Backfiring Vortec 4.3

[Jobey]

So I have a 96 w/t c1500 4.3 5 spd. I’ve owned this truck for almost 2 years now and put right out 100,000 miles on it. Gradually over time it started getting a high idle as I was slowing down but as soon as I come to a complete stop it will drop down to normal. It would rev up slightly then die back down to normal when it starts up as well,which I always thought was basically normal. Well, this morning when I fired it up it was idling at 5-600. After it warms up it would be maybe 650. Also whenever I was taking off it would backfire slightly between shifts and also when downshifting. Does anyone know what might be the cause of this? It’s never backfired before.

 

[Schurkey]

How old are the usual "tune-up" items--distributor cap, rotor, plug wires, plugs? Air and fuel filters? Timing checked to see if it advances? PCV, EGR checked for operation?

What is the fuel pressure?

Connect a scan tool, look at the data stream in addition to checking for "codes". Verify ALL the sensors, check the fuel trims, verify the IAC. Look at the misfire counts for each individual cylinder.

Intake gaskets have been a problem on the Vortec V6 and small-blocks; as has the poppet-style injectors.

 

[Jobey]

Distributor cap, rotor, wires and plugs are a couple months old. The thing that really bugs me is that it was running perfect the night before when I got home. Then I wake up and go to work and it’s like this.

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[Schurkey]

So it ran long enough to get up to operating temperature...at 300 rpm?

 

[Jobey]

That was right after I got back from work.

 

[RichLo]

300rpm??

Thats cranking speed not running speed. Should be around 800rpm at idle.

 

[Jobey]

I know. But that’s what it idles at right now. Which is why I’m asking for help. It runs great when I’m driving but whatever failed that is making this happen also makes it backfire between shift and when downshifting. I’ve had a lot of these trucks over the years and this is a new issue for me.

 

[Schurkey]

Have you inspected the distributor gear? Checked the intake gaskets? Verified the poppet-style injectors? What is the fuel pressure?

Have you verified all the sensors with that scan tool?

 

[Erik the Awful]

know. But that’s what it idles at right now. Which is why I’m asking for help. It runs great when I’m driving but whatever failed that is making this happen also makes it backfire between shift and when downshifting. I’ve had a lot of these trucks over the years and this is a new issue for me.

Intake gaskets have been a problem on the Vortec V6 and small-blocks; as has the poppet-style injectors.

As Schurkey recommended, you need to check that intake gasket for vacuum leaks.

 

[Casey916]

S

So I have a 96 w/t c1500 4.3 5 spd. I’ve owned this truck for almost 2 years now and put right out 100,000 miles on it. Gradually over time it started getting a high idle as I was slowing down but as soon as I come to a complete stop it will drop down to normal. It would rev up slightly then die back down to normal when it starts up as well,which I always thought was basically normal. Well, this morning when I fired it up it was idling at 5-600. After it warms up it would be maybe 650. Also whenever I was taking off it would backfire slightly between shifts and also when downshifting. Does anyone know what might be the cause of this? It’s never backfired before.

ame here man i cant figure it out