Purge Valve Replacement?

DWCaprice2017

Apprentice
Oct 25, 2019
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Hi, I'm Daniel with the '84 Caprice, 305, ccc q-jet:
I'm probably grabbing at thin air, but would a bad canister purge valve cause a rough (low) idle after the engine warms up?
I know to check the spark plugs and vac hoses, I'm trying to figure out what else might cause the roughness after about
10-minutes. Yeah...I know, a lot could!

No, I'm not getting a "Check Engine" light.

I found a couple of the tiny-opening hard plastic lines--very brittle, on the left (passenger side) of the engine, near the rear,
where do they start at? It looks like they're meant to be going to the same place.
 
A bad purge valve can cause idle issues, but so can bad PCV valve, brake booster, HVAC, oil pan gasket, etc. You need a Mityvac to test it and other vacuum parts for leaks.
 
Choke fully open when warmed up?
 
A bad purge valve can cause idle issues, but so can bad PCV valve, brake booster, HVAC, oil pan gasket, etc. You need a Mityvac to test it and other vacuum parts for leaks.
Thank you for the info!
Do you know of any YouTube videos that shows how to vacuum test those parts?
 
When I was a GM tech in the 80's, the first thing we did when the CCC cars came in for a rough idle was check the MC dwell. It needs to average 30* but should vary at idle from 25-35. If it wasn't fluctuating or was full rich or full lean and there weren't any vacuum leaks, we replaced the idle air bleed O rings and replaced the canister purge valve. The valve shouldn't be loaded with fuel. Take it off and shake it. If fuel runs out, it's bad.
 
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When I was a GM tech in the 80's, the first thing we did when the CCC cars came in for a rough idle was check the MC dwell. It needs to average 30* but should vary at idle from 25-35. If it wasn't fluctuating or was full rich or full lean and there weren't any vacuum leaks, we replaced the idle air bleed O rings and replaced the canister purge valve. The valve shouldn't be loaded with fuel. Take it off and shake it. If fuel runs out, it's bad.
Thank you!
I’m guessing the idle air bleed rings is inside the carb?
 
Thank you!
I’m guessing the idle air bleed rings is inside the carb?
They are easy to replace. You'll see a small cover that has two rivets holding it on. It's located between the choke and secondary air valve. They are aluminum rivets and very easy to remove. Just close the choke and use an alligator clip to keep it closed. I just used a small chisel to knock them off and then remove the cover. I didn't drill them out because I didn't want any shavings to fall inside. There is a special tool to set the installed height, but if you don't have one, just use a either a depth micrometer, tire depth gauge, or just a paper clip to reinstall it as it was originally. Then check the dwell. If it's still off, it's either adjusted by adjusting the idled air bleed or removing the plugs that cover the idle mixture screws. If there are ANY markings, numbers or letters on the idle air bleed, the dwell is set by the mixture screws only, not by adjusting the idle air bleed. To check the dwell, hook up a dwell meter using the six cylinder scale (no matter how many cylinders the engine is) to the green wire connector around the front of the carburetor. An old analog dwell meter works best because on a digital meter, it's difficult see the average because it changes rapidly. Even with the Tech 1 hooked up, we always used a analog meter to read MC dwell. I'll tell you now that if the mixture screw plugs need to be removed, you may as well just rebuild the carburetor because you have to turn the carb upside down and use a hacksaw to cut the throttle body below the plugs. This will take all of the sediment that's in the float bowl and put it everywhere.
 
You can drill out the baseplate idle screw plugs without having to use a chisel. PITA, but it's do-able. I know. I did it. The D shaped tool barely fits, but it still fits.

If you need O-rings for the idle air bleed valve (IABV), they need to be METRIC o-rings. They're a weird size. Anyway, here's what they look like. They don't always come in the aftermarket carb kits, but they do in the Delco power kits. There's a blank IABV, and ones with markings on top. If it's blank, you have to adjust those differently.

1733501534533.png


Here's a post I did about the IABV when I put an 85 carb together.

 
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