FRporscheman

Very interesting thread. Perfect timing for me, because I've chased down an issue in a friend's 968 to the ISV.

I tested it for leaks with my MityVac, and it doesn't leak (held 20 in Hg). I haven't opened it yet.

I reached in with a screwdriver and tried turning the door manually. It seems to be on a pretty strong spring, which holds it at about 1/4 open. I can open and close it with the screwdriver pretty easily/smoothly (doesn't seem to need cleaning) but it jumps back to 1/4. On my spare 944 ISV, there's no spring action, it just sits wherever you leave it. Is it supposed to have a spring?

gruhsy

Something doesn't sound right there

FRporscheman

So yours is totally free-moving?



I put in a hose coupler to connect the ISV hoses together, so I could start the car with no ISV. (I put a restrictor in first.)

Then I plugged the ISV into the car's electrical connector, and watched its behavior as I pinched and unpinched the ISV hose with the engine running. It did seem to open and close. I probed the wires with my friend's portable scope, and the pulse looked good for open/close signals. I saw the pulse width change as the ISV moved in response to me pinching the hose.

Both of my ISVs acted pretty much the same. Anyway, don't mean to threadjack, just wanted to know if there was a spring inside when you took it apart. Maybe the 968 units are different and they have a spring inside.

Just for fun, here's what the ISV signal looks like. I should've zoomed in...

gruhsy

All input welcome Thanks for posting that....nice test procedure.

Maybe some aftermarket ones have a spring? Not sure why they would though because it works by changing the power between 2 point. Ground one side it rotates clockwise...ground the other and it rotates counter clockwise. Power applied should hold it to one side or the other is how I am seeing it.

FRporscheman

It's a Bosch unit with a 1990 date stamp, so I think it's original.

At first I agreed with you, but now I think without the "centering" spring, the DME has no reference point, it doesn't know where the ISV started, so it doesn't know how much to close it or open it. But then again, it shouldn't matter, because if the idle is high or low, the DME will just try to close or open the ISV more than it is.

Maybe the 968 has the "centering" spring because the 968's idle is not adjustable, the computer sets it automatically. On a 951, you can adjust the idle, and in fact, you have to disable to ISV to set it right. So that must be it.

At any rate, both units operated pretty much identically, so I think it doesn't really matter. I think the issue with this car is something besides the ISV. Oh well, I wouldn't have learned all of this if I didn't take it out and test it! This thread gave me the idea so thanks!

(I put "centering" in quotes because it actually holds it at about 20% open, not 50%.)

gruhsy

Sorry I see I missed your first question.

Yes mine without power is totally free moving with no restriction. I wonder if the voltage supplied to it varies at all....probably not because if I am thinking correctly a variable voltage should only require 1 negative that would position the thing from open to closed. Would think it would be designed different internally too if that were the case.

I am going to go ask one of my engineer friends and will post what he say's if it helps.

Tom M'Guinn

Mine moves freely on the bench too -- no resistance or spring memory at all. Here's a video I did for someone a while back...

FRporscheman

Thanks Tom, great video. My spare 944 ISV behaves exactly like that. I'll see if I can take a video of the 968 one later, but the only difference is that it returns to a partially open position with no current applied.

LUCKY DAVE

If I were rebuilding one I would use a silicone O ring to deal with the heat. They cost about the same as Viton. I wouldn't use nitrile.

Laust Pedersen

To put the sealing of the ISV in perspective here is my data point on its effect:
Idle vacuum improved in the range of 0.5 to 1.0 in-Hg and still running in the closed-loop (fuel) mode.

As to the optimum O-ring material, here is what McMaster has to say about temperature ranges and chemical compatibility: https://www.mcmaster.com/#o-rings/=1782d02
The "AOS challenged" cars (track cars among them) will have oil and some fuel to deal with as well.

Laust

gruhsy

So I searched through the 0-ring store and I think McMaster might be easier to order from because the high temp silicone o-rings from the 0-ring store are not available in the 3mm x 33mm ID for the idle stabilizer.

I used 2 digital calipers to measure the size on the idle stabilizer for an accurate measurement. One of the calipers is a calibrated unit from work. Both calipers measured the same at 33mm

Here is the McMaster link for the 3mm x 33mm ID

https://www.mcmaster.com/#5233T201

here are also the links and sizes for the throttle body minus the giant o-ring that were in my Arnworx kit since o-rings are being ordered may as well get a bunch

https://www.mcmaster.com/#5233T51

https://www.mcmaster.com/#5233T54

https://www.mcmaster.com/#5233T59


And the tech info for the high temp silicone o-ring that are suitable for the air stabilizer...metric at the bottom

https://www.mcmaster.com/#o-rings/=17866ms

For use with:

Acetic Acid
Acetone
Ammonia
Ammonium Hydroxide
Animal Oils
Bases
Boric Acid
Butyl Alcohol
Calcium Hydroxide
Citric Acid
Diluted Salt Solutions
Ethanol
Glycerin
Methanol
Mineral Oils
Phosphoric Acid
Potassium Hydroxide
Sodium Bicarbonate
Vegetable Oils

markl951

I ended up ordering the Viton -218 O-ring Size: 1 1/4"ID X 1 1/2"OD X 1/8"CS from both the O-Ring store and and on eBay from Jet Gasket and Seal Company. That size seems to fit pretty well. I haven't put the AOS completely back together yet but when I do I'll add some lubricant to the o-ring to make sure it doesn't tear when pushing the cover back down over it.

I ended up going with Viton vs Silicone as I seem to drag a lot of oil though the intake. Viton is also more durable. Viton and Silicone have the same 400F max operating temperature. You can download detailed datasheets for Viton and Silicone from the O-Ring Store that have more detailed info than what you'll find at McMaster Carr.

-218 V75 Viton O-Rings
-218 S70 FDA Silicone O-Ring 70D

Given all that, either material will probably work fine. Maybe we should update this thread in a year or two after some heat cycles and report our findings?

gruhsy

I ended up ordering V75 Viton O-rings from "The O-ring Store LLC"

3mm x 33mm for the idle stabilizer

Leaks with old O-ring but not with new O-ring.

black944 turbo

I know this is an old thread. Laust you said your isv still leaked after new seals, would you recommend just using job weld or black silicone if your isv is working 100% correctly other than leaking? I am having 0 issues with my isv but it is leaking pretty badly.

Laust Pedersen

As I recall the O-ring (helped with silicone grease) worked well and the additional leakage was in the mounting of the electrical connector on top. I don't recall if the pins in the connector also presented leaks. In any case I fixed that problem with JB-Weld.