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Re: Mixture setting Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 14 Jun 2001 14:23:27
In Reply to: Mixture setting, Ed 9k 87, Thu, 14 Jun 2001 05:10:46 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I assume you mean that the spark plugs are black and sooty. This means you're running rich.
There are a few reasons why you'd run rich, and you'd want to address them before you start adjusting things (which you can).
Is your fuel pressure regulator OK? If it isn't regulating properly, you can get too much fuel pressure, which translates into running rich. Make sure the hose from the intake manifold the to pressure regulator is OK - not cracked or broken. Suck on the vacuum line running to the regulator - make sure you don't taste gas - that would indicate a leaky regulator diaphram. If you can stick a pressure gauge on, that would be best.
Make sure ALL the plugs look the same. If only one plug is black, then you may have a sticky fuel injector for that cylinder.
The next part is harder. If your O2 sensor is bad, it can be indicating the mixture as being lean, when it really isn't. If the O2 sensor says lean, the fuel control computer will supply more gas.
Get a Good digital voltmeter. Connect it to the signal wire from the O2 sensor. Don't disconnect the sensor - just tap into the line.
The sensor should output a voltage that swings between about 0.2 volts and 0.9 volts (or so). It should swing back and forth about 1 per second.
A rich mixture is the higher voltage 0.9 volts. A lean mixture is a low voltage (0.2 or even 0.0). If your plugs are showing rich, but the output of the O2 sensor is showing lean (0.2 volts), then you need a new O2 sensor.
Ideally, the output of the O2 sensor swings back and forth between 0.9 and 0.2 volts about once per second, with an average of about 0.5 volts. The voltage should spend about 50% of the time above 0.5 volts, and 50% below. If the voltage spends more time above 50% - say 75% of the time, that means the O2 sensor thinks the mixture is rich. If the voltage is stuck at 0.9 volts, the engine is running rich.
What you want to do is adjust the mixture screw on the Air Mass Meter (AMM) so that the voltage swings between 0.9 and 0.2 about once per second.
With the engine off, pull the connector off the Air Mass Meter and measure the resistance between pins 3 and 6 on the AMM. Write down this value - if you screw things up royally, you'll want to go back to it. If it isn't 380 ohm, turn the adjusting screw to get 380 ohms. 380 ohms isn't any magic value, it's just a starting point to adjust from. Some folks think if you set it to 380 all will be right. Not true.
Reconnect the AMM and start the car. With the engine at idle, AC off, fans not running, adjust the screw on the AMM until you get the signal out of the O2 sensor to swing around 0.5 as stated above. Once you've done that, pull the connector back off the AMM and measure the resistance between pins 6 and 3. You don't have to, but it's interesting to see how that compares to the value you used to have. I've seen an expert adjust the mixture, and although he started at 380 ohms, that particular car wanted to be up in the 700+ ohm range to run happily.
If you can't get the output of the O2 sensor to swing around 0.5, something else is wrong. But this is a start.
Good luck!
, Thu, 14 Jun 2001 14:23:27 <-- Viewing This Message
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