Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 924/944/968 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
Join Date: Aug 2013
Location: Dripping Springs TX
Posts: 9
87S Idle/AC interaction

I have the drooping idle problem where the idle dives to 400rpm or stalls when the AC is on. It will recover to 950 but will even dive when the gas pedal is lifted at 1000rpm. I have seen many 'repairs' listed that seem to solve the problem but I am trying to understand how the system works. For this car specifically, does idle kick up when the AC is turned on?? Mine does not. I see a wire that goes from the AC clutch to the DME. I assume it changes something in the idle condition. Does it raise the idle speed or make the ICV respond quicker? Any suggestions on how to solve this problem would be appreciated. Hate to replace the ICV when it's something else (pull manifold).
Old 09-26-2013, 09:42 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Registered User
 
Aussie944S's Avatar
 
Join Date: Sep 2011
Location: australia
Posts: 244
Garage
Bad icv will do this

Mine idle up for split second on engaging Ac button, then settles.

This is my MHO.... good luck.
Old 09-27-2013, 02:05 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered User
 
tamathumper's Avatar
 
Join Date: May 2012
Posts: 383
As a means of diagnosis, how does it act when you're driving and let off the gas and push in the clutch, like when you're approaching a stop sign? If it "crashes" when you let up on the throttle, it's likely the ICV. It's my understanding that the DME increases idle slightly when the AC is engaged. These cars, although equipped to have an AC compressor mounted on them, were not really capable of driving one without significant parasitic loss. I recall a tremendous drop in performance with the AC engaged.

In my experience with an '87 S, when things were running well the idle would descend smoothly to about 840, recover quickly if it dipped slightly below (if at all) and remain steady (with the AC OFF) or about 950 (with the AC ON). When I had a bad ICV, it would "crash" all the way down to the low hundreds and sometimes the car would die, regardless of the AC being engaged or not.
__________________
'78 924 "NA"
'87 944 "S"
'93 968 "SC"

Last edited by tamathumper; 09-27-2013 at 03:02 AM..
Old 09-27-2013, 02:58 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
AFM #725
 
Gawernator's Avatar
 
Join Date: Apr 2012
Location: San Jose, CA
Posts: 5,159
Garage
Sounds like the ICV to me
__________________
Watchdog Armory Racing AFM #725 Thanks to my 2017 Sponsors: JPH Suspension | AXO | Armour Bodies | BELL Helmets | Braven | EVLUTION Nutrition | Forcefield Armor | FunTrackDayz | Galfer | Motion Pro | MOTUL | Matrix Racing Concepts | ODI Grips | OGIO | Shorai Inc. | SPEEDMOB | Vortex | Woodcraft | Wicked Audio
http://www.gawerracing.com
Old 09-27-2013, 02:41 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
gtroth's Avatar
 
Join Date: Oct 2003
Location: CT
Posts: 621
I'd say your ISV is gummed up and sticking like mine was when it was doing that. You can try to remove it and clean it with carb cleaner, but since you have to remove the intake, it's a bit of a pain to do t all again if this doesn't fix it, so you might just bite the bullet and replace it. You might find less expensive non- porsche equivalents.
__________________
1995 993C4 guards red
1988 944S alpine white
Old 09-29-2013, 04:05 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
Join Date: Aug 2013
Location: Dripping Springs TX
Posts: 9
Ok, a little more information. I cleaned the ICV and tested it by applying 12v, it snapped open very quickly. Without AC, the idle is stable at 950 and does not crash. With AC on, it dives to 400rpm and sometimes stalls. I tested the wire/diode that runs from the AC compressor to terminal 40 on the DME, and it has continuity. My idle ignition advance is 8 degrees, on the low end of 10+/-2 listed in the book.

My question is: do other cars idle higher when the AC is on? If so, my DME may be at fault, as my idle does not increase w/ AC. If not, why does the DME recieve an 'AC on' signal? Does it change the rate of ICV activation? I'm OK with changing the ICV if need be but this does not add up. Could the ICV just be 'tired' enough that it won't respond to the extra engine load of the AC?

Thanks!
Old 09-30-2013, 06:42 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Just thinking out loud
 
mattdavis11's Avatar
 
Join Date: Nov 2001
Location: Close by
Posts: 5,155
I don't have a clue about the ICV, but if it's an electrical problem, you should be able to take the belt off to see what reaction the motor has without the compressor dragging the motor down. The compressor coil will still function as it should, and the DME would read it, but the internals will not rotate (obviously).

