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TBI high idle?

dyeager535 said:
Stay away from the idle screw unless you have messed with other things like cam, exhaust, etc., or you are fixing the idle someone else messed with.

TBI's idle is not adjustable...it has a set parameter it tries to reach, and anything that is done to modify the engine leaves the engine trying to still reach that RPM. Which means more fuel to get the mixture right, closing the IAC to try and reduce the air to the engine, etc. The proper way to set TBI idle is called the "minimum air setting", but again, unless the engine/TBI has been monkeyed with, you don't need to do this.

It's programmed by GM to work with the PCV. Put it back, find the real problem. If you can shake the PCV and it clicks, it's fine. Or so I've read.

The screw as far as I remember is on the drivers side front of the TBI, behind a small plug.
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It has factory manifolds... but no cat. Has a new oxygen sensor, new IAC and I've tried most all other vac lines trying to find a leak and I think its good minus the pcv flows like crazy and is increasing idle i think. When plugged it just sounds “normal” call me crazy but if i get rid of it and plug the vac hole... and then the computer has a set idle... wont it just open the throttle a bit more or squirt more fuel... read the oxygen sensor etc... to be tuned how it wants Air fuel ratio wise? Seems that dirty air from the pcv wouldn't be beneficial for it to burn... no? Just an epa thing I always thought.
 
badbowtie03 said:
It has factory manifolds... but no cat. Has a new oxygen sensor, new IAC and I've tried most all other vac lines trying to find a leak and I think its good minus the pcv flows like crazy and is increasing idle i think. When plugged it just sounds “normal” call me crazy but if i get rid of it and plug the vac hole... and then the computer has a set idle... wont it just open the throttle a bit more or squirt more fuel... read the oxygen sensor etc... to be tuned how it wants Air fuel ratio wise? Seems that dirty air from the pcv wouldn't be beneficial for it to burn... no? Just an epa thing I always thought.
Click to expand...

It's a calibrated vacuum leak. The ECM programming is tuned expecting it. If it didnt matter, idle wouldnt change, would it?

Worst case you swap the PCV out. But it sounds like it works, and you have a problem such as a cracked tbi gasket or somesuch.
 
Maybe changing the gasket should be done anyways... it is old! Haha. But also, i got a tach delivered today. Gonna hook it up and see what its actually at.

Back to the pcv though... is it supposed to flow a lot at idle? Also... the pcv has nothing to do with A/F ratio right? Isn't that all done with the 02 sensor? Maybe its a programmed leak for idle... but I’m thinking how could it hurt at idle to just be lower? If A/F is programmed with the 02 while running? Or is this insanely flawed thinking?
 
If it doesnt work properly with the pcv hooked up, something else is wrong. It's that simple. Don't overthink it.
 
badbowtie03 said:
Maybe changing the gasket should be done anyways... it is old! Haha. But also, i got a tach delivered today. Gonna hook it up and see what its actually at.
Back to the pcv though... is it supposed to flow a lot at idle? Also... the pcv has nothing to do with A/F ratio right? Isn't that all done with the 02 sensor? Maybe its a programmed leak for idle... but I’m thinking how could it hurt at idle to just be lower? If A/F is programmed with the 02 while running? Or is this insanely flawed thinking?
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The pvc is supposed to flow clear while at idle to scavenge the gases from the crankcase. Then it will close as you rev up the engine. There is definitely another leak somewhere causing the engine to rev up. It should idle around 600-700 rpm normally. Get the bad hose replaced. After you replaced the iac, did you reset it to get it here it needs to be? If not it will not idle correctly either.
 
chulisohombre said:
The pvc is supposed to flow clear while at idle to scavenge the gases from the crankcase. Then it will close as you rev up the engine. There is definitely another leak somewhere causing the engine to rev up. It should idle around 600-700 rpm normally. Get the bad hose replaced. After you replaced the iac, did you reset it to get it here it needs to be? If not it will not idle correctly either.
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Hey, no i didn't reset it. I will try to look up how. or if you have a link let me know thanks.
 
Was going to second or third putting the pcv back. It will cause all kinds of problems, even the pre 1970 motors had them, and I think the 50’s or 60’s engines used a draft tube to do something similar. Replacing it with a vent isn’t the same.

Sometimes the pcv valve is just plugged up with oil. Try spraying some carb cleaner or the like through it so it rattles easily when you shake it.
 
Anyone know how to reset the IAC? I finished the tach today and its about 750-800. But then randomly after driving I fee like it can sometimes be stuck around 1k... kinda different at times.
 
badbowtie03 said:
Anyone know how to reset the IAC? I finished the tach today and its about 750-800. But then randomly after driving I fee like it can sometimes be stuck around 1k... kinda different at times.
Click to expand...
700-800 is normal.
1000 is too high.
Do you have a spring to return the throttle back when you let off?
Mine was missing and it would do that
 
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