1997 K2500 454 Rough Idle, Loud Afterfire

[Manmeyco]

I have a '97 K2500 with the 454. Simply put, it doesn't idle right. When I start it up cold, it revs up to 2000 RPM, slowly settles to idle, and then either abruptly dies or practically dies before roaring right back up and going through it again. In order to keep it on I hold the gas slightly or rev it continuously. It's more likely to stall when cold, but it can happen pretty much anytime it idles. If I let it idle, and then put a small load on the engine (steer or jam brakes or put it into gear) without revving it'll also die, so making K turns is a nightmare.
2nd problem is the afterfire. It makes incredibly loud pops out the tailpipe irregularly - sometimes it'll just keep on popping, sometimes nothing and then a loud bang. The pops don't sound good and regular, but are completely random and never "in sync" with the engine's sound. It's legitimately so loud sometimes it hurts my ears and wakes up my family in the morning. The afterfire is a lot more frequent when cold, but sometimes happens when it's warm. Sometimes holding the engine at 1500 RPM is the only way to get it to stop popping, sometimes that's the only speed at which it'll pop. Sometimes it even feels like the engine is getting robbed of power until the pops stop - I'll put the pedal down, and as soon as the pops stop, the truck gets an extra 'oomph'. Might just be a perfectly timed upshift, but maybe not.

For some background, I replaced the head gaskets, intake gaskets, & exhaust gaskets. After that fix, it didn't run right, and my mechanic adjusted the distributor and tightened the pushrod rocker bolts. After that, it ran great for a few days, until the pops started, and it's been getting worse and worse (might just be winter rolling in?). My tailpipes have holes everywhere, and the flange where the front exhaust pipe meets the rear (under the cab) has a good 1/4" gap. My EGR hose is disconnected - when I took off the intake manifold I had to lift the whole thing up and rotate it to free it, and I forgot to put it back that way. I used to have the hose stuck into the port where it should go and wrapped in aluminum tape, but it didn't hold well and the engine was quite hard to start. So I taped off the EGR hose, as well as the port on the intake, and now it starts better (until it dies

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). Also note that I do not have any AIR or smog system.
I've heard elsewhere that exhaust has enough fuel to burn, but due to a lack of oxygen it doesn't, so could it just be the holes? I don't want to get a new set of tailpipes and then have them blow apart because it wasn't the problem.
Any clues as to what might be the issue(s)?
Sorry for the long post, just wanted to make sure I got the whole picture laid out.

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[Mangonesailor]

Have you checked compression? Have you tried checking for vacuum leaks with the IAC valve disconnected? Is it running rich or lean at idle?

Why did you replace the head and intake gaskets in the first place?

 

[Manmeyco]

Have you checked compression?

I myself did not, but after my mechanic fixed the valves he said it had good compression.

Have you tried checking for vacuum leaks with the IAC valve disconnected?

No, but when I unplugged it the engine idled (actually quite nicely) at 2000 RPM. The EGR port is just taped over, though, so that's probably leaking a bit.

Is it running rich or lean at idle?

Not sure. How can I check?

Why did you replace the head and intake gaskets in the first place?

Coolant was leaking into the cylinder through a crack in the head gasket, and once I was in there I replaced all the gaskets I had removed.

 

[Mangonesailor]

I myself did not, but after my mechanic fixed the valves he said it had good compression.

We can only assume that all cyl are even then. Hopefully,

No, but when I unplugged it the engine idled (actually quite nicely) at 2000 RPM. The EGR port is just taped over, though, so that's probably leaking a bit.

Yeah, you need to make a block-off plate or something. OR, you can use a ball bearing ball without the tube and tighten the nut against it to seal the port.

Not sure. How can I check?

By smell.

Coolant was leaking into the cylinder through a crack in the head gasket, and once I was in there I replaced all the gaskets I had removed.

Are you 100% sure the vacuum reference hose in the back of the intake is connected to the fuel pressure regulator and the intake? Doesn't have a hole in it?

 

[Manmeyco]

By smell.

The exhaust smells a bit like gas on a cold start. That means it's rich, right? Does that mean that my problem isn't fuel related?

Are you 100% sure the vacuum reference hose in the back of the intake is connected to the fuel pressure regulator and the intake? Doesn't have a hole in it?

I honestly don't know what you mean by vacuum reference hose, but I'm mostly sure the fuel lines in back are all good. Can't say for certain, as Chevy clearly didn't take the firewall into consideration when they jammed this massive big block into the engine bay. Anything behind the intake manifold (i.e. the distributor, ignition coil, fuel pressure regulator, etc.) is impossible to even see, let alone troubleshoot. But when I put the intake in, I did make sure to connect all the lines and such.


I also found another variable - the EVAP system. The outlet on the canister that says "air cleaner" isn't connected to anything, and the line to the purge valve isn't very tight. Going to experiment with plugging those lines tomorrow to see if it makes any difference.

 

[Erik the Awful]

Check to make sure your distributor hold-down is tight. The backfiring is a pretty good indication the timing is off, and the intermittence sounds like the distributor might be rotating on its own.

 

[Schurkey]

Is it running rich or lean at idle?

Not sure. How can I check?

By smell.

ABSOLUTELY NOT.

Lean misfire is going to smell rich.

Rich mixture that doesn't cause misfire promotes excess CO, and CO is colorless and odorless. Depending on the amount of richness/leanness, it might be reduced in the catalytic converter and have no trace by the time the gas gets to the end of the tailpipe.

Smell is NOT a reliable indicator of rich/lean.

You'd check for rich/lean by looking at the O2 sensor voltage. This is also fallible, because misfire for any reason will provide a false lean indication.

Fix the EGR
Fix the exhaust leaks from the converter forward.
See what happens.

"I" would connect a scan tool to look at short- and long-term fuel trim, O2 voltage and cross counts, coolant temperature, and anything else that caught my attention. A fuel pressure check would not be out-of-line, either. But we already know there's problems with the EGR and the exhaust system.

 

[7Brandon07]

I had a Silverado with a 350 it wouldn’t idle on its own or it would die you’d have to drive one foot on gas one on brake if you wanted to drive it. I changed the distributor ran fine ever since, the timing or vacuum leaks is something I’d consider investigating just my two cents

 

[Mangonesailor]

ABSOLUTELY NOT.

Lean misfire is going to smell rich.

Yeah, no. Lean and rich smell totally different. I've tuned enough carburetors to know the difference. If he does then he can tell.

 

[Erik the Awful]

Yeah, no. Lean and rich smell totally different. I've tuned enough carburetors to know the difference. If he does then he can tell.

I agree with Shurkey. Lean misfire smells like rich because the fuel doesn't get burnt. Had this happen just this weekend when our race car wasn't running right.

"It smells like fuel. It's way rich."

I pulled the plugs and saw the electrodes were coated white. "Nope, it's lean."

"Nope, it's rich. You can smell the fuel."

Pulled the data stream and it's way lean.

"Huh. It's lean. Why does it smell like fuel?"

"Because it's too lean to support combustion."

Blank stares.