erratic idle speed question

[dave m]

Any budding car mechanics out there?
I've got a 2.5litre 24 valve V6 petrol engine and its engine management system is playing silly b*ggers on me. The idle speed is normally constant at around 750rpm, but for no apparent reason it can climb up to 2 or even 3000rpm and either climb down again or stay there for a couple of minutes. Other times it will seesaw between 300 and 3000 rpm making you feel a complete idiot as you kangeroo up the road looking like a learner. And having your engine screaming away at 3000rpm at a red light next to some sports car who thinks you want to race can be a tad embarassing too. Is this likely to be electronic ie expensive or a dodgy sensor somewhere?

Dave

[GT98]

Well Rags is the resident Auto Mechanic here, but a quick thing you might want to do is dissconnect the battery for a couple minutes to "reset" the cars computer to see if that improves anything. Other thing might be the Fuel Filter might be clogged.

[SpiralDragon]

i hate carz.... cant seem to figure out how to drive one sucsesfully.....

[Greebe]

Could be a vacuum leak through the rebreather line (or elsewhere depending on engine design). Would also think that a fauly O2 sensor would cause similar operation.

[Gurm]

In many European cars (mind you I am referring to Mercedes, BMW, Volvo, Saab here, not Peugot, Rover, or some other little POS) the O2 sensor will do that to you at idle.

Or... a clogged fuel filter...

Or... hey wait this has all already been suggested.

- Gurm

[Dogbert]

I had the exact same problem in my cat (Peugeot 205 1100cc injection).
I found this on google trying to figure out what's the name of the failed part in my engine:

http://community.webshots.com/album/37911433dKedPX

Fuel Injection Trouble Tips 7

Erratic idling speed

1. Engine & battery earth connections
2. Air intake system for leaks
3. Auxiliary air valve/idle speed control valve
4. Idle speed & CO content
5. Fuel injection system wiring & connections
6. Coolant temperature sensor
7. Knock control
8. Air mass meter
9. Fuel pressure regulator & delivery rate
10. ECU & connector
11. Limp home function

I had problems with no.3 (idle speed control valve) on this list . Part replaced and problem is gone.

[GNEP]

What's a "Limp home function"?

Does this get you back in an emergency on 3 wheels?

[Dogbert]

Dunno, ask someone who knows about engines.

Here's an explanation about the idle speed control valve
And another one.

Here they call it "Stepper Motor" / "Idle motor". Took me a while to find the right term for that thing. In my car it's a Magenti Marelli.

[Drizzt]

Dog, unless it is a fake Idle Motor, it's a Magneti Marelli. Not a Magenti...

[KRSESQ]

If you try disconnecting the battery to reset the computer, you might have to leave it set for longer than a few minutes. On my Taurus I had to leave it disconnected overnight. The next morning I reconnected and all was good. Previously I had disconnected for 20 minutes and it had no effect.

Similar problems. Surging like there was an intermittent vacuum leak, dogging and hesitation when trying to maintain a constant speed. Replaced the throttle position sensor and every inch of vacuum line under the hood. Nothing worked until I disconnected the battery overnight, and that was accidental, because I needed the battery for something else (camcorder, of all things)!

Kevin

[GT98]

If it was an O2 Sensor, it should have turned on the check engine light...then again it is a European car and might not have the same engine control software as a US one

[dave m]

Actually it's Korean, a Kia Sedona, but I believe it borrows it's engine design from the newish Rover 75. Lots of interesting suggestions to look into, thanks. I do have a check engine light and I haven't noticed it coming on. I'll start with the disconnect battery, nice and simple.

Dave

[thop]

totally OT: gurm i think you should write a book, i already have a title for it

[Agent31]

Originally posted by GNEP
What's a "Limp home function"?

Does this get you back in an emergency on 3 wheels?

It tends to be a gearbox mode where by it automatically selects 3rd gear and keeps the revs low, like safe mode after a blue screen except more of a worry if your car is worth anything to you!

[GNEP]

Ahh! I see now... cheers A31