Ed Scherer

Stefan, I haven't done the intake refresh yet, so I don't know what's waiting for me in there (other than that which can be derived from external stimulus and observation ) when I do get to it.

With respect to the air leak... I can hear it leaking at the point noted in the original post.

Thanks for the info on the Y being a barbed fitting into the MAF boot. Now I can look and see if there's enough room to pull it out without removing anything else.

Bill Ball

Or try to snug a tie wrap tight around it. You might be able to sneak a long one around it and grab it with long forceps (sold at HF).

Jim M.

Ed, I'm having a similiar problem on my GTS. Fine when cold but after it warms up it starts to surge and sometimes stalls. I have a vacuum gauge in the console so I know that hasn't changed, therfore don't suspect a leak. By pure luck I did a battery disconnect and it fixed the problem for about three weeks. Problem came back and another battery disconnect again fixed it. At this point I suspect a failing LH.

Try the battery disconnect before tearing into the intake removal. YMMV

VehiGAZ

You will be this kind of happy and joyful when you do your intake R&R:

Ed Scherer

OK, I updated post #1 with what I believe is a better description of the current problem. I also updated the thread title, because I think my original description of the problem wasn't entirely accurate. This morning, I tried to be a little more observant and accurate as I noted what was going on.

The average idle speed really doesn't appear to be off that much: perhaps 650 RPM (I believe the '90 S4 should idle at 675 ± 25 RPM). But it's generally a little rough, and sometimes (at random intervals, but generally a few times a minute) it dives down to maybe 500 RPM or so, and then recovers. Opening the throttle a bit and trying to hold 1000 RPM, it is still rough, and still does the periodic RPM dives and recovers. More or less the same at 1200 RPM. Take it up to 2000 RPM, though, and it smooths out pretty much.

I think that "RPM dives" is a more accurate way to convey what's going on, as "surge," I think, implies a "rise/fall" sequence; what I'm seeing is a "fall/rise" sequence: RPMs dive, then recover. It seems as if the engine is starved of air or fuel for a fraction of a second and then comes back.




Responding to a few earlier posts now:

You can barely get to that one rear bolt, but it's doable with a ball head hex key. It actually wasn't as bad as I expected. My bolts weren't that hard to break loose; I've heard that many people have had seized bolts that snapped off. One of the other bolts (not the hard-to-get-to one) on mine came close to binding up about halfway out and I actually pulled a little thread out, but a thread chaser cleaned things up well enough that I had no further trouble (and I used anti-seize upon reinstallation to avoid any such incidents in the future).



Tried it this morning. Didn't help.



Tried this earlier this morning, after engine was fully warmed up. I thought I was seeing an improvement at first, but then... it seemed pretty much the same. Or maybe it was a little better, but not dramatically. With the O2 sensor disconnected, I'm pretty sure I could have driven the car. With the O2 sensor connected, based on an attempt a couple of days ago, I didn't even want to take the car out of the driveway because I didn't think I could keep it running; it was damn close to stalling when in gear, and it took both feet (one on the brake and one on the throttle) to get it back in the garage.

I'm not sure what to conclude from this.


No doubt, when I'm done doing the intake R&R. I suspect it'll be more like this, as I'm doing it: .

Stromius

Ed - Intake is doable. Just a lot of parts so get a sharpee and ziplock bags. No problemo. Have Roger get a kit to you. My experience is on RW: http://reutterwerk.com/forums/showthread.php?t=20872 .

I would say in hind sight, I wish I had been more careful with the CPS/CPS plug and thought about having the intake cleaned (it was OILY inside) and either rattle canned or powdercoated. But either way, I am happy I did this job.

It can be done in a day but basics are depressurive fuel, remove fuel rail and injectors, clean, remove, clean, change bits, reinstall. Can't say enough to track each bolt/nut/part in a marked ziplock. Great write ups by Sir Dwayne, Pirtle, 928OC D.C. etc.

John Speake

What vacuum do you have at idle on the front fuel damper ?

Ed Scherer

Yeah, maybe I just need to do it now. I could always just do it as expediently as possible now (i.e. don't get sidetracked by cosmetics) and do the refinishing (painting/PCing/whatever) later. If it really only takes a day, that's not so bad. Usually it takes me twice (or more) as long as it's supposed to, though.

Dwayne's writeup (1987 Intake Refresh; great writeup, as always) actually scared me off a bit, not because of anything looking difficult, but just the length of the procedure: somewhere around 347 printed pages, if I scale to 70% to generally fit two photos per page .

Thanks for the link on RW to your intake R&R.

Ed Scherer

I'll take a look next time I'm home and report back tomorrow.

Ed Scherer

Another question: since there's still rough running "RPM dives" even at 1000 RPM and 1200 RPM, etc., is the ISV still involved (or is it supposed to be involved) at those engine speeds? (I suppose it might be if it's stuck even partially open when it shouldn't be).

I guess what I'm thinking at this point is that if the air leak I'm seeing is minor (nobody has chimed in on at what point an air leak becomes significant enough to matter, although John mentioned measuring vacuum at idle, which I'll do at the next opportunity) and if the ISV can be eliminated as a potential problem, then I'd turn my attention elsewhere (LH ECU in particular).

Mongo

For some reason, this really doesn't sound like an LH issue, but more related to both an ISV failing and the vacuum or air leak you are experiencing. The ECU is getting fooled by thinking the ISV is letting in too much due to the leak and closes, causing that shutter you are feeling. Is there a posibility you may even have a cracked rubber intake boot?

Ed Scherer

Yeah, that's still a possibility. But harder to address than the alternatives. I'm looking for an optimal (particularly with respect to my time) path through diagnosis and repair.

I suppose, but I inspected it pretty well and didn't see or hear anything (other than the suspected leak in that area where the Y plugs in on the passenger side).

John Speake

I wouldn't get too hooked up on the small airleak you have as the posible sorce of the idle problem. If there was a significant airleak then the idle speed would always be too high.

Can you borrow an LH to save diagnosis time ?

Mongo

Now that you mention it John, you are right. Small air leaks would not really yield high idle, or issues he is having.

Go for the LH borrow.

Ed Scherer

That's a very important tidbit that helps bracket the cause of the problem substantially. Thanks. But there's still the ISV, right? Maybe I'll give it another blast of WD-40, and perhaps a dose of strong language.

I'd love to.

I'm not sure if I can find any locally, though. Of the two local 928 guys I know who have LH, one's car is in CA getting stroked, and the other's car is a modded GT that would probably require a chip swap. I'm a bit reluctant to do that on someone else's property.

I just PMed Louie awhile ago... I might just pop for one of your rebuilt LHs; even if it doesn't fix the problem, it would give me peace of mind knowing that I'd no longer have one of the units prone to failure.

Time=money, etc., etc. — therefore, I'm even willing to throw money at it (e.g., rebuilt LH) to save diagnosis time.

Are we down to the most likely candidates being ISV and LH? Or are there still lots of other possibilities?