Need help with exhaust questions on 1990 K3500 7.4L

[William Keiter]

So I picked up a 1990 K3500 4x4 with a 7.4L 454 engine last week and am looking it over when I notice it doesn't have a catalytic convertor installed and I live in an Utah county with smog testing. I have an appointment a a local exhaust shop and need to now what I should do for the meantime to pass smog and get it registered.

I have not had it but a week or so and don't know where I will be taking it long term engine and exhaust wise. Just looking for suggestions. Sounds like o2 sensor was replaced about a year ago.

 

[studigggs]

https://www.dmv.org/ut-utah/smog-check.php

 

[William Keiter]

https://www.dmv.org/ut-utah/smog-check.php

I think I already checked the local county website and the DMV.org is outdated as they stopped safety inspections from what I read. Really looking for ideas other than just adding a cat so I don't forget when I am at the shop.

 

[Schurkey]

Depending on how GM certified your truck, you might be buying TWO catalysts.
My '97 has two, originally leading into a dual-inlet, dual-outlet muffler. I don't know how the '90s were certified. The GM service manual would have that info.

The shell of the new catalyst(s) will be stainless steel, but any pipes they replace will likely be "aluminized" mild steel. Aluminized steel won't last as long as the OEM pipes. It would cost extra to have new stainless steel pipes installed. Pretty-much the same deal for the muffler. The aftermarket replacements don't last as long as the originals. unless you pay extra for Stainless.

The later, Vortec 7.4L exhaust had a VERY restrictive muffler. I don't know about the earlier trucks. The Vortec muffler had big inlet and outlet tubes, but the internal tubes that you can't see when the muffler is installed, were very small. The "hot tip" seems to be to scrap the OEM muffler, replace it with a pair of aftermarket mufflers installed "on edge". Oval cases installed vertically, side-by-side--sort of like this: (o)(o)

Crap. My illustration looks like a pair of boobs instead of a pair of mufflers viewed from one end.

 

[William Keiter]

Depending on how GM certified your truck, you might be buying TWO catalysts.
My '97 has two, originally leading into a dual-inlet, dual-outlet muffler. I don't know how the '90s were certified. The GM service manual would have that info.

The shell of the new catalyst(s) will be stainless steel, but any pipes they replace will likely be "aluminized" mild steel. Aluminized steel won't last as long as the OEM pipes. It would cost extra to have new stainless steel pipes installed. Pretty-much the same deal for the muffler. The aftermarket replacements don't last as long as the originals. unless you pay extra for Stainless.

The later, Vortec 7.4L exhaust had a VERY restrictive muffler. I don't know about the earlier trucks. The Vortec muffler had big inlet and outlet tubes, but the internal tubes that you can't see when the muffler is installed, were very small. The "hot tip" seems to be to scrap the OEM muffler, replace it with a pair of aftermarket mufflers installed "on edge". Oval cases installed vertically, side-by-side--sort of like this: (o)(o)

Crap. My illustration looks like a pair of boobs instead of a pair of mufflers viewed from one end.

Nice illustration . I looked up under the truck and looks like previous owner went with a straight steel pipe from crossover removing cat(s) and leading to a cheap muffler and cut short ending just past the cab under the passenger side of bed.

I will see what they say about one or two cats when I am at the shop then likely see what it would cost to run new stainless pipe and add a couple mufflers as you describe above. Hoping the shop is knowledgable, just moved to the area and found a shop on yelp.

 

[b454rat]

Pretty sure even the big blocks had one cat. I had an 88 1 ton and an 89 1 ton both 454s both had only one cat. I think I even saw a 3500HD in the junkyard with only one cat.

 

[andy396]

It is a single converter and it goes *after* the muffler

You must be registered for see images attach

 

[b454rat]

weird, both of mine were before....maybe was changed out.

 

[William Keiter]

It is a single converter and it goes *after* the muffler

You must be registered for see images attach

Thanks so much, this is great! I would have never guessed the cat was behind the muffler on this system. I thought they were always before the muffler closer to the front of the exhaust. I will be sure to bring this with to the shop.

 

[andy396]

weird, both of mine were before....maybe was changed out.

That could be. I do know 96+ were all before the muffler. I’m guessing they put it after the muffler to keep the temps low as to not overload (and over heat) the catalyst with hydrocarbons and whatnot. That way they could get away with a small/inexpensive cat and still meet the emission requirements at the time.