I finally diagnosed my erratic idle. I did some searching and found in a thread by holding my finger over the iac hole the car kept running so I assume I have a vacuum leak. It bogged down quite a bit but it did keep running. I connected everything back together and then I sprayed throttle body cleaner all around the upper intake where it meets the lower and the best I could around the egr tube but no fluctuation in idle. I'm wondering if the sleeve around the tube is keeping the tb cleaner from getting to a possible crack? Any advice is greatly appreciated!
Marc

http://www.aa1car.com/library/vacleak.htm

PRESSURE VACUUM LEAK DETECTION TECHNIQUES

Another way to find an elusive vacuum leak is to pressurize the intake manifold with about three lbs. of regulated air. This can be done by attaching a regulator to your shop air hose, then attaching the hose to a vacuum fitting or the PCV valve fitting on the intake manifold, carburetor or throttle body. Do not apply too much pressure or you may create new leaks! With the engine off and air flowing into the manifold, spray soapy water on suspected leaks. If you see bubbles, you have found the leak.

You can also use the opposite technique, which is to apply vacuum with a hand-pump to various vacuum hoses and circuits to see if they hold vacuum. But this technique means tracing the entire circuit to see where it ends, and disconnecting and plugging any parts of the circuit that do not "dead end" against a diaphragm or valve.


* Intake manifold gasket vacuum leaks Try re torquing the intake manifold bolts, working from the center out in the recommended tightening sequence. If that fails, the intake manifold will have to be removed and the intake gaskets replaced. Sometimes the mating surface of the intake manifold or the heads will not be flat (check both with a straightedge). If warped, the intake manifold and/or heads will have to be resurfaced on a milling machine. Another problem to watch out for here are heads that have been milled or resurfaced to raise compression. To maintain proper alignment between the manifold and heads, metal also needs to be machined off the bottom of the manifold where it mates with the block, otherwise it will sit to high and the ports and bolt holes won't align.

Loosen the two 10mm bolts heads that hold the EGR tube to the upper plenum of the intake manifold. Cut a strip from an aluminum can 1 1/2" wide and the length of the can. Slip that in between the upper plenum and EGR tube and re-tighten the bolts. See if your idle has dropped. If so your leak is in the EGR tube. If not look elsewhere.

If your engine runs with the IAC passage covered your leak has to be substantial. There are not very many places that can leak that much vacuum. It's not any of the small lines so don't worry about them. The EGR tube I would say is about a 75% likelihood. 25% to be the IAC tube that runs from the throttle body to the intake. There is an internal O-ring that can get displaced.

Yeah it's a pain to get to the bolts on the bottom of the manifold. Take off the dist cap and rotor to get them out of your way. Yes they can be reached with the intake on the engine.

I've used smoke machines to find vacuum leaks, but those things are very expensive. I normaly use carb/choke cleaner. It's very effective. Just try not to use it around extreamly hot parts, and try to do your testing before the engine gets real hot. Let it warm for a little bit, but don't do this test after a 1 hour drive.

As for that smoke machine, I've seen some cheap novelty machines at stores around halloween time....I wonder...

quote Originally posted by Gokart Mozart: http://www.aa1car.com/library/vacleak.htm PRESSURE VACUUM LEAK DETECTION TECHNIQUES Another way to find an elusive vacuum leak is to pressurize the intake manifold with about three lbs. of regulated air. This can be done by attaching a regulator to your shop air hose, then attaching the hose to a vacuum fitting or the PCV valve fitting on the intake manifold, carburetor or throttle body. Do not apply too much pressure or you may create new leaks! With the engine off and air flowing into the manifold, spray soapy water on suspected leaks. If you see bubbles, you have found the leak. You can also use the opposite technique, which is to apply vacuum with a hand-pump to various vacuum hoses and circuits to see if they hold vacuum. But this technique means tracing the entire circuit to see where it ends, and disconnecting and plugging any parts of the circuit that do not "dead end" against a diaphragm or valve. * Intake manifold gasket vacuum leaks Try re torquing the intake manifold bolts, working from the center out in the recommended tightening sequence. If that fails, the intake manifold will have to be removed and the intake gaskets replaced. Sometimes the mating surface of the intake manifold or the heads will not be flat (check both with a straightedge). If warped, the intake manifold and/or heads will have to be resurfaced on a milling machine. Another problem to watch out for here are heads that have been milled or resurfaced to raise compression. To maintain proper alignment between the manifold and heads, metal also needs to be machined off the bottom of the manifold where it mates with the block, otherwise it will sit to high and the ports and bolt holes won't align.

Good info. Thanks Mozart. When the weather gets warmer I'll take off the intakes and check both of them with a straightedge. While I'm in there I'll be able to get a better look at the vacuum lines as well.

quote Originally posted by phonedawgz: 25% to be the IAC tube that runs from the throttle body to the intake. There is an internal O-ring that can get displaced.

I don't remember there being an o-ring. Good possibility I am missing it. Can someone please show a picture or diagram where it goes and where a new one can be purchased if it is gone?
Thanks, Marc

[This message has been edited by Gokart (edited 03-31-2011).]

quote Originally posted by 1fatcat: I've used smoke machines to find vacuum leaks, but those things are very expensive.

https://www.fiero.nl/forum/Forum1/HTML/081547.html Cheap and easy

You have to take the throttle body off and look at the surface that bolts to the intake manifold. If you look at the bump right under the main bore you can imagine where the throttle body passage is. It's the size of your little finger. There is a large tube that runs from the intake manifold to the throttle body. That large tube slides into the passage in the throttle body. The o-ring is in an internal machined ring inside the throttle body. The tube slides into the the passage and the o-ring is supposed to stay in the machined slot.

Not a Fiero throttle body but you can kinda picture the main bore above and then see the IAC passage below it. The manifold only mates to a circle around the main bore. The O-ring passage portion of the throttle body is below the manifold surface.




This is the tube that runs from the intake manifold and fits inside the throttle body.

[This message has been edited by phonedawgz (edited 03-31-2011).]

TKS phonedawgz. Ratings to all and thanks for the advice.