worf928

Ok. So, I have no idea what that describes. There's not much to mounting the ISV to the bottom of the air guide. The two bonded rubber buffers used for that are the same as for the air box. There should be no need to substitute anything.

In both cases above, I was referring to the electrical connections.... because if your idle control loop is not activating you will get the symptoms described.

One very easy check is to see if the idle switch was set-up properly during the intake rebuild. You should be able to hear it click when you turn the throttle linkage every-so-slightly and then it should click when the linkage returns to rest.

ROG100

Sean is indeed talking about the two rubber buffers that mount the ISV to the TB. Actually they do not have a part number anywhere in PET for the ISV buffers. However, you are right that they are similar to the air box buffers. Similar but not identical. The buffers for the airbox are diamond encrusted at circa $25 each so I use a Mercedes sourced identical buffer for circa $6.

worf928

I wasn't sure and guessed.

Yup. I went looking for them a while back and didn't find them. So, I measured the old ones and they are within a MM or two of the airbox buffers give-or-take shrinkage over the years. Hard for me to say that they aren't identical. In any case their height isn't critical in this application as it is with the buffers on an early '87's intake for the fuel rails.

Yup. They are available from various sources in various flavors for around that price.

safulop

Yes it occurred to me later you must be talking about the wiring harness. The TPS was also changed for a new one. This contains the idle switch right? I think the malfunctioning TPS may have caused part of the wild idle behavior I was having prior to the fix-ups. Together with the frozen ISV they made a very interesting situation for the idle.

You're saying the car will have a steady idle with the idle control loop inactive? I just torture-tested it by idling home in a massive traffic jam in the hot sun. It was once again around 700 and stable. It idles about 200 rpm higher in neutral than in Drive. But, it has been doing that since I bought it in 2010. So in that sense, I've come full circle.

SeanR

The reason some of us question how good they are, is each time they have had the car you come on and ask questions about why something is not correct. It's not that we are bashing on them but lordy, if it's not right when they've worked on it and you have to come here to ask, i'd start questioning their competence.

Do they have the Bosch KTS "hammer" to do diagnostics with? Should have been test number 1 after they put it all back together, to ensure that the TPS is getting proper idle and WOT contacts, to check if the ISV is working etc.

You idle should be a rock solid 650 for auto and a tad bit higher on a 5 speed. It should stay there from cold to hot. Moving to 1000 when hot tells me there is an air leak due to expansion. Most always in the intake or vacuum lines. Have them dig back in and find the issue, after all, that's what ya paid them for.

safulop

^^^^^^^^^^
OK well, I don't want to waste breath defending them too much; they really only do what I ask them to do and they are not perfectionists about how the car runs. All I am saying is that I have not found out any mistakes or screw-ups they've made. I don't mean to say they are great 928 guys who can put one into concourse condition -- I would not go that far. But they do know how to reassemble an intake on a wide variety of cars without making mistakes.

Unfortunately they lost their Bosch Hammer when the whole place burned down (with my previous 928 inside) back in 2010. But according to them, the diagnostics from an S4 are so primitive they can do just as well with a voltage and resistance meter, which they use judiciously.

worf928

Yes.

It's more-complicated than a yes/no answer.

worf928

Or the resistance of the temp-II loop is changing.

Or its two or five things that are wrong that are convolving. Given that the shop "are not perfectionists about how the car runs" who knows ...

safulop

OK well, things are improving with my idle as the wonky new ISV gets broken in, or something. I know now that it is not a vacuum leak, unless there is such a thing as an intermittent vacuum leak. The idle stays good when the car is started but not driven. I can let it warm up and the idle stays down low. After I start driving it around, it eventually gets stuck into the condition where it is about 100-200 rpm too high. But the time this takes varies, and is increasing. Last night I made it all the way home, with a stop at the store, on a good idle. This morning it got stuck up in a high idle halfway to the office.

Speedtoys

Ya..heating can change how a vac leak works.

Did you put a pressure test on the system yet?

safulop

OK well, I eventually took this car back to the shop for a checkup and... there is nothing wrong that can be easily identified. There are no vacuum leaks, as I had suspected. The only thing wrong is, the idle speed.

So, the only thing that could cause the idle speed to start low and suddenly jump up a bit when warming up is the ISV, no? At any rate, I am just going to roll with it like this because it runs fine otherwise and I have no other recourse to try to figure this out. We will see what happens at smog check in the next little while.

safulop

I found this info on a VW forum:

Symptoms of a faulty ISV or one that is getting progressively or intermittently faulty are:
1. Idles low or rough when cold (improving after warm-up).
2. An idle "jump" for a second or two (internal problem in ISV).
3. Erratic idling (surging) and stalling, especially when the air conditioner compressor cuts in by not compensating for the extra load. The symptoms would likely appear slowly over a period of time, but getting progressively worse.
4. A real tell-tale sign of a faulty Digifant ISV is to let your foot off the throttle pedal while running (particularly higher revs). If the rev needle drops immediately to the 500-600 range before stabilizing at 800 or so, there is a failure occurring. Sometimes the idle will drop so low that the car stalls. A correctly working ISV should drop to about 1000 or 1100 rpm when letting your foot off the gas pedal and then the needle should slowly ease down to normal idling speed (about 800).

My car idles 100 rpm lower at least with the A/C running. And it also behaves weird as described in #4 above, when I take my foot off the gas while running, sometimes. Like the rpms drop way down and then recover, as described. And it idles lower when cold. So, my symptoms indicate a partially faulty ISV, as I suspected.

FredR

Just one of the many components that get tired and then fail with time. The nature of the beast is to be able to respond promptly to controlled inputs but once there is "stiction" of the shuttle it simply cannot respond as intended and the result is a jerky response.

Some 16 years ago just after I purchased my late S4 the idle went and the main agents diagnosed that I needed a new ISV. I did not buy into this and so decided to have a crack at the inlet manifold for the first time and needless to say found the usual medley of issues. For the ISV I found the shuttle was sticking and that there was a dry black powdery substance fouling it. I cleaned it with cotton buds and cloths until it seemed to move freely once more- put it back and it was just fine.

The same ISV is still in there to this day but now it also has a little bit of a surge that I put down to a tired ISV. I can live with it albeit I find it a bit annoying and doubtless will change the thing out in the not too distant future.

Rgds

Fred

safulop

Or in my case, that was already failed brand new in the box. This is a *new* ISV that is apparently not working correctly.

It was earlier this year during some troubleshooting that Rich Andrade offered his wisdom, that just because a part is brand new out of a box does not guarantee it is a good part.

dr bob

If you haven't already done so,

Check (Ohm Meter, at the controller terminals...) that the idle switch (called "throttle position switch") is true with throttle closed. The ISV is active only when this switch detects that the throttle is in fact closed.

For those with '89+ cars, the MPG display in the digidash will go high (90 MPG) on trailing throttle and RPM's greater than about 1250 or so. With the switch not working, the MPG number won't jump as high or as obviously.