressure, then no it shouldnt spray out (at least not gush out). Even if it comes spraying out, it doesnt tell you if you have a problem or not. Your truck needs a very specific fuel pressure in order to start. Itll be higher for starting and once it starts itll come down a little and hold there. If youre off by a few psi, it may not start.

Not sure what pressure an early vortec needs...TBIs were like 7-8psi, TPIs were in the 30s I think, and the current vortecs are in the 50-60 range. I would assume yours is the last.
 

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^What he said.

Also, if you lose pressure while the key is On but the engine is not running, it does NOT attempt to repressurize by running the pump again. It primes when you turn to On just one time and doesn't run the pump again until the engine is running.
 

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^What he said.

Also, if you lose pressure while the key is On but the engine is not running, it does NOT attempt to repressurize by running the pump again. It primes when you turn to On just one time and doesn't run the pump again until the engine is running.
Good point. Another thing to note is that if the check valve in the pump is shot it may take a few key cycles to build pressure. A good pumping system should build pressure almost immediately. The check valve is what prevents fuel from draining back into the tank when the engine is off. Can make for long hard starts.
 

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Yup, I have a bad check valve and have been living with it for years by priming it 2 or 3 times before starting.
 

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Thats not true, it SHOULD NOT prime EVRY SINGLE TIME YOU cycle the key. All my L31's DONT prime everytime. If you the key in the RUN position and then turn the crank with your hands at the balancer the pump will cycle twice per crank revolution.

Just because the pump is running doesnt mean its making 60-66psi of pressure, I have had pumps make 60psi at idle, then not push enough fuel during acceleration.

Spend the $20 a get a fuel pressure gauge.

Did you try dumping soem fuel down the throttle body? If it fires then you know you need a pump.

peace
Hog
 

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I've never heard of a GM that doesn't prime every time you turn it from Off to On. If the pressure is high enough the pump still runs, but the regulator sends the additional fuel back.
 
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