Why does idle speed drop on turning the wheel?

[NDavies4]

When the car is fully warmed up (>80 deg), stationary, the AC off and the gearbox is in 'Drive' the car idles at 600 rpm as expected. If you then turn the wheel towards full lock the revs drop to 500 rpm: Why?

Reason for the question: Have been suffering from the dreaded warm engine rough idle/hesitation problem for some time. Have tried various solutions to date, but recently felt the problem was finally resolved by the replacement of the crankshaft position sensor. Had the car out for a spin last night though and just as I was parking up had the rough idle back but only when I was stopped in 'Drive', turning the wheel etc. as described above (i.e. when the revs fell to 500). I must say the rough idle was relatively mild compared to days of old, but definitely there.

 

[43622]

Hi Neil,

The most probable reason for the idle speed to drop a little is due to the additional load created by the power steering pump as you turn the wheel, this as far as I know should be quite normal, however if it is causing running problem then it may be worh checking out.
It may be worth checking the power steering reservoir level too.

My s class (1991 420) had a really noisy steering pump especially when cold, you could feel the vibration through the wheel however when it had a service and a fan belt, it cured its self !!! Odd ! I asked the garage if they had done anything and I was told they just 'topped' it up !

hope you get it sorted !

andy

 

[NDavies4]

Thanks for the explanation Andy - makes sense.

Don't think the power steering pump/reservoir is the cause of my problem though; it's more the side effect of it lowering the idle revs by 100 rpm that makes the rough idle noticeable.

Main thing is the idle problem is intermittent and tends to disappear whenever car is booked in to see a mechanic. Hope now I've finally got myself a technique to bring on the bad idle when required.