Alpine951

been trying to track down lumpy lean idle and lean cruising past year. When I jumper the ISV port and try to increase the idle it won't increase. The main focus of my testing has been leaning towards a big vacuum leak but have not been able to find it. Would not being able to adjust the idle be an indicator of a large vacuum leak that it is so big that the tiny screw on the TB wouldn't have any affect?

Dougs951S

I had the same issue, investigation lead me to discover that the ISV was not getting power because the PO hacked up the harness that fed power to the valve. My solution was to delete the ISV and not look back. I have a rock solid 1000 rpm idle and the car is running the best it ever has.

944Phil

Your ISV might be dead or in need of a cleaning with carb cleaner.

Alpine951

but when I have a jumper wire or not the idle speed does not change at all while adjusting the screw.

divil

Do you hear a difference when you attach the jumper wire? Check when the engine is cold - that way the ISV should be mostly open and you should hear the difference when you disable it. If you don't notice any difference then it might be stuck open.

raleighBahn

Yes that is strange. As other posters have said, you should hear it when you jump the ISV. By hear it meaning slight change in engine rpm when you jump it.

After that, are you saying that no matter which way you turn the idle screw (on top of throttle body), it's not changing the idle?

Are you still on original throttle body seals?

divil

I was thinking more of the hissing noise that you can hear when it's open, since if you have a rough idle to begin with you might not notice a change in rpm.

Alpine951

I hear no change in idle speed when jumpering. The TB seals where changed out 7 or 8 years ago. Recent multiple intake pressure tests show no leaks at the TB.

raleighBahn

Interesting. Have you done any of the electrical test steps on the ISV?

MM951

I need to do some searching, but IRRC there is a inline fuse somwhere in the harness coming from the diagnostic port in some years. I have the same issue, though I was able to achieve a 800rpm idle in all conditions, I am nearly bottoming out my idle adjustment screw

Alpine951

No, but I am thinking of just removing the isv and capping of the intake and maf pipe.

raleighBahn

Yes that is definitely a good idea - fuse or no fuse, getting a multimeter across those ports would be prudent.

Humboldtgrin

Have you checked the condition of the o-ring at the throddle body to intake manifold?

Techno Duck

I had a similar problem on my 911 3.2 which uses the same ISV. What i found was the operation of the ISV was intermittent. I think there must have been a dead spot in the armature because if i tapped the ISV down or up (moving it up or down on the shaft) it would work or stop working depending on direction of my tapping. I have gotten the ISV out before without pulling the intake, you can snake it out from the front with the intercooler pipe and air box (if you still have it) out of the way.

You can hook up a 9v battery to the leads and alternate the polarity to open or close the ISV with it still installed on the car, you should hear and feel the ISV open and close.

Its possible you cant adjust the idle because the idle screw is at its max adjustment range due to the ISV being stuck in position.

Alpine951

No, but under a pressure test there is no indication of leak there or when spraying some starting fluid there.