Tech Area

Running Rich

Question: How do I tell if it's running rich? And the MAP sensor - can someone go in depth about how it works and if the bucking is related to fuel mixture?

Answer: This is both simple and complex.

1st, what is fuel mileage? If 10 gal's yield 200 miles, 5 gals yield 100 miles (with tripmeter divided by gals used....not by tank indication), then mileage is 20 mpg. Could be good for certain engine/trans/car use combos but may be rich for others.

However, if pulling the plugs shows a light tan color on the porcelain, then it would be driving habits, not fuel mixture, that would make someone suspect a rich condition. Only sooty black porcelain truly indicates a rich mixture.

2nd, with unleaded fuels, all exhaust tips look "sooty", regardless of a rich condition or not. "Cakey" soot in the tips is usually caused by excessive oil burning (which normally reduces mpg).

Above is the simple part!

Below is the complex part:

The MAP (Manifold Absolute Pressure) sensor measures the rise and fall of the pressure (lets call it vacuum...for a later reason) in the intake manifold as the throttle valve is opened and closed. As the valve is opened, pressure (vacuum) falls. The MAP converts this to an electrical reading for the ECM. Along with the TPS (Throttle Positioning Sensor), the ECM then knows how much MORE fuel to meter.

There are 2 other important sensors concerning fuel metering. The Coolant Temp Sensor lets the ECM know if the engine is cold (requiring more fuel...or if faulty, may demand more fuel even if warm), and the Oxygen Sensor which lets the ECM know the final oxygen content in the exhaust gases (in other words, how efficient the combustion mixture...rich/lean...is).

All these are analyzed by the ECM to determine how much fuel to deliver.

Now, back to the MAP. If there is a pressure (vacuum) drop caused by a vacuum/air leak, gasket/lines (not related to the accelerator position), then the ECM will increase the fuel mixture resulting in a rich condition.

Actually, a "bucking" is usually caused by a lean condition or accelerating from too low a speed for the gear selected (manual trans).

Sorry this cannot pinpoint a definite source for your problem, but hope it gives you some insight as to what to look for.

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