Off-idle stumble resulting from ported EGR

Bohdan L Bodnar bohdan at
Thu Jun 1 16:10:45 GMT 1995 you've perked my interest.  Try the following as a failure

1). EGR valve has carbon deposits
2). Normal EGR control vacuum (AVERAGE vacuum) slightly off idle and no load
    on engine is about 5" Hg
3). You open the throttle
4). Backpressure transducer applies control vacuum to the valve;  however, the
    deposits prevent it from opening (or cause it open VERY slowly)
5). More control vacuum is applied
6). Valve snaps open, "over-EGR-ing" the engine
7). Engine stumbles, valve now operates more-or-less normally, control
    vacuum drops back to roughly what it should be.

The 5" Hg is typical for engines at very light load and just off of idle.
Higher readings suggest a malfunction in the transducer or restricted valve or
EGR tube venting to atmosphere instead of the valve assembly.

Incidently, some PWM controlled EGR valves don't have a control vacuum line;
the EGR flow is inferred from MAF readings (and probably a bunch of other


Bohdan Bodnar

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