(automatic transmission, 4.3L V6 engine): 1GNDT13Z1M2

Hi guys,

I don't want to bore you with too much background information, so I'll try to keep it short(ish).
basically I have been given the task of fixing up a '91 Chevy S10 blazer 4x4 that belongs to a friend. This car is very rare in my country. In fact, it's the first one I've ever seen here in 30 years.
I'm not a mechanic, but I've serviced all my cars and motorbikes and have somehow always managed to figure things out and get things working with the help of the internet, good fellows like yourselves, and some service manuals.

This car (I call it the Urban Ninja) was born in the U.S, then sold to Germany, and now it's in Portugal.

My involvement started after the owners had a GPL conversion done on it and failed the emissions test required to certify the GPL installation. I was asked if I could install a catalyser on the car so that it would pass the emissions test and could be used with Gas instead of Petrol (I'm a computer programmer, so the fact that they would ask me to install a catalyser is so deliciously random, that I decided to go with the flow). I replied that, in the first place it was strange that there was no catalyser (as I was told, I haven't had a chance to confirm yet because it rained all day and I didn't feel like getting under the car to have a look), and second, if it was nicely tuned up and everything working fine, there should be no reason to fail the emissions test, even without a catalyser.

Things were left at that for about a week, and then I was told that the car suddenly became sluggish, backfiring and with extremely high consumption rates (almost a litre per mile - later I find out that this is third-hand information and may be exaggerated). I asked them to bring me the car to have a look. So they get ready to do this 50 mile trip, and stock up on extra fuel, figuring the 1 hour journey is gonna cost over $100 in fuel, but end up having a smooth ride, only consumed the top two sticks on the fuel gauge and no sluggishness or backfiring. Right now I'm thinking possibly some dirt got stuck in MAF sensor messing up the readings and then just blew away and things returned to normal. Or maybe some deposits around the O2 sensor? Or maybe a bad connection somewhere? Or maybe the information was highly exaggerated in the first place.

Today I was told that the mechanic changed the fuel mix to make it richer just before going to the emissions test. Now why would he do that?... but the real question is, how did he do that? this car has no carburettor, so no mixture screws to mess around with... how would one go about changing the fuel mix?

The car starts, idles and runs fine, but has a very rich-fuel smell inside the cabin. both 'abs fault' and 'check engine soon' lights were on. I took a few things apart and found an Obd1 connector lost behind a bunch of wires and read the codes:
2-4 : Vehicle Speed Sensor (vss) or circuit. This should be the origin of the abs warning.
4-3 : Knock sensor circuit (because, maybe it did behave erratically after all?)
4-4 : Oxigen Sensor.

Second question is: Could a faulty oxygen sensor cause this behaviour that would in turn throw the knock sensor error too?

(btw, of all the s10 photos i've seen, nobody seems to have the same console and dials as this one... I wonder why)

Thanks