Idle issue

Stoonly

'90 245 has started an erratic fast idle issue, revving to 2-3k on start up, nothing mechanical so at first thought idle Air Control valve faulty? swapped valve from our other 245, no change, is it ECU???

ramrod

How does the car drive?

Especially under load

Stoonly

Drives ok under load, its manual so when clutch is engaged it over revs?

ramrod

Diagnosing problems on these cars is never easy, there’s a lot that can be wrong.

You can show a video of the idle to help get a better idea

Most often, idle trouble is cause by a leak in the vacuum side (excessive air being drawn) and unmetered by the fuel injection system.

Do you have a vacuum gauge?

Also where abouts are you from?

Forg

Most often, idle trouble is cause by a leak in the vacuum side (excessive air being drawn) and unmetered by the fuel injection system.

Inspect any hoses/fittings to the intake manifold for cracks or holes?

ramrod

Good idea, although often times the leak is not visible.

Even the o-rings that shrink (not noticeable) cause air leaks

Forg

Some people use a smoke or steam generator of some sort to detect leaks don't they? Visible smoke or steam wafting out of ... I've never seen it done so I don't know what/how ... and if you see it being sucked in somewhere then there's your leak?

dmc

A water spray bottle is another good test. Once all hoses checked spray a mist or jet of water across the intake manifold where it joins the head, if the idle changes you will hear it. Use a spray bottle though, not a hose, if there is a leak you do not want a lot of water getting in there!

Have you checked the codes?

Major Ledfoot

Throttle cable adjustment is good and cable moves freely?
Throttle butterfly not gummed up or sticky?
IAC valve hoses all good? (I've seen rock-hard brittle ones...)
Throttle switch is clean?
Is the speedometer working? (If it's not, the ECU won't be getting a vehicle speed signal, so idle will be all over the place - lack of VSS input even when stationary will make the idle speed go high on startup with LH 2.4)