Dialing in your pre-smog-check carbureted EEC machine.


GSGregg
01-08-2011, 07:28 AM
I did this on an '85 S-10 Blazer 2.8L w/ E2SE carb, but it can be done on any carbureted, computerized smogger with a Mixture Control Solenoid (or whatever someone else wants to call it).

Mixture adjustment is definitely hit-or-miss, due to ECM compensation, so I decided to 'fool' the computer. A stoichiometric (the 'ideal' 14.7:1) air/fuel ratio will generate 450mv from the O2 sensor. After checking/repairing the usual suspects and warming the engine, I disconnected the O2 sensor and connected a DVM (2-volt scale) to the sensor wire so I could read its output. Then, having bought a little 'project box' from RadioShack and mounted a pair of potentiometers in it (one high-value and one low-value for fine-tuning---sorry: it's been several years, so I don't remember the exact values; probably something like 50 Meg and 1Kohms) in series with each other and a AA battery, I adjusted the output from that box to approx. 500mv. and connected it to the harness-side of the O2 sensor connector.

Since the ECM goes into Open Loop at idle, I put an appropriate spacer between the throttle lever tang and the TPS stem---to tell the computer that the throttle was off-idle so Closed loop was induced---and started the engine.

When the ECM detects the ideal fuel mixture, the Mixture Control Solenoid is cycled at 50%, which can also be read as 30 degrees on a 6-cylinder dwell-meter scale (or 45 deg, on a 4-cyl. scale, or 22.5 deg. on an 8-cyl. scale, or 72 deg. on a 5-cyl. scale, etc.---in my case, 30 degrees was my target).

I adjusted the 'pots' to feed 450mv. to the ECM (very sensitive; current draw was somewhere around 64 micro-amps) which gave me my 30-degree/50% at the MCS (measured at the green single diagnostic terminal, if I forgot to mention it) and adjusted the idle-mixture screws until the O2-sensor voltage (on the DVM) was also 450mv.

Removed the TPS spacer, unhooked everything, and took it to the smog joint; the Tech said it was the cleanest-running 2.8L he'd seen in a long time.

(I am ooooh, soooo SMART!) Hope someone can benefit from this.

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