Car question

[glatt]

Originally Posted by BigV

glatt, when the car's all warmed up or whatever conditions are needed for this rough idle symptom to appear, would the roughness disappear if you gave it a tiny amount of gas? Like just enough to raise the idle, as if the idle were, say 100 rpm greater?

Not sure if only 100 extra rpm makes it go away, but a couple hundred more do make it go away. If it goes from 700 to around 1000, the vibrations go away.

[tw]

Originally Posted by glatt

but a couple hundred more do make it go away. If it goes from 700 to around 1000, the vibrations go away.

That is about the point where an engine changes from its idle mode operation to normal mode operation. More accurately, point centers closer to 1200 RPM. Its not a sharp transistion. But at 1000 RPM, idle mode parameters are changing to normal operation mode.

What exactly is that point? Decelerate while not touching the accelerator. It begins entering idel mode when fuel injectors go from no operation (no fuel into the engine) to providing fuel for idle mode.

Idle for most cars is 800. But again, what determines that idle? Its computer times the engine by monitoring only the crankshaft or camshaft sensor. Computer only reads that sensor to adjust fuel for 700 (or 800 on most cars) RPM once the engine is warm.

Asking if 700 is too low is a valid question. Sometimes that spec number is on a label glued under the hood or attached to the door pillar (that label covered when the door closes). I have never seen a shop manual not provide that number even when no adjustment is possible. Since a wrong number could be symptoms of a specific problem.

Anything you might do to change idle will be compensated for by the computer. So if idle should be 800, well that would imply a master clock problem in the computer. And therefore might explain roughness only at idle only because the computer wants an idle that is too low.

Well, touch the accelerator to raise RPMs up to 800 may or may not report something useful. Says little by itself. But in combination with other facts, may imply a suspect.