QuestionQUESTION: I've got a 1986 Porsche 944 NA, 5 speed, that will occassionaly start idling at around 1600 rpm, and then immediately back to around 900 rpm, and then right back to 1600 and back to 900 and on and on... Actually, it has turned into the norm, where it now only occassionaly idles correctly at 900 rpm or so and the majority of time is doing the 900-1600 idle "dance"---a real pain. I've checked all hoses and they all look to be good, and the fact that it will occassionaly go back to the correct idle leads me to believe that it could possibly be the 'idle control valve" that is the culprit, not a vacuum problem at all. However, is there a way of checking this out prior to just buying a new ICV and installing it? $200 and about 5 minutes replacement time would be CHEAP at this point if it IS the problem, but I'd rather test first, rather than just replacing and hoping, know what I mean? This has been a well maintained vehicle, so things like spark plugs, wires, etc. are all in good shape, so can I test the ICV for proper operation and/or are there also other items I should test as well that could be causing this idle fluctuation? Thanks!!
ANSWER: From many posts on the net:
To test the ISV.
There is a Test Plug located near the Fuse/Relay box.
Engine warm. You jump pins B & C.
Adjust the idle with largest screw on Throttle Body.
Remove jumper.
Take a short drive and see if problem is resolved.
There is a procedure to attempt to clean the ISV as well.
Otherwise its most likely a vacuum leak and there are plenty
of sources or points to check, especially where a hose connects
to a component the end of the hose tends to harden if not break
in time on a 22 year old car.
If you can find someone with a "smoke machine" you can find
leaks quickly.
If you can't find component schematics for the 944 I on the net
I would search for a used Shop Manual which will save loads of time
in trouble shooting.
---------- FOLLOW-UP ----------
QUESTION: Also, went to test this yesterday morning and found what I assume to be the test plug---a 5 pin plug just in front of the fuse/relay panel accessed by removing a small cover with two phillips head screws, right? Anyway, there is no way for me to tell which pins are B & C as there are no markings on either side of the plug. Located and purchased a Haynes repair manual, but of course there is no mention of either the test plug or the Idle Control Valve, so that $27 could have been better spent. So can you help me out in determining which pins are B & C? Thanks.
AnswerI do not know of any factory procedure or other fool proof test for the ICV other than PS (parts swapping).
Try this:
1) Set the Idle as outlined above; jumping B&C takes the ICV out picture. Warm up the engine. Air conditioning should be off.
Locate the test plug. When looking at the test plug: terminal at 9:00 is "+", terminal 11:00 is "A", terminal at 1:00 is "B", terminal at 3:00 is LED and terminal at 6:00 is "C". Place jumper between B&C.
Try to adjust the idle to 900 +/- 50. Remove jumper and take vehicle for ride. Park and see if car idles.
2) If that does not resolve your issue, some have claimed they can test a faulty ICV by tapping on it when the engine is running. So if you want to give that a go, warm up the engine, take a screw driver and tap the handle end on the ICV and see if the idle problem is resolved.
3) As I recall, when 12v is applied to the unit it will "click" and you will be able to see a slice or gap through the ports.
4) Some claim when they cleaned the ICV it resolved their issue, some say it did not make a difference. There is no factory procedure to clean it, and there are different claims as to the correct way to do it. Here is how I cleaned mine: note the ends of the ICV and hoses they connect to, mark them if you have to. Remove the ICV. Take a can of WD40 and spray it in one end until it runs out the other. Reverse the procedure spraying in other end. Take some brake cleaner and spray that through the unit (you are trying to dissolve and flush out the gumming residues). Then take shop air, low pressure, and blow the unit dry. Follow up with spray of an aerosol silicone spray. Reinstall and see if that solves the idle problem.
If you have ruled out the ICV, the most "common" reason for erratic idle seems to be a vacuum leak and some of the leaks you can't simply reach with your hand, you have to remove things; this is were a smoke machine comes in handy.
Here is a list of other culprits for erratic idle listed in the "Shop Manual".
Air flow sensor or AFM
Ground and plug connections
Fuel pressure
DME, DME relay
Ignition
Fuel injectors
Intake temperature sensor
02 Sensor
Engine Temp sensor
Idle speed switch on TB
and many others.
A Haynes manual or other "aftermarket" publication is not a "Shop Manual". A Shop Manual is the manual published by Porsche used in authorized Dealer repair facilities. You use to be able to purchase them through the Dealer's Parts Dept. Sometimes you can find them for sale in web forums, sometimes on auction sites, Porsche Swap Meets, or electronically on websites .... who are not authorized to do so, such as a torrent; I do not recall any in case you are wondering.
