Mrmerlin

you need to pull the vacuum off the front damper feed,
where you have it is after the Eductor so its not a valid number.

Also your erratic running is due to the aircleaner base being off and hot air being drawn into the MAF
as temps change so does the air density,

NOTE the MAF air is now heavily influenced by the exhaust manifolds heating up air that is then drawn into the intake.
Install the aircleaner complete with the tubes and retest.
reconnect the Brake booster line and tap into the front damper vacuum line for accurate vacuum reading.

How old is the O2 sensor is it a factory part or a universal?

Make absolute sure the wheel throttle cable is able to reset to idle,
NOTE from you plugging the cable end back onto the arm the cable looks tight.

jcorenman

When logging, include "MAF Signal (absolute)" - that is the (vertical) load scale on the fuel map, not MAF percent.

One thing that strikes me is that shortly after 3:00 on thew video, ISV% climbs from 35% (functionally closed) up to around 60% (2/3 open, roughly) and back to 30% with no real effect on idle speed. That doesn't make sense.

The video doesn't show Sharktuner once the engine warmed up, but you indicated that idle increases once LH goes closed-loop. Prior to that, logged NBO2 shows "lean", and what we saw previously was that O2-adjust was running in the 0-9% range (positive) once warmed up. That's well within the -20 to +20% range, it just indicates that fueling (without adjustment/adaptation) is 4-5% lean. It could be an aging MAF or an unmetered air-leak bypassing the throttle (which would also cause high idle), not sure but also doesn't matter.

And with adaptation disabled, the engine is going to be running 4-% lean during warmup, which will reduce the idle speed a bit. Once it goes closed-loop, I would expect to see O2-adjust adding 0 to 9% fuel, NBO2 alternating between rich/lean, and uncontrolled idle speed would increase a bit as the mixture got normal. (Enabling adaptation-- which is the normal case--- would add that 4-5% fuel all the time, again well within the -20% to +20% range-- but that has nothing to do with idle).

What I am seeing here (as far as the video goes) is no control at all by the ISV. Whether closed (30%) or open (60%) has no real effect. Yet somehow there is enough air bypassing the throttle to idle the engine well above the target of 675 RPM. How is that idle air getting to the engine?

You also reported that the mechanical idle-stop is backed nearly all the way out yet still idles high (which makes no sense), and screwing it in even a partial turn does increase idle speed-- that part does make sense. You also reported that controlling ISV manually with Sharktuner did increase idle speed as ISV was opened further, which also makes sense. What doesn't make sense is that an ISV% of 30-35% should kill the engine, yet there is enough air getting past the throttle to idle at 800-900 RPM.

How is it doing that, getting through or around the throttle plate to idle well above normal?

Mrmerlin

Take off the plenum sides and see if they are warped put DC 111 on the rubber seals put the plenum sides on a piece of 400 grit and sand the area adjacent to the seal groove many you have uneven PC material and the plates are not fully sealed also check the RF corner of the intake make sure it’s not hitting the water bridge this could lift off the intake from fully seating

Darklands

Maybe as a first step tape around the plenum sides? 10 minutes work and if Michael have sucess with it he can go further!

PorKen

How about putting the S4 lower plenum/throttle body on there to rule out a problem with the GTS lower half?

(Currently a combination of S4 upper and GTS lower.)

Michael Benno

Good suggestions everyone. I hope to follow up with you all later this evening.

Kevin in Atlanta

Haven't we sucked all the possibilities out of it being vacuum?

Couldn't it be wiring at the MAF or idle control valve?

jcorenman

Stan, thinking further on this: Michael has done multiple smoke-tests, with no leaks to outside found. What he described was capping the MAF, pressurizing the plenum (via the venturi loop I think), no smoke escaping and held pressure. A leak from the plenum side-plates, or the upper-lower plenum for that matter, would be a leak to outside air and would show on a leak test-- no?? (I admit no direct knowledge, so this may be wrong).

