All,

Update:

I reached out to Rich, He gave me 3 tests I am comfortable doing:

1. Throttle Switch Idle Pin 3 .01 Ohms from pin to ground. Press the pedal 60% throttle and reading was beyond the range of 2000 Ohms
2. Throttle Switch Full Throttle Pin 12 7.18 Ohms at no throttle, 7.21 ohms at 60% throttle (Scale 2000 Ohms)
3. Temperature II Sensor testing Tested Pin 2 @84 degrees (1.76 Ohms) and @104 degrees (.7 ohms). Same results with PIN 1. (Scale 2000 Ohms). I should have run two more tests at higher temperature.

Not entirely sure how to interpret these results. The temp sensor appears to be failing. I replaced it over 12 years ago.

Once tests were done, I dialed down (lean) the orange Idle screw on the throttle body. When I started the car, the idle was perfect, 750 for about 5 minutes. Drove it around the block and it reverted to 1500 RPM idle. (no throttle cable binding) Warms up and goes back to 1500.

One last observation from 6 years ago. I was living in PA (cold environment) and the car would not fast idle at all. Could not drive it until it warmed up. That led me to replace the Idle Sensor Valve (ISV). I tested the new ISV and it does work (I even repaired the old one after it was removed).

So if I can get some advice, I appreciate it.

Boaf

 

Hi @Gandelfthegray

Yes Steve, these are similar symptoms. But mine needs to cool of for 30 minutes to go back down to normal idle, then when warm, right back up.

Bob
ob, my 86.5 is acting in a similar manner. Out of curiosity, if you drive some distance the car will be idling high (1500 rpm or so), now if you shut the car off for a matter of say 5 minutes, will the car (still hot) start and idle normally? This is how mine acts. I can drive to the bank (idling high when I arrive), shut off the car, return a short time later and start the car and it will idle at proper rpm until I drive again. Soon as I drive and reach the next street light it's back to idling 1500 rpm. Curious as to what you find on yours and also if I have the same symptoms.

 

Replaced the Temp 2 sensor, oh well same result: Warm up and goes to 1500. I will remove the "brain" this weekend and send it to Electronik Repair, Inc. Last step before shipping to my shop.

Happy Festivus.

Boaf

 

Was the new idle valve a Bosch or a Lowe? There have been reports of new-out-of-the-box Lowes causing high idles.

 

Non Bosch. I have the old one and it now works. I will replace it with the original. I am running out of options here.

Thanks Dave.

 

did you make any headway on this?
My '86 does the same thing as gandalfthegray
I also recently did a top end refresh and vac line replacement
I also replaced the IAC, but don't know if it was Bosch or Lowe
Still idles about 1200 when warm

 

My culprit was the new throttle position sensor cable had the pins switched. I replaced my ICV with a Lower and then also purchased a Bosch taken from a BMW neither fixed the issue until I dug into the wiring diagram and starting checking everything. That's where I discovered my new TPS sensor cable had the WOT and Idle pins switched at the connector by the diagnostic port. Fixed that and my idle, now I'm right at 675/700...Now my issues is lean vs rich. If you PO messed with the wiring like mine did. Get the wiring diagram and make sure all the wiring is correct before you over troubleshoot and buy unnecessary parts. If you need the wiring diagram for an 85-86 let me know.

 

thanks for the info
If you could send me the diagram, and would be very helpful
I doubt the PO messed with the plug, but you never know
thanks again

 

Here is the 1986 wiring diagram. Let ne know if you can download and access. If not send me a PM and I'll try and send it that way.

 

got it thanks
I may need a degree in electrical engineering though

 

This was extremely helpful check it out.

Reading The ‘83/’84 Wiring Diagrams (“For Dummies”) - Rennlist - Porsche Discussion Forums

 

Dave Roger,

Just making sure I removed the correct module (the one without the Vacuum). Roger, is this it and you can test? Thanks man.

Boaf.

 

Correct the one w/o the vacuum is the LH and controls the fuel and the ICV

 

Thank you!!!

Boaf

 

Fellow 928ers,

Update: I worked with Pete with ElectronikRepair, he tested my LH, all was well. He replaced some resistors. He also returned my unit with a known working LH from his shop. Both were installed and tested. The results:I installed the new unit and the RPM’s started at 1100 which is what you would expect when cold. But after 25 seconds up to 1500. Continued to let the car warm up to operating temperature and sprayed carb cleaner all around the intakes for vacuum leaks, there was no change in idle speed. Turned the engine off.

Put my old unit in and started it up. It went to 750, nice, the engine was still warm. But as soon as I touched the gas, it went up to 1500 and stayed there. Any thoughts on that? ISV? There is no binding that I can see or feel including the cruise control.

Any further thoughts / tests would be greatly appreciated. It's been over a year working on this.
Boaf.