My 95 S320 has recently developed a surging idle. I read through
previous posts and tried a few things but problem persists.
At idle, in or out of gear and hot or cold, idle surges between
500-700 rpm with an occasional peak to 1000. I disconnected the MAS
but noticed no change at all. Voltage at idle with A/C on is about
14.10. Plugs and coils changed last year and car has about 80K miles.
I read about the "overload relay" and how that should be checked
first. I looked behind the fuse box and did not see any relays with a
fuse on the top as described in previous posts. Is it different or
located elsewhere on this model?
Any ideas would be appreciated!!
There is no over voltage protection relay as on older models. This is
taken care of by a more modern power supply design.
The idle speed is controlled by the same circuit as the cruise control,
which means that the throttle position is controlled by an electric
There could be varios reasons for the surge. Here are some:
1) The throttle position is monitored by a potentiometer within the
same unit with the electric motor. If the motor or the potentiometer is
mechanically loose, or if the potentiometer is defective, the idle will
2) If the throttle is sticking, it will surge
3) If the mixture is too lean it may surge
In any case, the surge is probably due to too slow reaction in the idle
control loop (the engine reacts too slowly according to the ECU's
commands), or there is something else wrong in the loop (f.ex. loose
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