GregBBRD

I've had two or three that were shorted internally and damaged the contol circuit inside the brain, requiring a replacement brain.

I have had quite a few, over the years, that were "stuck", which we cleaned and reinstalled, only to have them restick again. This is common enough that I don't fuss with them. If we confirm they are stuck, they get replaced.

A few weeks ago, we had an engine come in with a known idle problem. It was scheduled for a complete "top end" refresh. The idle stabilizer looked great. We cleaned it and tested it, re-installed it. A few days later, the idle problem was back...and the idle stabilizer was stuck. We "zapped" it with 12 volts and it worked again, only to stick again in a couple of days.

I got the pleasure of removing the intake a second time....on my nickel. The idle stabilzer still looked great and was perfectly clean.

​​​​​

Kevin in Atlanta

Like I said - given the labor to go in a second time I will always replace the flappy actuator and ISV during a top end refresh. Cheap insurance.

Speedtoys

...I said it first...

Mrmerlin

Here is a picture of how a new looking unit can fail


https://rennlist.com/forums/928-foru...-it-works.html

matt968

If you decide to replace your ISV be sure to keep the old one until you confirm the new one works properly. Myself and another Rennlist member had the exact same issue after replacing the ISV. The only solution for both of us was to put the original ISV back in. I personally put 4 different Bosch units in (one I purchased directly from Porsche). I made the mistake of not keeping my original unit and ended up buying a used original one online.

merchauser

I may have missed something: what issue are you referring to?

interested in knowing why? quality control on new parts that bad?

worf928

worf928

Hyperbole Fred. Hyperbole.

I've seen sticky ISVs and a very few that were stuck then unstuck when cleaned and powered up. Those I have replaced more-often than not. Unlike Greg, I've never seen one that was just 100% dead. And I've never seen one that was, as Sean puts it: "... so gummed up and dirty they could not be saved. No matter how much I tried to clean them up " When I do, it will get tossed.

So, it's a mystery to me, the operating conditions that cause them to be so gummed up that they are mechanically stuck. Maybe it's folks squirting WD-40 into the ISV's long hose thinking they have an ISV problem to solve.

Mrmerlin

the other reason they stop turning is when the pivot shaft gets rusted,
this can happen from sitting in a garage where the temperature swings are big and the air has over 50% moisture,
water droplets will form on metal parts and corrode.
I have had 2 of these that were frozen from rust,
one of them I cut open for the thread link I posted above.
A few more that were slow turning were tossed out

FredR

I surely appreciate that- my "desert story" was spot on- I was like the road runner being chased eastwards out of Riyadh- fortunately the roads were empty and I had a 5 lane motorway to myself- unfortunately I did not have a 928 to power me home just a crappy Mazda saloon- not as glamorous as Tom Cruise though- more like Tom Tit!

When this issue struck me some 20 odd years ago what surprised me was the material that caused the problem. For the life in me I have no idea what it was- it was a bone dry powdery greyish substance and yet no sign of any oil even though I saw the expected pile of oil in the lower manifold. I was hoping someone like yourself might just know what the crap was. Never seen it since so it was probably a fluke of some kind.

Given my ISV was 25 years old I decided to give the Lowe unit a try whilst I had the manifold off the car to do the knock sensors. It did not solve my small idle issue and upon completion, the idle was pretty much the same as it was with the previous original unit so if the Lowe unit takes a dump I would probably stick the original unit back in given how much I like pulling the inlet manifold!

matt968

I may have missed something: what issue are you referring to?

Sorry. This is the original (long) post I was referencing.
https://rennlist.com/forums/928-foru...e-problem.html

Here is the "Readers Digest" version:
We both had the same issue where the new ISV's worked properly from cold start but when the car reached operating temp the idle would just switch from normal (675 +/- 25) to around 800-900. Obviously the first thought is an air leak. However, I had previously replaced every hose and connection plus smoke tested the intake several times. I was certain it was not an air leak. We both eventually came to the same (independent of each other) conclusion. The valve opens and closes based on duty cycle. At normal idle my new Bosch valve held correct idle around 41%. (At cold start, duty cycle was about 45% and slowly dropped to approx. 41%). Then for no apparent reason duty cycle would fall to less than 20%. My understanding is that at around 35% the valve is fully closed. If you could look inside the valve at 35% duty cycle the air passage should be completely blocked. And as the duty cycle continues to fall, the movable vane continues rotating but begins to re-expose an opening in the air passage, therefore, INCREASING air flow which results in an increase in RPM. If you look at your ISV with it off the vehicle you can see the air passage is partially exposed. I believe this is what the valve looked like at such a low duty cycle. This is its "fail-safe" position...meaning, If the valve were to lose power for some reason, you would still be able to start the vehicle. To confirm this, when my S4 had the problem and switched from normal idle to the higher idle I tried disconnecting the ISV electrical connector. As expected, it made no change in the idle. In fact, I could start it and drive it with no problem...other than the idle would be around 800-900. Incidentally, with the ISV electrical connector connected and the car operating I read 12v at the valve and it operated at about 97Hz. Voltage drop on the ground side of the electrical connector showed less than .1V.....unfortunately, I did not measure current at the time.

interested in knowing why? quality control on new parts that bad?

I have no idea why the duty cycle would fall that low. I personally only used Bosch ISV's when swapping them out (all provided by Roger except the one I bought from Porsche). The other Rennlister tried the Lowe and a Bosch with the same results. It wasn't until he tried putting his old one back in that fixed his problem - so I tried the same and it fixed mine as well. It's odd that we both had the exact same problem and that we each only changed the ISV when the problem started. Also, thinking it was the LH as the source of the problem I had my LH rebuilt by Ott's and it made no change. But honestly, I do not know what is involved with a "rebuild" so I do not know if this would have had any affect.

I don't think it has to do with quality control of the ISV's since Roger said he sells plenty of them and has not had anyone else with the issue. The other Rennlister lives in England and I do not know where he got his parts from.

Just remember to save your old parts until you confirm the issue you are working on is fixed.....something I know, but neglected to do this time!

Incidentally when I replaced my original ISV it was working fine! I only changed it because I had previously performed a full intake refresh and this was the only item I did not change. Knowing what I know now and having removed my intake a dozen or more times....I would wait until the ISV needs replacing.

merchauser

^^^^^ thanks for the explanation.

Boaf 32

Gents,

Been struggling with an Idle problem on my 86 1/2 928S for 5 years.

Symptoms:

1. When cold, put the car in gear and RPM'S drop and the car stalls.
2. Have to restart it and let it warm up for 5 minutes before driving it. If I try to drive it before operating temperature, it stalls.

Is the ISV on the passenger side and accessible without Removing the intake? Good resources said yes, with some knuckle bleading.

Thanks
Bob

davek9

First question: What is the idle speed when cold (before you put it into gear) ?

And yes it can be replace w/o removing the intake runners from the heads, you will need to remove the side Plenums, MAF and center "T".
Note:: a bad / sticky ISV will cause the ISV driver circuit to fail requiring an LH repair.

Dave K

bureau13

For what it's worth, as long as the screws on a couple clamps are clocked such that you can reach them with a long screwdriver, you don't even need to take the center T out, just the airbox, MAF and side plenums. My problem was not getting to it, it was finding one that actually worked for my car. I tried 3 or 4 different new ones (none of them were Bosch though...can you still get those new?). They were all SUPPOSED to work, and none of them kept my idle under 1500. What worked was buying a used Bosch. Which isn't a great long-term solution, but it's held up for about three years, so my fingers are crossed.