Question on Rear O2 and Catalytic Converters

[LostX]

So a little background on the truck is its a 96 model k3500 with the 7.4 vortec engine. My issue is that at some point some previous owner gutted both catalytic converters (they are hollow). I am planning on redoing that portion of the exhaust with new catalytic converters soon. The issue i have is that even though the cats are gutted, and there are numerous exhaust leaks before and after the cats, the computer has never thrown a light or code for any rear O2 issues. I can verify the rear O2 sensors are in place and connected to the harness. If I put new cats in I would like to have that portion of the engine computer that verifies catalytic converter efficiency working.

Is it possible a previous owner had the factory 96 computer tuned to remove that feature, or is there another thing I could be looking at?

 

[RichLo]

More than likely the computer has been tuned to delete the rear O2.

Has the truck been modified more than just gutting the cats?

 

[stutaeng]

Most likely yes, O2s deleted from the tune. There's no way I can see deleting the cats and/or O2 sensors and the CEL not coming on. Can you confirm by checking with codes with a handheld code reader? Is your CEL lightbulb working?

If no codes and you go new catalytic converters (functional), then you should check your tune...

 

[LostX]

The truck does have a lift kit, but the drivetrain, engine, trans is all stock. Someone put a dual exhaust on a long time ago judging by the broken welds and corrosion on the exhaust.
I've owned the truck for 5 years and frequently check with a handheld and have never seen a rear 02 code.

I tried looking at both sides of the engine computer to hopefully see a label saying "tuned by blackbear" or something but I couldn't find anything. If I plug a handheld into the OBD port it does pull up the correct VIN number for the truck.

One other thing is that the previous owner installed a junkyard 454 from a 97 truck. I don't know what happened to the original. One issue I've come across is that the fuel trims are always 10 to 15 percent adding fuel, no matter what I do to try to fix it (I had installed new 5.0 injectors and upper manifold gaskets to no help). One thought that crossed my mind is that the previous engine was modified and the computer tuned accordingly. But with the new junkyard stock motor the trims are all out of whack.

I just don't have the tools to check the tune of the computer. I own all the moates components to tune and write to 95 and earlier computers. so I'm a little stumped on what to do.

 

[stutaeng]

There are some guys here that know about tuning. They can hopefully chime in on how to check. The CEL will come on if the fuel trims are over 15%. If you are below that number, you are technically "OK." But your LTFT and STFT should be as close to 0 as possible, for the given bank. Is that true on your truck?

The post-catalytic converter O2 sensors are there to check that the catalytic converter is actually working and the chemical reaction is occurring when the gasses pass though it. Without cats, the working O2s will pick up on that the first closed-loop drive cycle I would imagine. Can you pull up the live data for the rear O2s?

 

[LostX]

Long term trims are always around 15% (adding 15% more fuel). When the LTFT reach 20 is when it throws a code, which rarely happens. I'll try to take a video of what the scanner says all 4 O2 sensors are doing in closed loop.

 

[88monteSS]

My 97 454 had one hollow cat & one functional cat. It never threw a code on the stock PCM & it was like that for who knows how long. According to Justin @ blackbear that was normal on the 1 tons.

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