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Rough Idle 5.7 Vortec

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Hello everyone, Im new to the forum. I have a 1996 GMC K1500 5.7 Vortec 180,000K. My problem is it has a rough idle and it is hard to start. I have to let the pump run 30 sec before I start it. Off idle and during acceleration it runs fine. When I drive the truck the engine light will go on and off. I scanned it and came up with P0300, P0172, P0175 ans P0141 codes.

 

Recently I changed plugs, new dist cap/rotor, wires, fuel filter, air filter and battery.

 

Checked fuel pressure: keyon/not running 62psi then drops to 20psi after 3 seconds. Truck runniing at idle 55psi and @1500rpm's 60 psi.

Checked for vaccum leaks and fuel leaks non found.

 

What should my next step be in diognosing this problem.

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if your fuel pressure is only 55 psi at idle then this will cause the engine to run rough and cause the random misfire P0300 code

 

with the fuel pressure dropping within seconds of turning the ignition on, you will have a very difficult time starting this thing... if it does at all

 

the Vortec fuel injectors need 61+ psi at all times for the poppet valves at the tip of each injector to function properly

 

Based on my experience, I believe you need a new fuel pump. When you do, get an ACDelco or a Delphi... the others are just not as reliable.... and you don't want to be doing this all over again anytime soon.

 

Also, run some good injector cleaner through it.

 

Good Luck ....

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if your fuel pressure is only 55 psi at idle then this will cause the engine to run rough and cause the random misfire P0300 code

 

with the fuel pressure dropping within seconds of turning the ignition on, you will have a very difficult time starting this thing... if it does at all

 

the Vortec fuel injectors need 61+ psi at all times for the poppet valves at the tip of each injector to function properly

 

Based on my experience, I believe you need a new fuel pump. When you do, get an ACDelco or a Delphi... the others are just not as reliable.... and you don't want to be doing this all over again anytime soon.

 

Also, run some good injector cleaner through it.

 

Good Luck ....

+1...replace your Fuel pressure regulator too.

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Is there any test I can perform to check the fuel pressure regulator.

Pull the vacuum line off and if there is fuel present its toast. That's the usual sign, as the diaphragm fails and fuel leaks through. But thats not the only way they can fail...they can also get weak...no real easy test for that...

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Ok, pressure regulator ok, but Im confused thee truck starts and runs fine I drive it everyday, but the fuel pressure at idle is 55psi and not running 62psi then drops with rough idle. Should I change the fuel filter and spend big bucks or is there elswere I should investigate. Plugged CAT, o2 sensor??

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Ok, pressure regulator ok, but Im confused thee truck starts and runs fine I drive it everyday, but the fuel pressure at idle is 55psi and not running 62psi then drops with rough idle. Should I change the fuel filter and spend big bucks or is there elswere I should investigate. Plugged CAT, o2 sensor??

Not without a code pointing you in that direction.

 

Try running some seafoam...a full bottle in the tank.....you may be getting sticky injectors.

 

Its a cheap thing to try, and probably your last resort before replacing the pump.

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Replace the spider injector(fuel injector assembly) they are known to go bad. The poppet valves stick and leak down. You can run all the injector cleaner you want throught them but unfortunately it won't work. It's best to replace as an assembly especially since you are getting a P0300 which is random misfire and not a specific cylinder. There are 2 choices for the assembly in the aftermarket (autozone, advance auto parts, etc.) One is the original replacement with the poppet valves and the other (i strongly recommend this) is one that got rid of the poppet valves and put electrical valves that don't rely on pressure to open. This is also the one that you would get if you go to a GM dealership because it is an updated part. This is an expensive part but is well worth it. The good news is that both assemblies come with new fuel pressure regulators! It is possible to get just the runner for the poppet valve style instead of the assembly but like i said you don't have a specific code so it is tough to say which one or ones to get. This should also take care of the code P0172 as well. Hope this info helps, i have done this a few times, in the shop and on my dad's truck. Good luck!

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I tried Seafoam, no luck. Im finally getting the chance to take this truck out of service and pull the upper intake. Also have a new fuel pump. I'll let you know how it comes out

 

Thanks

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Replace the spider injector(fuel injector assembly) they are known to go bad. The poppet valves stick and leak down. You can run all the injector cleaner you want throught them but unfortunately it won't work. It's best to replace as an assembly especially since you are getting a P0300 which is random misfire and not a specific cylinder. There are 2 choices for the assembly in the aftermarket (autozone, advance auto parts, etc.) One is the original replacement with the poppet valves and the other (i strongly recommend this) is one that got rid of the poppet valves and put electrical valves that don't rely on pressure to open. This is also the one that you would get if you go to a GM dealership because it is an updated part. This is an expensive part but is well worth it. The good news is that both assemblies come with new fuel pressure regulators! It is possible to get just the runner for the poppet valve style instead of the assembly but like i said you don't have a specific code so it is tough to say which one or ones to get. This should also take care of the code P0172 as well. Hope this info helps, i have done this a few times, in the shop and on my dad's truck. Good luck!

 

 

Do NOT do this until after you have done the pump, your problem is pressure related thus far, not at the injection point.

Throwing parts at it will not help, testing the pressure as you hav done clearly shows the pump is not supplying enough pressure.

Cheers

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Thanks johnnyquick

 

Right now Im not ready to push out hundreds of local greenbacks for this truck its my daily work truck and it runs fine with the one exception of rough idle, The SIL comes on and goes off at random intervals . Service Tech at local GM dealer recommends same. He say's the truck won't idle right or run at all if the pressure isn't between 60-65 PSI. Im trying a new pump this weekend.

 

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