New Y-pipe and cat for the old girl

[GoToGuy]

L30 - 5.0 vortec, L31 -5.7 vortec, L35 - 4.3 vortec.
As sown above the L31, 5.7, has one O2 senor ahead of each cat, and one O2 sensor behind each cat, 4 total. The drawing is bad example. I don't know why they have 3 sensors ahead of one cat in the 4.3 configuration. The " NA1 " is the RPO option for what emissions level it came equipped with. ( NA1 Emissions level less than 8500 lbs.)
The TBI engines, OBD1 and earlier have more simple emissions systems. The LO3 -5.0, LO5 - 5.7 have single cat.
So if you have to comply with emissions or just choose to be compliant, knowing what level or RPO option will simplify your choice. My friends 98 Tahoe from Texas has cats in different location from my California emissions 98 Tahoe. I can understand adding more for stricter rules, but other than thermal activity, changing the location of cats has me confused.
So when your looking at the parts diagram the notes are often using RPO codes for model application. NA1 less than 8500. NA4 above 8500. NB2 emissions CA tier 2. And so on.

 

[L31MaxExpress]

L30 - 5.0 vortec, L31 -5.7 vortec, L35 - 4.3 vortec.
As sown above the L31, 5.7, has one O2 senor ahead of each cat, and one O2 sensor behind each cat, 4 total. The drawing is bad example. I don't know why they have 3 sensors ahead of one cat in the 4.3 configuration. The " NA1 " is the RPO option for what emissions level it came equipped with. ( NA1 Emissions level less than 8500 lbs.)
The TBI engines, OBD1 and earlier have more simple emissions systems. The LO3 -5.0, LO5 - 5.7 have single cat.
So if you have to comply with emissions or just choose to be compliant, knowing what level or RPO option will simplify your choice. My friends 98 Tahoe from Texas has cats in different location from my California emissions 98 Tahoe. I can understand adding more for stricter rules, but other than thermal activity, changing the location of cats has me confused.
So when your looking at the parts diagram the notes are often using RPO codes for model application. NA1 less than 8500. NA4 above 8500. NB2 emissions CA tier 2. And so on.

I do not know why GM used 3 sensors ahead of the cat on the 4.3 and 305, but every last single cat setup on a Vortec I have looked at is like that. From the C/K trucks, S/T trucks, M-vans and G-vans, they all have the same 3 02s ahead of the cat.

Some L05s had the cat behind the muffler or no cat as well. The ones without a cat generally do not use an 02 at all either. Completely open loop calibrations.

 

[GoToGuy]

I remember working on Camaro or GTO at Ford and the gm parts called it a resonator, but it called the farther back one muffler.

 

[FighterOfTheFoo]

I know this is old but checking to see if moving the O2 sensor location has any ill effects. Wanting to replace my Y pipe and everyone I see has the bung by the cat instead of by the driver's side manifold. Looking at a magnaflow 23256 for a 1995 k1500.
Some say you need to upgrade to a heated 02 and some say it runs fine.

 

[Schurkey]

The farther downstream the sensor is, the harder it is to keep it hot, and the slower it is to react to changes in combustion quality.

Many new cars have the O2 sensor and catalyst built-into the exhaust manifold, for the fastest warm-up, and fastest reaction.

 

[someotherguy]

I know this is old but checking to see if moving the O2 sensor location has any ill effects. Wanting to replace my Y pipe and everyone I see has the bung by the cat instead of by the driver's side manifold. Looking at a magnaflow 23256 for a 1995 k1500.
Some say you need to upgrade to a heated 02 and some say it runs fine.

The way the factory set these up seems to vary a bit - but I'd be surprised if you didn't already have a heated o2 on a '95 1500. By '94, many of the GMT400 light trucks (automatic transmission anyway) went to heated.

Richard

 

[FighterOfTheFoo]

The way the factory set these up seems to vary a bit - but I'd be surprised if you didn't already have a heated o2 on a '95 1500. By '94, many of the GMT400 light trucks (automatic transmission anyway) went to heated.

Richard

I've seen people say they went to a heated O2 in the collector by the cat at some point but mine is a single wire on the driver's side. Converting to a 3 wire doesn't seem hard but I'd rather save a little money if it works as is. Mine is an nv3500 so maybe that's why. It's also an odd duck with a build date of 8/95 but still a TBI, I'm surprised the 96s weren't out by then.

 

[someotherguy]

I've seen people say they went to a heated O2 in the collector by the cat at some point but mine is a single wire on the driver's side. Converting to a 3 wire doesn't seem hard but I'd rather save a little money if it works as is. Mine is an nv3500 so maybe that's why. It's also an odd duck with a build date of 8/95 but still a TBI, I'm surprised the 96s weren't out by then.

It's still a bit of a mystery to me why they chose one over the other style of o2 sensor in the last couple years of TBI. My '94 C2500LD 5.7/4L60E had a heated o2 from factory, '94 C1500 5.7/NV4500 was non-heated. '95 3500HD 7.4/NV4500 non-heated also. This led me to believe the manual transmission trucks ran the "old" ECM (not needing automatic transmission controls) and that's why they have the non-heated o2. So yeah; yours being NV3500 is probably a big part of that decision process at manufacture.

Richard

 

[L31MaxExpress]

I've seen people say they went to a heated O2 in the collector by the cat at some point but mine is a single wire on the driver's side. Converting to a 3 wire doesn't seem hard but I'd rather save a little money if it works as is. Mine is an nv3500 so maybe that's why. It's also an odd duck with a build date of 8/95 but still a TBI, I'm surprised the 96s weren't out by then.

TBI was around until 1999 on these trucks too. Yours a Mexican model by chance?

 

[L31MaxExpress]

It's still a bit of a mystery to me why they chose one over the other style of o2 sensor in the last couple years of TBI. My '94 C2500LD 5.7/4L60E had a heated o2 from factory, '94 C1500 5.7/NV4500 was non-heated. '95 3500HD 7.4/NV4500 non-heated also. This led me to believe the manual transmission trucks ran the "old" ECM (not needing automatic transmission controls) and that's why they have the non-heated o2. So yeah; yours being NV3500 is probably a big part of that decision process at manufacture.

Richard

1995 454 would have had the newer model P4 PCM regardless if it were an automatic or manual. GM needed it to run the linear EGR valve. 4.3L was also equipped the same way. Only the SBC/Manual transmissions from 93-95 used the old C3 style ECM.