It might be the compressor. I have good compressor parts for your 944S here in S. Austin.

My compressor drags the motor down, no idle kick up. It's an early car though, and the PO's mechanic could have disabled any idle kick up if there ever was one.

Most cars do idle higher with the ac on.
__________________
83 944
91 FJ80
84 Ram Charger

Last edited by mattdavis11; 09-30-2013 at 08:17 AM..
Old 09-30-2013, 08:11 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
tamathumper's Avatar
 
Join Date: May 2012
Posts: 383
Does the idle drop the entire time the AC is "on", or is it only a problem when the AC compressor's clutch is engaged?

Although I thought from memory that there was an AC "bump up the idle" function, I see no reference in the 944 workshop manual to an idle speed adjustment of any kind, not just related to AC. The closest verbiage I could find for the 944S is on page D24/28-29, and states

"Information for idle speed and idle speed CO test:

a) Idle speed

944 S engines have adaptive idle speed volumetric efficiency regulation since 1987 models. This means that the idle speed of all versions does not have to be adjusted. The system of cars with adaptive idle speed volumetric efficiency regulation should be adapted each time the DME control unit had been disconnected on permanent positive or after each inspection.

Idle speed control value for engine at operating temperature: 840 ± 40 rpm"

BTW, have you considered that an idle of 950 seems high and might indicate a vacuum leak that gets compounded by a parasitic loss from the compressor?
__________________
'78 924 "NA"
'87 944 "S"
'93 968 "SC"
Old 09-30-2013, 08:26 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
9FF 9FF is offline
Registered User
 
9FF's Avatar
 
Join Date: Sep 2011
Location: Palm Beach, FL
Posts: 1,414
I doubt it's the ICV, iirc that basically acts as an throttle air bypass letting more or less air into the intake to keep idle stable when the engine is cold and the TPS shows closed. It's like an old school choke and only works when the engine isn't fully warmed up. So if this issue happens all the time, not just on cold starts, I cannot see what the ICV has to do with it.

I have heard and read comments about the ECU increasing idle with the AC switched on but as stated ^^ I also cannot find any reference in the manual and it doesn't happen on any of my cars. I think the only thing that ECU signal does is tell the supplementary fan to run at full speed when the AC is on.

My guess is your AC compressor or fan is drawing too much load and causing a dip in power enough to stall or almost stall at idle...or your alternator regulator is on its way out...or your TPS is dead.
__________________
Mike A
9TECHNIK | TRANSAXLE ÄRA
1986 944 (Street); 1986 944 (Track); 1986 951; 1989 951 (3.0L 8V); 2000 996 Cab.

Last edited by 9FF; 09-30-2013 at 01:20 PM..
Old 09-30-2013, 01:14 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Registered User
 
Join Date: Aug 2013
Location: Dripping Springs TX
Posts: 9
I think I was wrong on the idle speed, it's just above the first tick mark below 1000, so probably more like 850.

When the AC is on, if the engine does not stall, it will recover back to normal idle speed. It seems like a transient problem. It will even dive when you hold the engine speed to 1000 and let off the gas pedal. To complicate matters more, I was reading a Bosch Fuel Injection book by Probst, it says the motronic system also uses ignition advance for quick speed response and then follows with the slower ICV idle air increase.

If no one's cars are seeing an increase in idle speed when the AC is running, I guess it may be normal and indicates my ICV is just slow to respond. We just rebuilt the motor this summer and had the PERFECT opportunity to replace it. Trouble is, you have to draw the $$ spend line somewhere. Bad enough we needed a replacement short block due to a severely failed #2 rod bearing (lost on the track).

Sometimes you say 'what a great car' when driving it.....then you look at how the engine is designed and say 'what a piece of trash, amazing it runs at all!'
Old 09-30-2013, 02:02 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
Registered User
 
gtroth's Avatar
 
Join Date: Oct 2003
Location: CT
Posts: 621
ISV is not the same as the ICV. The ICV on the 8V engines is an on-off valve - opens when engine is cold. The ISV on the 16V engines is a proportional valve driven by a pulse width modulated signal and provides dynamic idle adjustment to hold the idle at 850rpm, warm or cold. It's in a feedback loop, so if it's slow the response lags and the idle can see big errors, even stalling.
With AC on, idle should be higher. ISV opens a little to raise the idle.
__________________
1995 993C4 guards red
1988 944S alpine white
Old 10-01-2013, 06:30 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Registered User
 
