IAC tube vac leak? (Page 1/2)
Additivewalnut DEC 02, 09:04 PM
86 2.8, had a real high idle ~1500 RPM, figured it was the intake gaskets, replaced those since they're going on 38 years old.... and the idle is still high. Same spot. The throttle body gasket was replaced when the coolant lines were deleted about 3 years ago so that should still be fine. EGR and all it's related items have been deleted so there's a plate on the intake with a new gasket. The only vacuum lines left are to the MAP, regulator, and PCV; tested MAP and regulator lines by plugging them, no change. Haven't checked the PCV line yet, figure the only way to plug that would be taking the intake off again. Put in a new IAC and adjusted it by jumpering the pins on the ALDL. 1500 rpm idle and a hesitation on sudden throttle opening.

I blocked the hole in the TB and got the idle down to a solid 500-800rpm, then got my highly advanced vacuum leak locater apparatus (an old piece of large vac line) and poked around listening for suction noises and found it to be the loudest at the IAC. When feeling around on the back of the TB where the tube goes in, I can feel it sucking in my finger, so I have to assume that whatever seals that tube in has gone bad in some way. Would a leak from that tube cause such a high idle and if so, how do I repair that? Spraying carb cleaner around it doesn't seem to change the idle at all but it's the only place I hear or feel suction.
Patrick DEC 02, 10:57 PM

I believe there's a rubber O-ring in the TB that seals against the metal tube.
Additivewalnut DEC 02, 11:58 PM

quote
Originally posted by Patrick:


I believe there's a rubber O-ring in the TB that seals against the metal tube.



Can I replace that with a specific part or do I just have to try and find an O-ring that'll fit in? Or put a bunch of RTV on the tube and shove it in there LOL
theogre DEC 03, 12:34 AM
IAC Motor should have a Flat Gasket where it screws into the TB. New units also have dry thread sealer too.

the TB has a hole somewhere that sucks when IAC is open but should be somewhere in the filter air path.
If leaks in other places, could be missing a plug to stop air flow @ that point. GM & others has various "freeze plugs" thing that are not actual freeze plugs.

Example: Duke Block have 2 "fake" plugs... 1 is a "rear" end of cam so oil doesn't leak. 1 small one behind the cam gear seals full oil pressure & you drill a 1mm hole to get more oil to that gear per GM TSB covered in cave. Both are there to seal some hole to make the block but not needed or wanted otherwise.

I've seen other carbs & TB w/ similar metal plugs to seal holes the same way. Many "Carb Kit" have extra plugs seal them after cleaning or if seal rusted thru.
Never seen TB for "Fiero" V6 enough to notice.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Patrick DEC 03, 01:55 AM

quote
Originally posted by theogre:

I've seen other carbs & TB w/ similar metal plugs to seal holes the same way. Many "Carb Kit" have extra plugs seal them after cleaning or if seal rusted thru.
Never seen TB for "Fiero" V6 enough to notice.



I'm not sure why you're going on about "plugs". The OP is talking about #5 in the diagram and the hole for it in the back of the TB.






quote
Originally posted by Additivewalnut:

Can I replace that with a specific part or do I just have to try and find an O-ring that'll fit in? Or put a bunch of RTV on the tube and shove it in there LOL



Unless someone can advise otherwise, I'd take the tube to the hardware store and find an O-ring that fits the tube snugly... and hopefully you can insert it in the groove inside that TB hole. Make sure of course to remove whatever is in there now, if there's anything still there.

[This message has been edited by Patrick (edited 12-03-2023).]

theogre DEC 03, 02:53 AM
I see...
I followed vac map etc on VECI label & alldata & still don't see what that does.

Don't use RTV. Loose "boogers" often get stuck in important areas & very hard to remove if doesn't wreck the engine before you try to fix that.

[This message has been edited by theogre (edited 12-03-2023).]

Patrick DEC 03, 03:30 AM

quote
Originally posted by theogre:

I followed vac map etc on VECI label & alldata & still don't see what that does.



The labeling for #5 in the diagram I posted is incorrect. Air that the IAC valve allows to pass reaches the intake through that tube.

theogre DEC 03, 09:27 AM
So basically IAC flow bypass the Throttle & upper intake completely? Very Interesting while thinking Arte Johnson's Wolfgang. (Laugh-in is on ShoutTV) FSM etc diagram(s) for the TB doesn't show same for IAC plumbing.

VECI labels & other vac maps & more are often missing details. VECI label is on/in the engine bay of every vehicle cover vacuum to Emissions parts. FSM etc just echos same data for some things & maybe add other maps for cruise etc.

Yes, a leak there will cause problems if engine even starts/runs. That & brake booster line can have a big vac leak & engine barely starts if starts @ all. And may not set "code(s)" because either vac leak too big & won't start or just small enough ECM can't set them.

O-rings from most "normal retail" & plumber stores may not have the right size O-ring. O-ring kits from HF etc may not have it too.
Try Grainger & related that mostly serve Industrial buyers. May even have a store near you "hidden" in industrial park.
(Graybar is about same but mainly for electric supplies.)

Standard rubber may work. Try Viton because doesn't get fuel directly, engine heat, parts/car cleaners & crap in the air can "eat" standard "rubber" rings.
Grainger caries many Viton parts. HF etc Viton Kits may have but often not.

[This message has been edited by theogre (edited 12-03-2023).]

Additivewalnut DEC 03, 11:26 PM
I think we need to add a new item to the list of possible vacuum leaks. After blowing cigar smoke into the intake we found it was coming out of the CONNECTOR for the IAC. I didn't even know that was possible, but here we are. After fixing and adjusting the IAC it had a solid 1000rpm idle, still a little high but livable, but after restarting, it went right back to 1800 and held there steady. So I adjust the IAC again, does the same thing. For my third attempt I get it adjusted, let it sit at that 1000rpm idle, then unplug it. Now on restarts it holds that idle perfectly, and on a cold start it idles around 700. Not really sure what this all means, possibly something funky with the wiring? ECU taking a crap? Who knows. I'm just happy it's quiet at idle now.

Here's a video of it running rich to compensate for the air leak after it was fixed. We reset the ECU after this LOL

https://youtu.be/41-wSbiQn1c?si=mHCE0r5-uTLvfBIq
Patrick DEC 04, 01:01 AM

quote
Originally posted by Additivewalnut:

After blowing cigar smoke into the intake we found it was coming out of the CONNECTOR for the IAC.



Out of the electrical connector?


quote
Originally posted by Additivewalnut:

For my third attempt I get it adjusted, let it sit at that 1000rpm idle, then unplug it. Now on restarts it holds that idle perfectly, and on a cold start it idles around 700.



I did the same thing years ago with a '94 Hyundai Elantra when its idle wouldn't stop surging up and down. I just left it that way. Worked perfect and never set a code. You may or may not be as lucky not setting a code with the Fiero.