1998 chevy 305 to 350 swap pcm options

[94chevc1500]

Hello all I read through the posts about people who have done the swap and no one has asked what I am thinking. My truck is a 98 chevy 2wd 4l60e the previous owner swapped in a 350 it was a 305 originally. It runs fine but I want it to be right my question is will a computer from the same year 4x4 truck suburban tahoe or 2wd truck suburban tahoe work?

 

[L31MaxExpress]

Hello all I read through the posts about people who have done the swap and no one has asked what I am thinking. My truck is a 98 chevy 2wd 4l60e the previous owner swapped in a 350 it was a 305 originally. It runs fine but I want it to be right my question is will a computer from the same year 4x4 truck suburban tahoe or 2wd truck suburban tahoe work?

It will not work unless you go to the 350s dual catalyst exhaust and 02 sensor configuration. The 4x4 PCM will not work, need a 2wd version. The transmission will not shift correctly with the 4x4 settings.

305 has a single cat, 3 x 02 sensors in front of the cat and 1 behind it. The 350 has dual cats and 1 sensor ahead and behind each cat. The 350s PCM will not be happy getting the wrong readings from the 02 sensors in the 305 exhaust. 4.3L to 350 swaps have the same problem. I have tuned a couple of V8 Astro and S10 swaps keeping the V6 exhaust, but it requires using TunerCatsOBD2 to correct the issue. HPTuners and EFILive cannot do it.

Best bet is to find a donor 350 2wd 4L60E truck, pull the whole exhaust system and the PCM. For the exhaust I would find a factory 8-lug truck exhaust if you can. PCM needs to be from a 4L60E truck though.

 

[L31MaxExpress]

The 8-lug factory exhaust is on the left. Some of the Express vans have even larger 3" OD piping, but the 2-3/4 OD of the 8-lug truck should be plenty. Either way you go you will be extending the 02 sensor wiring and reconfiguring the 02 sensors into the correct locations as they are pinned into the PCM. The 350 is breathing through a straw with the 305 exhaust and two straws with the 1500 exhaust.

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[L31MaxExpress]

Here is an image showing the exhaust/02 configuration changes. I think the image I have is Express/Savanna van, but the trucks are setup the exact same way. Oddly enough #2 in the diagram is actually a threaded plug. Some trucks used threaded plugs and not all of the 02 sensors. Many of the trucks and vans were built without rear 02 sensors and they were turned off in the tune. This went well into the GMT800 years as well. I have OE rear 02 deleted tunes from GMT400s and GMT800s and even a tune that GM ran on the CNG trucks and vans that deleted the MAF sensor. The more variations I run across the more odd I find the changes.

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