Aussie944S's Avatar
 
Join Date: Sep 2011
Location: australia
Posts: 244
Garage
Quote:
Originally Posted by gtroth View Post
isv is not the same as the icv. The icv on the 8v engines is an on-off valve - opens when engine is cold. The isv on the 16v engines is a proportional valve driven by a pulse width modulated signal and provides dynamic idle adjustment to hold the idle at 850rpm, warm or cold. It's in a feedback loop, so if it's slow the response lags and the idle can see big errors, even stalling.
With ac on, idle should be higher. Isv opens a little to raise the idle.
+1^
Old 10-02-2013, 02:37 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #12 (permalink)
Registered User
 
Join Date: Oct 2006
Location: va
Posts: 2,036
Garage
Maybe the ISV is sticking. Try to spray some carb cleaner in thru the hose without removing the intake...
Old 10-02-2013, 03:45 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #13 (permalink)
Registered User
 
Join Date: Aug 2013
Location: Dripping Springs TX
Posts: 9
I was reading another string that talked about a diode in the AC compressor wiring. They showed the odd rectangular plug that is near the compressor and said it was a diode. In the wiring diagram, a diode is shown in the green/red wire going to the DME, but it is shown close to the DME rather than out on the end at the compressor. Our wires were damaged so I eliminated that rectangular plug and re-terminated them to conventional crimp spade. Is it possible that was the diode shown on the diagram and it's removal somehow messes up the DME operation of AC idle?

Again, the ICV responded during testing and it was cleaned with carb cleaner off the car.
Old 10-02-2013, 11:56 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)
Registered User
 
Join Date: Oct 2006
Location: va
Posts: 2,036
Garage
I would try to restore the electrical system to the original configuration. The change you describe might be the cause of your idle speed issue.

It is a good thing you have the knowledge of the change. If a previous owner did this it would take a lot more detective work to find the problem.
Old 10-02-2013, 04:31 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #15 (permalink)
In the Fires of Hell.....
 
kdjones2000's Avatar
 
Join Date: Oct 2007
Location: Phoenix, AZ
Posts: 2,917
Garage
DSDoug, did you ever figure out the issue here?

I have pretty much the same issue on my '87 944S, and am looking for possible solutions.

It's really annoying to have this minor niggle in an otherwise well running engine.
__________________
NASA/PCA Instructor
'88 951S - with LBE, Guru chips, 3Bar FPR, 1.3mm shimmed WG, 3120 lbs, 256 RWHP, 15 psig boost
1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
1987 944S - Restored salvage title, ready to live again
Old 09-10-2018, 02:57 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #16 (permalink)
Registered User
 
Join Date: Nov 2018
Posts: 2
I'm also very interested to know the result here. i'm having a similar issue with my '97 944S and need some help. In the pic below, is that pointing to my ISV?? If its too difficult to confirm from this photo i can try to post another. Let me know and thanks.


Last edited by ZePorshe; 11-05-2018 at 05:07 AM..
Old 11-05-2018, 04:36 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #17 (permalink)
In the Fires of Hell.....
 
kdjones2000's Avatar
 
Join Date: Oct 2007
Location: Phoenix, AZ
Posts: 2,917
Garage
The solution for me was to ensure that the TPS idle switch was clicking every time. The arrow is in the general area of your ICV, you just have to remove the intake manifold to get to it.
__________________
NASA/PCA Instructor
'88 951S - with LBE, Guru chips, 3Bar FPR, 1.3mm shimmed WG, 3120 lbs, 256 RWHP, 15 psig boost
1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
1987 944S - Restored salvage title, ready to live again
Old 11-05-2018, 05:16 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #18 (permalink)
Registered User
 
Join Date: Nov 2018
Posts: 2
Here's another picture with a closer angle. From what I've read my problem is likely the ISV. Can you point me in the direction if this isn't correct?

Old 11-06-2018, 08:44 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #19 (permalink)
In the Fires of Hell.....
 
kdjones2000's Avatar
 
Join Date: Oct 2007
Location: Phoenix, AZ
Posts: 2,917
Garage
Yes, that's it. New they are $$$
__________________
NASA/PCA Instructor
'88 951S - with LBE, Guru chips, 3Bar FPR, 1.3mm shimmed WG, 3120 lbs, 256 RWHP, 15 psig boost
1987 924S 944 SPEC racecar - '88 pistons/DME, short 5th
1987 944S - Restored salvage title, ready to live again
Old 11-06-2018, 03:27 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #20 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 08:41 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.