After finding several posts on rough idle, i attacked the ISV cleaning this past Sunday to see if that would smooth out my idle. Thanks to all for the tips. It was definetly "gummed up". However, it did not seem to solve my low idle problem. With the A/C on the car runs much smoother and revs are around 800 rpm. Without the A/C the idle flutters between 725-750 rpms but has not stalled yet. 2 Questions:
1) any other ideas? I just had the major 60k service done recently at 54k (new plugs, wires, rotors, etc..) and i cant believe my Porsche dealership did not see this especially after I spent $1400 for the parts & service.
2) What is the normal idle speed? 800 seems to be smooth to me.
Thx again to a great Porsche site. Joe 96 C4 Cab

 

1) try a can or two of "Techron". Search the archives for more info.

2) Shop manual sez 800 +/-40 RPM.

 

For what it's worth - I had this problem on a previous '97 993 cab and the cleaning helped, but did not cure the problem. A Porsche tech told me to change the entire unit (about $125). I did the swap and the car ran great afterward. IMO, not a bad risk at $125 to try a new unit.

 

I have exactly the same problem. I only have 24K, so I didn't expect the ISV to be dirty, but I cleaned it anyway. It had no effect on the stumble in the idle. It is annoying, but as we run the AC a lot here, not a big factor. As long as it is not a symptom of something serious like a bad plug wire, I'll wait until I can borrow another to try it out.

 

Thanks for all the info guys. I think i will try swaping a new ISV for the $125, although i am a bit skeptical. I would think if the ISV was bad then why does it idle fine when the A/C on? Also, Is there a manual idle adjustment screw somewhere?

 

c4-toy,

I have the same exact problem and I'm debating whether I should change the ISV as well. The lowest price I've found for the ISV is $168 + 15% from Sunset Porsche.

To answer your idling question, in looking at the 1996 service information book it clearly states that the idle speed is 800 +/-40 rpm WITHOUT the A/C and 880 +/-40 rpm WITH the A/C which explains why you have a higher idle with it on.

 

[quote]Originally posted by c4-toy:
<strong>Thanks for all the info guys. I think i will try swaping a new ISV for the $125, although i am a bit skeptical. I would think if the ISV was bad then why does it idle fine when the A/C on? Also, Is there a manual idle adjustment screw somewhere?</strong><hr></blockquote>


Hi:

LOL,...manual idle adjustments were deleted in '84 when they started using Motronic Engine Management. Now, its all done in the software.

Your ECU commands the idle and ISV operating changes when the A/C compressor is triggered.