Idle's Irregularly

[COFFEE]

I have a V8 Limited and all of a sudden they car wont idle smothly.
Considering all the leads and plugs are new i simply checked they were on proplerly and crossed them of the list.
I unplugged the air flow sensor and the car idled fine, plugged it back in rough( up and down etc ).
Got my mates sensor and swapped them over as his car runs fine.
When i plugged it in the idle went up and down again ?
It's got me a little stumped.
Can any one make any suggestions as for things to look at ?
I have since had the leads checked and tested, they are ok and the plugs look fine and the gaps are good.
I am hoping it's nothing wrong with the ecu.

Has anyone had a similar problem that can point me in a starting direction ?

Regards

Stephen

 

[Pro]

sounds like someone's done a headgasket....

 

[mycarhasposessedme]

Checked for a vacume leak

 

[qca]

check for cracked or broken / loose pipe between inlet manifold & air flow meter
then check diagnostic codes (self test)
also check the dist caps for cracks , rotors for cracks and coils for cracks or tracking
(marks left where spark travels )
hope this helps

 

[Lextreme II]

How about doing some test with the idle speed valve? You can test it with a test meter. Test if the ohms are consitant with the repair manual.

 

[COFFEE]

Thanks guys,
I would be suprised if it's a head gasket.
Only on the grounds that the car has done 100,000km's and i have had it for half that.
I have never seen the car get hotter than half way, so i would suspect that unlikely.
I have checked between the sensor and the throttle for leaks but couldn't notice anything.
I think thats more the right track though ! for some reason i keep think it might be leaking from the manifold ?
Idle valves a good one ! did'nt think of that.

Thanks again.

Regards

Stephen

 

[Peewee]

I'm with the vacuum leak.
A crack in the piping somewhere.

If the AFM gets unplugged, I think the ecu will either go into a fail safe mode, or a limphome mode, and just lock the ISCV in a certain position, and hence the idle hunting disappears.