worf928

@Michael Benno If the idle problem is due to false air then you didn’t do the smoke test correctly. If the smoke test was done correctly then the problem cannot be false air.

Michael Benno

53minutes to remove an intake!




@worf928 you are probably correct I did not smoke test correctly. Help me understand what I did incorrect and help me with some guidance on how to do it correctly

Here is what I did:

1. Disconnected the large vacuum hose that goes to the brake booster
2. Inserted the output of the smoke machine into above hose such that smoke was going into the upper intake and through the venturi into the intake boot.
3. I had the MAF inserted and plugged with a rubber large boot used for pressure testing the intake. (see picture in post 20)
4. I let the system fill up with smoke for about 15min and verified system was full by removing the vacuum lines at the tank vent diaphragm and front fuel damper and see a small stream of smoke come out.
5. I reconnected all the lines and checked for smoke using a bright pencil light.

What did I miss? How do you do you smoke testing?

FredR

Michael,

The scenario Dave is talking about is for if and when a false air scenario [aka a leak to the outside world] is in play.

In your case it is not false air rather it is one of excess air but it is metered air nonetheless.

jcorenman

Michael, I think we are getting lost in the weeds here. I am sure your smoke test was fine, for what it was-- a check that there is no unintended connection between the intake and outside air. No split hose, no leaking o-rings, etc.

To repeat: There are only three normal pathways for idle air: throttle plate, ISV loop, and brake venturi. All three are "metered", i.e. air flows through the MAF and gets measured. And only one-- the ISV loop-- is controlled to set the idle speed. And one of those-- the throttle plate-- seems much more likely to be the culprit.

Which brings me to a question for those here with a lot more experience than I have. The throttle-shaft bearings seem like really nice little bearings that live an easy life: They rarely get over a few RPM, don't get particularly hot, and aren't highly loaded. Do those bearings ever fail? Is there any reason to change them?

Mrmerlin

Jim the shaft bearings on the flappy and throttle shafts have a single O ring seal,
so once the bearings are this old they turn to brittle and all of them leak .

NOTE since the O rings get brittle they allow dirty air through the bearings,
this adds increased wear to the bearings as well

The new bearings are now double O ringed, with an O ring at each end of the bearing./
So replacing the bearings when doing an intake refresh is more or less SOP for the flappy shaft and the throttle shaft.

Mike is that intake powder coated? the coating looks like spray paint,
this will make the injector bores have a wider opening.

Kevin in Atlanta

Fwiw, my smoke tester can reveal leaky throttle shaft leaks. I would assume OP's would was well. My limited experience with those leaks is they don't raise the RPM as much as we seeing here.

I know this has been asked but I didn't see a response. Have we disconnected the ISV?

worf928

Actually, the purpose of my post was not to cast aspersions on your procedure for smoke testing. It was to make plain the logic of the diagnostic tree so as to avoid diving into the wrong rabbit holes, some openings to which have appeared on this thread.

Based upon your description I have no doubt that your smoke test was done correctly. Based upon how long you left it "smoke filled" you don't have a false air issue. You have a "bypass issue" as per Jim's, Greg's, et. al., posts.

After Greg's post I went back to the top and looked at your as-installed throttle plate in the air guide. And, yep, sure-enough it doesn't appear to be right. That and all the "messing" you had to do with the stop screw is evidence that points to the throttle plate. Occam's Razor.

Since you already have the intake off again, go ahead and make that right.

If it was *me* and the intake was still on - since I hate with a passion guessing about stuff - I'd plug in my spare brain case with PEM-filled ECUs and run the Shark Tuner at idle so as to see the ISV duty cycle, Mass-Air Sensor voltage, 02, etc., so as to gather more data before getting out wrenches.

Michael Benno

Thanks again for all the comments above!

Today, I fiddled with the throttle plate a bunch trying to get those two punch marks on the plate to be equidistant from the throttle shaft. And, while trying to make the tightest seal on the throttle body. I used a high-intensity light inside the throttle body to look for light leaks. Of course, I backed off the stop screw to allow the plate to find the tightest seal.

Here is where I ended up. This first photo shows the throttle plate with the two dots properly aligned. The throttle plate opens and closes smoothly without binding on the edges at rest.


This second photo is showing the amount of light leaking at this position. And you can see the slight increase in light leaks.


Next up is to set the idle set screw, Greg advised that I need to set the screw such that a little airflow is allowed such that the ISV has some room to work with, but not so much that there is too much air for the ISV to properly adjust. Which would seem to be what was happening to me previously. Take note of the clocking of the throttle plate stop screw (4 o'clock in this case), in this position it is just barely kissing the throttle arm.

In this next photo you can see the increase in clocking of the screw from 4 o'clock - to - 6 o'clock. Something like 3/8ths of a rotation.



Feel free to provide input on this step. I tried as best as I could to follow the guidance. But this is certainly one of those steps where years of experience come into play (which I do not have).
ISV
I have 3 ISV's to choose from all three bench tested the same and have the same rest and full open position. Given these are just motors with brushes that wear, I installed the newest one.

1. 1992 OEM Bosch with 70K miles on it
2. 2011 Lowe with unknown miles
3. 2022 Lowe that is still new

jcorenman

Michael is smart, just not patient. He bought a Sharktuner in early March, when we were gone for weeks and then recovering from covid (not unrelated...), which will ship Wednesday. (My mistake was to not turn off the online-order page while gone).

So hopefully, in a week, he'll know what the ISV duty-cycle is and which way he needs to move the mechanical idle-stop. (Pro-tip: the ISV active range is 35-70% or thereabouts, and you want to see around 50% with the engine just happily idling in normal conditions. But you knew that).

jcorenman

I knew there was a reason that I didn't like to mess with throttle plates. Not for Solex's, Webers, Holly, whatever. Nothing but trouble there.
Personally, I would take a Bosch anything over whatever else you offered. It's a rotary solenoid, no brushes-- just a coil, magnet, and a couple of bearings. What does happen, is that they get yucked up with intake vapors and the like. But if they are clean, and respond freely to a bit of voltage, then they are good to go.

worf928

And expected voltage from Mass-Air Sensor is ?

Mrmerlin

NOTE for the ISV I suggest to spray some PB blaster into the unit to let it soak the shaft that the vane rides on,
when the unit gets wet from condensation the rod will rust and cause the vane to bind.
I also suggest to reuse the bosch unless the vane feels like its stuck.
The PB will dissolve the corrosion on the vane shaft

android606

@Michael Benno : I can't wait to see how this goes. I have an '88 with an idle problem almost exactly the same as what you show in your video.

Just on the off chance there's any info in there that's useful to you, the thread about my problem is here: https://rennlist.com/forums/928-foru...urrection.html

Michael Benno

Hi @android606 I'll follow up with you in a bit. I've been a bit preoccupied with 928-troubleshooting, work, and other not 928 related activities.

SwayBar

This is an excellent thread - my 87 idles at 850 RPM and looks like I'll need to remove the intake too and get fresh/correct injector o-rings.

Other than that, it runs really strong.