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Old 04-18-2006, 12:17 PM
BrianH's Avatar
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Join Date: Jan 2003
Location: Chicago
Posts: 46
126 Fuel Mixture Adjustment

I had the 560sel dynoed and we found the car running a little lean. Is the F/A adjustment the 3mm screw on the top of the air flow meter? If it is, I assume clockwise is richer and counter-clockwise is leaner (like old CIS 911s).
Any enlightenment would be appreciated.
BTW the car was 159hp at the wheels or roughly 219hp at the crank.

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Old 04-18-2006, 04:25 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,241
Whats a "little lean"?

What was the measured range of A/F at WOT?

The mixture screw just affects the base mechanically derived mixture at idle and low load and "adjusting" it will have little or no effect on the WOT mixture, but it can screw up emissions, driveabililty, and fuel economy.

Exhaust probes don't always yield accurate A/F ratio depending on what the catalyst is doing. On O2 sensor equipped engines catalyst activity stops at WOT enrichment once stored oxygen is consumed.

Also, oxygenated fuel typically means the actual A/F is leaner by the mass O2 fraction, which is usually around 2 percent .

Some Mercedes engines don't have WOT enrichment. The W140 V12 beginning in its second year of availability comes to mind. Advertised power was reduced from about 400 to 389 IIRC.


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