Vacuum Lines for Power Steering and ISC Valve
for Toyota 4A-G 20v AFM 1993
14 March 2004
The port circled by the red colour square is producing pressurised air (blowing out) to the power steering rack and pinion's vacuum idle up valve / compensator.
The power steering rack and pinion's vacuum idle up valve / compensator then "return" the vacuum / pressurised air to the intake plenum (surge tank).
The way it operates will be described below: -
When there is NO steering movement, the valve is closed, blocking the pressurised air (blowing) to reach the intake plenum (surge tank). This idling is not affected in anyway.
When the steering is being turned, the valve is opened and the pressurised air will blow through and reach the intake plenum (surge tank). The extra air will then be sucked into the engine by means of engine's aspiration (vacuum) through the ISC Valve (Idle Sped Control Valve) and cause the idling to increase to compensate for the engine's loading due to the power steering pump pulling the engine slow.
Since my power steering rack and pinion does not have a vacuum idle up valve / compensator, hence this port is closed. When this is done, there will not be any return from the power steering rack and pinion's vacuum idle up valve / compensator, hence the little hole at the intake plenum will also be closed. Else the engine will breath in extra air which is not measured by the AFM (Air Flow Meter).
For the case of 20v MAP (blacktop) the story is slightly different as the MAP sensor (vacuum sensor in reality) is measuring from the 4 throttles' equalised passage, which is far far away from the intake plenum, hence there will be no difference to idling or performance, except dirt / contamination, because these air did not pass through the air filter, when being sucked into the engine.
But a friend's 20v MAP was found that: -
1) the pressurised port goes directly into the intake plenum
while
2) the vacuum lines from the power steering rack and pinion is looped.
Whenever he turns the steering, the engine will idle up accordingly. This is very strange! It suggests that OEM ECU have very high intelligence built in.
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