Engine Hunting On Idle

[galaxychap]

hiya guys

 

I noticed today after parking outside work that my engine was hunting while idling she dips to 800rpm then goes up to 1000rpm then with a little press of the pedal she stays at 900rpm.

 

Any ideas as to what it could be

 

Thanks

[johnb80]

Idle air valve needs replacing or cleaning, while you're at it clean the throttle body too. Usually a good squirt of carb cleaner and a good brushing with a tooth brush (use the wifes) does the trick.

 

Regards - John

[galaxychap]

Thanks john can someone tell me what it looks like and where i find it.

[GSMGuy]

Thanks john can someone tell me what it looks like and where i find it.

Normally pink, and found in the bathroom.... lol - Sorry couldn't resist!!!

 

Mike

[johnb80]

I don't know the layout on your Gal but if you look at the pipework you will find a pipe that goes from the air cleaners side of the throttle body to the manifold side of the throttle body. Somewhere along this pipe you will find a small cylindrical device around 150mm long and perhaps 40mm dia, this may be attached directly to the manifold or air cleaner housing. It's a small valve that allows more of less air into the engine at idle, if they get gummed up they stick and you get problems like yours.

 

Regards - JB

[Guest vr6galaxy]

You have more chance of spotting Lord Lucan in your local Tesco's than you have of finding an ISV on you engine :o you dont have one ! the idle speed is set by the PCM which gets a voltage signal from a Potentiometer on the side of the throttle body, this voltage signal is then used to control the idle speed, idle air volume control, fuel after-run shut-off and fuel load enrichment

if this fails then the PCM relies on the MAF sensor and engine speed sensor for information on idle speed, fueling etc

it may be worth your while to check the wireing to the switch block on the side of the throttle body? or just get the codes read to narrow down the cause of this hic up! IIRC the switch cannot be bought as a seperate item but only with the throtlle body :o

another thing to check is the crank breather valve! this is known to cause to cause idling problem and roughness at low engine speeds

http://www.kiter.plus.com/images/PICT0146.JPG

 

this is the valve, but it may be located in a slightly different place on your engine as this picture is of a VW Corrado engine so it's layed out slightly differently to ours

[johnb80]

How does it increase or decrease the idle speed if it doesn't have control over the air supply ?

 

Regards - John

[Guest vr6galaxy]

From what I can gather the butterfly valve in the throttle body has a set opening at rest allowing a set amount of air through, the PCM then adjusts the fueling to suit giveing you a tick over

[johnb80]

I don't think they'd get away with that, mixture is pretty critical with cats and the idle would never be smooth. All the cars I've worked on that have fuel injection of an idle speed control / air valve call it what you like. The ECU has direct control over the injectors and needs to be able to control the air as well. In some vehicles MCC Smart for example where they have drive by wire systems, the throttle butterfly can be controlled by the ECU and thus doesn't need a seperate valve.

 

Regards - JB

[Guest vr6galaxy]

In the past I have had the throttle body plug and the MAF sensor unplugged while working on the engine, the tick over didnt alter even after the engine have been revved! on one occassion I drove to town and back 19/20 miles like this and didnt notice any change in the tick over :o

how the idle speed is maintained is a question I have asked on a couple of dedicated VR6 forums and been give the same answer! I will continue to ask as I want to know the answer :huh:

[johnb80]

The base idle is normally set with the Idle control valve defeated, centred if you like so that it can add air or reduce air. The ECU can then increase fuel and air to increase speed and of course adjust the mixture, I'm still convinced there will be a valve there somewhere.

 

Regards - John