SOURCE: my chevy s10 failed emissions
Have your cat. converter checked out. I just had one of these for the same reason in my shop today.
SOURCE: my 1991 chevy truck S10
also check your alternator, mine did the same and it ended up being the alternator
SOURCE: chevy s10 engine running fine then idles real fast all by itself
Your going to have to get the ECM scaned, there are to many things that can cause that. most places like Auto Zone or Part Source can scan your S 10 free of charge and can also give a how to's if you want to replace the part your self.
Good luck and hope this helps
SOURCE: 1991 chevy s10 4.3 v6 new rotor and cap new spark
Check your knock sensor, and timing.
The distributor is controlled by the computer and springs, I can't
recall but I don't remember seeing springs in there. Regardless, check
anyways to see if there is any binding to your counter weights in the
distributor.
Start with finding out why your check engine light is coming on, this
way there is no expensive guessing work. Here is a link for you to
retrieve the DTC codes and post me back. Be glad to help you get your
S-10 running 100% again soon. 
http://www.troublecodes.net/GM/
Recheck your timing and you'd notice more difference if you increased
your static timing advance from the stock setting of 10 Deg BTDC to 15
deg BTDC or so. Be sure when you adjust timing to disable the ESC
circuit according to the manual. Otherwise the timing will be off scale
and you won't even be able to see where it is with a timing lite.
The connector you're looking for is located in the passenger footwell,
sometimes covered by the carpet/floor mat. It is a black connector, that
has a tan/black wire in each side. Disconnect this wire and you will be
able to see the base timing.
The ECM will control the timing with that wire connected. When it is
disconnected the timing advance is controled strictly by the ICM
(Ignition Control Module) located on the base of the dizzy.
How to set:
The vehicle emission control information label, which is found
underhood, will often contain specifications or procedures for checking
and adjusting timing that have been updated during production.
Set the parking brake and block the drive wheels, then warm the engine
to normal operating temperature. Shut the engine OFF and connect the a
timing light to the No. 1 spark plug (right front 2.8L engine, left
front 4.3L engine or front plug on in-line engines):
If using a non-inductive type, connect an adapter between the No. 1
spark plug and the spark plug wire; DO NOT puncture the spark plug wire,
for this will allow a voltage arc.
If using an inductive type, clamp it around the No. 1 spark plug wire.
If using a magnetic type, place the probe in the connector located near
the damper pulley; this type must be used with special electronic timing
equipment. Do not under any circumstances pierce the insulation of a
spark plug wire in order to connect the timing light.
Clean off the timing marks, then label the pulley or damper notch and
the timing scale with white chalk or paint for better visibility. If the
timing notch on the damper or pulley is not visible from the top, the
crankshaft should be bumped around using the starter or turned using a
wrench on the front pulley bolt, in order to bring the mark to an
accessible position.
Model vehicles equipped with EST, the electronic spark timing must be
disabled or bypassed to prevent the control module from advancing timing
while you are attempting to set it. This would obviously lead to an
incorrect base timing setting.
There are 2 possible methods of disabling the EST system, depending on
the type of engine:
On 2.5L engines, ground the "A" and "B" terminals on the ALDL connector
under the dash before adjusting the timing.
On all other engines using the EST distributor, disengage the timing
connector wire. On a few of the earlier model vehicles, the 4-terminal
EST connector must be disengaged from the distributor, but most later
model vehicles are equipped with a single wire timing bypass connector.
On these later model vehicles the bypass wire is usually a tan wire with
a black stripe. This wire usually breaks out of the wiring harness
conduit adjacent to the distributor, but on some vehicles it may break
out of a taped section just below the heater case in the passenger
compartment.
Start the engine, then check and adjust the idle speed, as necessary.
The idle speed must be properly set to prevent any centrifugal advance
of timing in the distributor.
Aim the timing light at the timing marks. Be careful not to touch the
fan, which may appear to be standing still. Keep your clothes and hair
along with the timing light's wires clear of the fan, belts and pulleys.
If the pulley or damper notch isn't aligned with the proper timing
mark, the timing will have to be adjusted.
TDC or Top Dead Center corresponds to 0° mark on the scale. Either B,
BTDC, or Before Top Dead Center, may be shown as BEFORE on the scale,
while A, ATDC or After Top Dead Center, may be shown as AFTER.
Loosen the distributor base clamp locknut. You can buy special wrenches
which make this task a lot easier on certain models. Turn the
distributor slowly to adjust the timing, holding it by the body and not
the cap. Turn the distributor in the direction of rotor rotation to
******, and against the direction to advance.
Once the timing is properly set, hold the distributor to keep it from
the turning and tighten the locknut. Check the timing again after
finishing with the nut in case the distributor moved as you tightened
it.
If applicable, remove the plug and connect the distributor vacuum hose.
Shut off the engine and reconnect the EST wire (if equipped), then
disconnect the timing light and tachometer. 
Also you may want to inspect the EGR to see if it's stuck, which may be
causing our issue when the engine is under load.
SOURCE: I have a Chevy S10 V6 1987, I have no spark from
check for ignition start supply to the coil or series resistor. maybe help.
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