1991 chevy s10 pickup 4.3 engine

First I recommend changing spark plugs, air filter, and change the oil & filter. If these are dirty or fouled up it will kill your millage and help to fail smog.
You can reduce both NOX and CO emmissions by adding denatured alcohol to the gas tank. Run it down to a bit under 1/4 tank and add 1 1/2 gallons. It makes the engine run a bit rough, but really clean.
If you still fgail SMOG then you need a new catalytic converter.
I used to "trick" my 86' with alcohol, but just rebuilt the engine so I put a new Cat on it. It passed emissions with WELL BELLOW numbers.

Good luck.

Many things can cause this, so I'll just try to hit the high spots.

1. Poor fuel ignition (bad plugs, wires, cap, rotor, etc.
2. Improper Ignition Timing
3. Lean air fuel ratio mixture
4. Defective Catalytic Convertor
5. Defective Air Injection System.
6. Low Compression

These are general things that can cause this condition, but they may not be applicable to your particular vehicle.

Try a good old fashioned tune up: Plugs, wires, cap, rotor and points (if applicable) and a new air filter.

Wouldn't be a bad idea to also change the oil and oil filter.

The final item (#6) is a sign of a well worn engine and if this proves to be the primary problem, I'm afraid a engine rebuild is in order.

Anyway, good luck!

The problem with emissions is there are so many things that can help out a little or a lot, Have you had the car checked for any issues with the exhaust such as 02 sensors, converters, muffler, charcoal canister. Try using a higher octane gas that will burn better, this will create a leaner exhaust and also install plugs that will create a hotter spark to ignite the gas during combustion. Here in SC we don't have the emission laws and it's not used as freq. as other states. Good Luck. By the way. I work for Bosch which makes the 02 sensors and I suggest buying the aftermarket because these are built and made the same but the OEM is 2x's as high if not more.

For the 1991 Chevrolet S10 or T10 Pickup 2.8L V6:

Firing Order:
1-2-3-4-5-6

Cylinders:
.......(back)........
|..5..|........|..6..|
|..3..|........|..4..|
|..1..|........|..2..|
.......(front)

Distributor:
....(back)
....3...4.(+)-screw
2...........5
(+)1...6
....(front)

Let me know if this helped, or if you have additional information or questions. Feel Free to contact me at FixYa.com!

991 Chevy S10 firing order
The firing order for a Chevy s10 depends on which engine you have.
1.9L engine the firing order is: 1-3-4-2 . Distributor rotates counterclockwise.
2.0L engine the firing order is: 1-3-4-2 . Distributor rotates clockwise.
2.5L engine the firing order is: 1-3-4-2 . Distributor rotates clockwise.
2.8L engine the firing order is: 1-2-3-4-5-6 . Distributor rotates clockwise.
4.3L engine the firing order is: 1-6-5-4-3-2 . Distributor rotates clockwise.

spark plugs /

You need four things for an engine to run. 1. fuel 2. spark 3. compression 4. timing (spark at the right time. By your post it seems you have fuel and spark and you probably have compression unless you blew a head gasket. That leaves the timing. Check the timing and/or the timing belt or chain whichever you have.

i know it sounds silly but the garunteed to pass stuff at the auto stores work.

high hydrocarbons indicates head gasket(1 or both) leaking , carbon monoxide is a worn out catylitic convertor which both could lead to the oxy sensor getting a false reading

Have your cat. converter checked out. I just had one of these for the same reason in my shop today.