Driveability Corner

Motor,  Feb 2006  by Warren, Mark

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Fig. 3 is a recording at idle and 2000 rpm with no load and no vehicle speed, varying the EGR opening from zero to 100% open. The top graph is STFT, the second graph is LTFT, the third graph is EGR duty cycle, the fourth graph is MAF, the fifth graph is rpm and the sixth graph is idle air control (IAC).

Throughout the testing there was little change in STFT. The LTFT did increase from 1.6% to 4.8%, which was not enough of a change to help in diagnostics. The MAF reading at idle did increase from 5 to 7.5 gms./sec., as it had earlier. If the MAF measures only the incoming air and is not affected by EGR downstream, why the change? With a little thought, the answer was obvious. At 100% EGR at idle, the engine misfired badly. The misfiring cycles consumed air and fuel but contributed nothing to engine rpm. Therefore, the engine had to consume more air to maintain rpm to compensate for the misfire events.

Note that the last third of the Fig. 3 recording shows the effect of changing the EGR opening at 2000 rpm.

In conclusion, excessive EGR had a substantial effect on fuel trim on the MAP system. Excessive EGR had little or no effect on the fuel trim of the MAF system. It's always important to identify which system you're working on, MAP or MAF, and remember how each system responds to different operating conditions.

Copyright Hearst Business Publishing Feb 2006
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