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Okay, I'm stumped sorta. I went and got my 1991 S-10 Blazer 4.3l smogged and failed badly.

Timing set at 13 BTDC. I know that should be at 0.

15mph/1447prm = 6.3% CO2, 1.3% O2, 624 ppm HC (max is 121), 12.45% CO (max is .70), 114 ppm (max is 1006).

25mph/1402 = 6.2% CO2, 1.3% O2, 725 ppm HC (max is 101), 12.63% CO (max is .90), 220 ppm NO (max is 866).

I've pulled the following codes.

42 – Bypass or Electronic Spark Timing circuit - The EST signal did not change when the ECM applied bypass voltage to the ignition module.

43 – Knock Sensor Ciruit - Knock sensor signal was either above 3.5 volts or below 1.5 volts for at least 20 seconds.

44 - Oxygen sensor or circuit – lean exhaust detected. - Oxygen sensor voltage was under 0.25 volts for up to 4 1/2 minutes of closed loop operation.

Now, I think correcting the timing will fix codes 42 and 43, and replacing the O2 sensor should fix code 44.

Am I way off base, or are there any other things I should check?
 

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estrada42 said:
Okay, I'm stumped sorta. I went and got my 1991 S-10 Blazer 4.3l smogged and failed badly.

Timing set at 13 BTDC. I know that should be at 0.

15mph/1447prm = 6.3% CO2, 1.3% O2, 624 ppm HC (max is 121), 12.45% CO (max is .70), 114 ppm (max is 1006).

25mph/1402 = 6.2% CO2, 1.3% O2, 725 ppm HC (max is 101), 12.63% CO (max is .90), 220 ppm NO (max is 866).

I've pulled the following codes.

42 – Bypass or Electronic Spark Timing circuit - The EST signal did not change when the ECM applied bypass voltage to the ignition module.

43 – Knock Sensor Ciruit - Knock sensor signal was either above 3.5 volts or below 1.5 volts for at least 20 seconds.

44 - Oxygen sensor or circuit – lean exhaust detected. - Oxygen sensor voltage was under 0.25 volts for up to 4 1/2 minutes of closed loop operation.

Now, I think correcting the timing will fix codes 42 and 43, and replacing the O2 sensor should fix code 44.

Am I way off base, or are there any other things I should check?
fix timing and the o2, might pass then
 

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Yep you're having ign misfire causing a lean condition which
the o2 also sees as a false lean, sending voltages to the ECM
to increase injector on time. You need to set that timing at 0
degrees in base timing mode. You should get rid of that old
o2 sensor and change the oil and replace the PCV. The
oil now has a uncertain amount oil unburnt gas in it as well
as acids from post combustion and semi post combustion.
The PVC will try and pick this crap up and send it through,
hose going into the intake under the TBI, making for a dirty
a/f mixture that the engine will which will also cause a smog
problem..

When I go to do a smog on my 92, I time it, clean out the EGR,
install a new cap, rotor, and plugs. I change the oil the very
day of the smog test, so the oil is as clean as possible and
i replace the PCV with a new valve.

I also have a new cat I keep handy just for smogs. Once I
pass it comes off and back into the box. I also have a new
Bosch o2 sensor I stick on just for smogs, then once it's
passed it comes off and goes with the new cat until next time.

Getting the cat hot is one of the most important things, Take
it for a hard run right before the test to heat the cat up they
let it idle while you fill out the paperwork or are waiting for
the test. Don't shut it off so the cat can cool down..

With as rich as you have posted above, you may just have
to get a new cat as the substrate may be corrupted from
the rich mix sticking to the cat's substrate. You may luck out
you may not.

Anyway I hope this helps. This is how I prep my 92 for a
smog. I used to do Cali smogs a few yrs back, and have
picked up what works on a 4.3 TBI and what doesn't if
the engine and computer are decent shape.

Sorry for the extremely long post, but it's cheaper than
failing again, and I used to charge a certain amount
for the info. However I do not in any way guarantee you'll
pass
. This is just my common pre-test prep.
 
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