Engine rotational speed

Is the throttle closing completely, or is something (the dashpot maybe) keeping it from closing?

Do you hear any air leaks or see any loose hoses?

Dave Hinz

Hi Dave,

Thanks for the reply, and yes I have cleaned the trottle house (not sure of the name here), and everything around it (where the air comes in). And when I look at it and the motor is running; everything seems ok to me.

Thanks again.

Has your car, in the past, had a "surging" in the idle speed, where it goes up and down a lot? There is a valve which controls idle speed, I wonder if it might be stuck open? Have you had any problems like that?

Dave Hinz

I think the valve Dave is referring to is the Idle Air Control (IAC) valve. This is the controlled element in a servo system which actively controls engine idle speed to maintain it at one of several "set-points" (to use a control system term), depending on several factors. The other main elements of the servo system are the Electronic Control Unit (ECU) and measures of (primarily) rpm, temperature and throttle position.

IACs are prone to becoming sticky and hence not controlling properly. In particular they are prone to sticking fully open or fully closed, due to the way they work (oscillating shuttle). I had both cases before I eventually fixed mine. The "open" case gave idle revs in the range you describe (2000-3000 rpm). Before it sticks completely an IAC will generally give a loud metronomic 'CLICK' of about 3 Hz at tickover. It may do this for weeks or months before eventually deciding to stick in one position or the other. Spontaneous recovery is also possible, though it will be a matter of time before the problem reappears.

Ultimately, you may need to replace the IAC valve but not before trying to unstick it. I recommend INJECTOR CLEANER, which is designed to dissolve the same sticky residue that is affecting your IAC valve. This is by far the simplest approach, as disassembly/reassembly is minimal.

You can buy injector cleaner from most high street automotive stores or motor factors. Simply use it as per the instructions on the can (typically spray into the air intake with filter removed). From my experience (SAAB and other makes) you will need to spray significantly more liberally than the instructions indicate.

NOTE: Terminology and design details may differ depending on the particular type/brand of fuel injection system fitted.