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Ok, how do you adjust for a lean mixture?

4857 Views 8 Replies 4 Participants Last post by  jhodg5ck
See previous thread on adjusting idle....

They said they adjusted for a lean condition, but the idle would increase if they did that... So they reset it back to the lean condition and said it was a bad idle control valve.

I'm fairly competent with cars, but this is new to me.
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by turning the CO adjusting hex bolt clockwise. It is just in front of your air metering plate. 3 mm hex key..insert, push down till it bottoms, turn in fractions of a flat to adjust.

I would check your voltage @ the air idle valve after adjusting the mixture/CO (btw, this only effects up to 2500 RPM mixture wise).
When was your O2 changed last?

Does the car have a rough idle?

If it idles fine why do they think it's lean? Usually when these cars are horribly lean they will surge @ idle.

If you don't know when the O2 was done last it would be wise to swap it out. I'm partial to the 4 wire conversion on our cars.

I've dropped that method in favour of adding an extra O2 bung and running a wideband O2 sensors/gauge. 3 Wire sensors are mighty slow to respond and w/ the A/F ratio reading out in real time it doesn't get anymore accurate. Also very handy for seeing if your EHA is set properly (about 2500RPM).

guarddog said:
So just take the 4th grey wire and clip it and run it to the ground on the battery, correct?

Also, I can use a sensor from a mustang, right? I might have one of those around--I have 95 Mustang GT, so that should work fine, right (last of the 302 5.0 liter engines.)

How do I tap into the lambda and where is it on a 420 SEL 1991. I found an article talking about the emissions, but couldn't find out where the port is for it.

I'd like to install a wideband, but they are still kind of spendy for a one shot calibration of Air/Fuel ratio. I guess I could run it on a dyno for $60 to get an air/fuel reading... Would that be better than reading the millivolts?

Here's an article on setting via the lamda/duty cycle:

Yes, the 4 wire off of a later Mustang should work fine...and you are correct, just run the extra ground to your negative battery terminal.

As for setting w/ a wideband you can have the extra bung installed (should be minimal cost) and then running @ the dyno will let you set things Very accurately..or, ideally if you can find someone w/ an Innovate hand held wideband you'll be Set:)

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