Michael shared some ST log info in the video, and also shared a file with me spanning warmup, which confirmed that LH was trying to reduce idle speed (very low ISV% numbers), and AFR-- judging from rich/lean NBO2 indication and the resulting LH O2-adjust correction-- was well within the normal range.

In other words: Somehow the engine is getting enough air through or around the throttle plate to idle at 900-ish RPM, which appears to be metered air, i.e. passing through the MAF. It could be a busted ISV, not likely a MAF issue (but never rule that out), it could be a breather-hose that somehow bypasses the throttle (some weird Japanese-GTS thing?), unlikely to be an external leak (e.g. plenum sideplates or injector o-rings). To me, it looks exactly like the thottle-plate is being held open by a too-tight throttle cable, but that is not the case.

928Myles

I don't recall anything mentioned earlier, and I could be way off the mark through ignorance...

What mechanism does the system use to adjust idle when the AC is turned on? Could there be something in the system / wiring that makes the engine think this is on & it raises idle accordingly?

We seem to have fairly much covered all the mechanical options.

Myles

Kevin in Atlanta

Let's have some fun. Thinking outside the throttle body.

Leave the MAF harness connected and separate it from the throttle body. Start the car.

What's the idle then?

​​​​​​

Michael Benno

Hi all, thanks for all the suggestions. As Jim mentioned above he has been helping me by analyzing the Sharktuner data plots, which are an excellent troubleshooting guide. Everyone has been so helpful and I am grateful for your support. As Jim mentioned, we have good faith that there is no air leaks from the outside of the intake and we are focusing on too much air getting around the throttle. I am hoping to dig in today and re-evaluate the ISV air route and the throttle plate adjustment, again.

Pressurized Intake. To me, this seems sufficiently airtight.

You Tube

See below for responses to your great suggestions:

Myles, the ISV is used to adjust idle when the AC load is activated.

Kevin, Good suggestion I took wondered the. When looking at the data logs and the MAF single it’s pretty consistent, meaning the wiring connection is also consistent. Same with the ISV signal in the Sharktuner, I am able to manually override the IVS % and it responds as I adjust it. So I am thinking the IVS control (and voltage) is working properly

Ken, the maniform from the 1987 S4 (2R) was dimensionally identical to the 1992 GTS (4R). I know there was a discussion on the 4R having larger openings at the head interface. However, when I compared the two I had side by side there was not difference so potentially there may be manufacturing variances that are not defined by the 2R and 4R designation. The 2R manifold was used from 1987-1990. The 4R manifolds were used from 1991 and up. The difference between the 2R and 4R is the epoxy coating used on the inside, which tends to flake off and get ingested in the motor.

MB: Good catch, I figured that out later and moved it to the front damper as you suggest.

How old is the O2 sensor is it a factory part or a universal?
MB: O2 Sensor, I have two OEM sensor, a new one and one that is about 10K miles on it. I have tried both and neither has an impact on the idle condition.

Make absolute sure the wheel throttle cable is able to reset to idle [/QUOTE]
MB: Check, all idle tests were done with throttle plate cable disconnected

MB: I have vacuum tested the fuel dampers and pressure regulator. They all hold vacuum for several hours. I have vacuum tested the booster, the check valve and the accessories attached (HVAC, Reservoir, Flappy), all hold vacuum. I have tested with the booster line plugged to eliminate those systems and again no impact.

MB: FYI the starter is not the OEM starter, it is a Hi-Torque mini starter and they do sound different. Maybe this is what you are hearing. I am not opposed to doing a compression test, I haven’t done one before. But how is it that low compression in one cylinder would cause high idle?

PorKen

Uh huh, but the idle was high before and after just about everything except the lower plenum was changed, so...

10,000 posts!

Kevin in Atlanta

I asked about that and the response was in the two years of ownership he could not remember when it idled below 850.

uraniummetallurgist

Maybe your tachometer is reading incorrectly...........

Kevin in Atlanta

@Michael Benno If that's true, this was an issue long before you owned it. Have you reached out to the previous owner about the high idle?

It had nothing to do with anything you changed except that your work might have made it worse?