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# Mass Air Flow sensor - page 8 / 59

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At 900 RPM a 4 cilinder 4 stroke engine makes 900 RPM * 2 strokes/revolution * 60 minutes in one hour = 108,000 strokes. So if the ECU reads 480 mg/stroke at idle, it equals to 108,000 * 480e-6 = 52 kg/hour air flow.

480 mg/stroke is the value you should read with a 1.9L engine during idle when the EGR is closed. One full cilinder filling is 0.48 liter. Air is about 1290 mg per liter. So one could calculate 0.48 * 1290 = 619 mg/stroke. However, the cilinder is not fully filled as it has to suck in the air in a limited amount of time. Therefore the value is a little lower. (If the turbo would be really working at idle, the air flow could be higher than 619 mg/stroke.)

The output voltage of a Pierburg MAF during idle and closed EGR is about 1.6V. A Bosch MAF reads about 2.2V in this situation. With a closed EGR you should measure about 480mg/stroke. With a 78% opened EGR you should read about 290 mg/stroke. If the values you find are much lower, something is blocking the air flow or the MAF has gone bad. The air flow can be blocked by a clogged air filter, a clogged intercooler, a (partly) closed anti shudder valve, a clogged intake manifold or bad valves. The intake manifold may clog due to soot/oil as exhaust fumes are fed back into the engine by the EGR and oil is fed into the engine by the CCV. It's also possible that the EGR is stuck open.

Below is a curve of the mass air flow of a 1.9L TDI industrial engine (AFD).

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