After I changed my vacuum hoses and venturi delete my RPMs would drop real low for about a second after I let off the throttle, to around 2-300 RPMs where the car would seem like it would stall, and then it would go back up to a normal idle. The weird thing is that when the car is cold it works fine but after a few mins then it does it. I tried cleaning the ISV with carb cleaner and it helped a little bit, only drops down to around 4-500 now. Im just wondering if this would be a symptom of a vacuum leak or do I need to replace my ISV? Im thinking of plugging the ISV connection to the manifold and see if it is really the ISV because I thought it supposed to stop working after the car warms up.

 

1. Connect a Dwell/Tach on the coil (Green is +, Black is -) and start the engine. Idle should be 840 +- 50 RPM.
2. Put a jumper between pins 'B' and 'C' on the diagnostics connector. This will disable the ISV and shift idle control to the mechanical idle screw on the Throttle body.

Note: If there is any significant mis-match between the mechanical idle and "ISV" idle, engine RPM could rise or fall significantly. It may be necessary to either open or close the mechanical idle screw on the Throttle Body before the car will idle with the ISV disabled.

3. Use the idle by-pass screw on the throttle body to set the idle to 840 +- 50 RPM.
4. Remove the jumper from the diagnostics connector. Note: Idle may rise or fall a tad due to the ageing of electrical/electronic components, but idle should be within + - 50 RPM.
5. Load the engine with the A/C, headlights and fog lights. Idle should not change significantly.
6. You're done.

 

Well I think its the ISV now for sure. I just pluged the ISV connectors in the manifold and near the throttle body and then it didnt do the RPM drop after it was warmed up. But I think I'll still do what you said just to double check. Thanks

 

I'm having almost the EXACT same problem.. just did the venturi delete+changed ALL vacuum lines.. sometimes mines really really bad.. down to 300rpm or something... Maybe we just had that horrible a vacuum leak before that we didn't notice it with the ISV not working properly?

One complication, my new (3 week old) idle control screw, well half the head on it has broken off when I was trying to adjust it, so for now I'm out of ideas as to what to do about it..

 

Well guys, I also am having the same problem. Mine started after I installed my APE MAF. It got really bad to where the car was stalling, then I redid all of my vac, and that seemed to help, but then it came back. Well today, I plugged the ISV bungs, and the car almost wouldn't run. I adjusted the idle control screw accordingly, and it seemed to drive well. I hooked the ISV back up, and the car is running fine. I think my idle screw was so tight that the ISC was covering for it, but it just couldn't compensate all the time. Not sure if this is the case or not, but it makes sense to me.

9 out of ten stops are good, the idle stays where it should. HOWEVER, 1 out of 10 stops, the car will still loose the idle and drop down, then bounce up and around until it settles at idle. It now only does this maybe 1 out of ten stops or so, a huge improvement over what it was doing before. I wonder what could be causing this problem......... Oh, and it doesn't matter if the ISV is hooked up or if the holes are plugged. Doesn't change anything.

 

For MAF equipped cars, this problem can be improved by "clocking" the MAF -- try different positions for the MAF by twisting it 10-20% at a time until the problem is minimized.

 

Wow, it would have been nice if that was in the Autothority instructions.....

Thanks for the info!

 

I have not used the Autothority MAF, so can't say for sure it suffers from the same problem (I assume so). Clocking is a bit of a band-aide. It addresses the issue described on speedforce racing site here: http://speedforceracing.com/products...irflowkits.php

 

Sorry to bring up an old thread but I'm having the same problem and I like to use the search!

Mine too happened when I deleted the venturi setup. It seems like deleting the venturi setup causes these cars to not idle as well. I wonder if when porsche deleted the venturi they did something else to compensate?

Anybody have any idea? My car was perfect before I deleted the venturi.

 

Mike the 87 and newer ISV units operate completely differently as compared to the older 85 86 units do
the newer version doesn’t have brushes that wear out the armature but rather turn against a spring and the fail mode is for the center pivot rod to rust and bind up the vane.
Spraying PB blaster into the later model unit will usually restore operation as this will dissolve the corrosion

Note it would be prudent to have the computers checked for driver damage just to rule out that possibility of them being damaged from a stuck Vane