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Old 07-23-2010, 04:04 PM
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TBI Idle Issues

My modified 4.3 V6 in my 92 chevy has developed a Idle issue, it idles at 2500 RPM and won`t come down. I already checked for vacuum leaks and found none, the only other thing that comes to mind is the IAC valve but I hadn`t checked it yet, too hot out there.

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Old 07-23-2010, 04:58 PM
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That would be where I would look after the preliminary vac leak inspection.almost sounds too hi for an iac but you know fact is stranger than fiction.maybe the butterfly sticking or cable.
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Old 07-23-2010, 05:04 PM
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I would check codes- even w/o a MIL on. A bad base gasket will cause it and is common...

You can remove the vacuum lines one at a time, plugging at the vacuum source to see if the idle speed changes.

If you disconnect the IAC, does the idle change? If it goes higher, that's normal but no change could mean that it's bad or dirty or the circuit is bad.

If you remove the IAC for cleaning, plugging the passage should make the engine all but stall unless there's air getting in elsewhere from a vacuum leak.

You can use a timing light to see what the shape of the injector spray pattern is like. If you have a "dripper", you know what to do.

The temp sensor could cause a high idle, but should throw a code.

Brake booster check bad? PCV valve? Charcoal can/hose? Cruise control linkage? A/C switch "ON"?
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Old 07-23-2010, 05:56 PM
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unplugging the IAC will stop it wherever it is at . The engine speed will stay constant as it is different from a FORD as a ford requires a magnetic pull to keep the plunger at the desired cfm rate for engine load. A gm IAC has a bi directional motor that has a gear reduction and it only moves when a pulse current is applied in a forward or reverse polarity so it will stay in a fixed position when not energized.
The safest way ( to avoid an accidental fire... trust me) is to visualy inspecting for a vacuum hose that is leaking.You can carefully spray at one in question and if it is leaking you should note a change in engine rpm.
A temp sensor may or may not throw a code on a 92 unless it is shorted or completely open circuit.However a simple live datastream is a good way to verify.If it is flaky.. verify CTS by checking the resistance at a given temp according to the chart for that sensor. if the sensor checks out there is a problem with the pcm or the wiring has a problem
Leaky injectors will cause black smoke and not a fast idle problem, throttle bodys are dribbly and sloppy by nature
The idle air control would be a good place to start but it sounds more like a very big air leak like a brake booster hose off or a throttle body gasket sucked in, pcv valve hose off .
The IAC can be commanded to extend by jumping the 2 pins in the ALDL ( diagnostic connector) . Jump the 2 pins that give you trouble codes and then turn on the key. It should extend the plunger . If it is removed from the throttle body when you do this the ram may come all the way back out.you may be able to look down the corner of the Throttle body where it lives and see the plunger tip move into the passageway that it controls without pulling it out. (Recommended) This is part of the gm procedure for adjusting the minimum air rate on a throttle body. I would focus on an air leak, hoses ... the base gasket especially or a fully retracted IAC that is stuck.
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Old 07-23-2010, 07:25 PM
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Update, I did a IAC test and it proved bad. It pretty much wouldn`t move. I ran to town and got another one and it`s the best it`s ever idled. I replaced the original IAC valve when I did the rebuild 30,000 miles ago and it never did idle all that well and I had thought it to be the mods I did during the rebuild. Oh well, live and learn. Thanks for the help guys.
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Old 07-23-2010, 11:15 PM
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Usually when an IAC valve sticks open, there's a loud hissing monster in there!
I had one do that on a 305 TBI and it was like what the h*ll is that? Man, didn't take long to find it.
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