Hard Rough Idle Problem

[chevyaddict]

Hi All - As long as I've owned my 81, it has had an EXTREMELY rough idle. It runs so smooth otherwise when I'm driving it I never really gave it much attention to fix. But now I'm kind of tired sitting there at lights looking like I have a drag racing cam in the car. I've put new carb on it and the problem is the same so I'm thinking it may be a significant vacuum leak but for the life of me I can't find one. How does one test for such things? Its a bit worse now also because when I go to take off I get this significant hesitation which I believe is because there is a surge of air without comparable fuel (just like a big vacuum leak). Any suggestions?

 

[TimAT]

Long list of stuff to look at on an 81. Start with the usual stuff- plugs, plug wires, distributor cap/rotor, and timing.

A vacuum leak can be tough to find, but a couple cans of spray carb cleaner will help. Engine at idle, air cleaner on (makes it a little easier), then lightly spray the cleaner around the base of the carb, along the sides of the intake, around the EGR valve, and then along all of the vacuum hoses- including the brake booster.

From there, you'll need to look at TPS voltage, MC solenoid dwell and idle air bleed. All are adjustable, but you need to determine if your carb has the plugs in it over the idle mixture screws and follow the factory service manual's steps for adjusting them.

Little tip: fill a spray bottle with a couple drops of dish soap and water, start the car at night or in a dark area, then spray the water/soap mix over the plug wires. A bad wire will show sparks like July 4th,

 

[chevyaddict]

Thanks for the suggestion on the carb cleaner; I'd forgotten that little trick! Its not anything electrical given the car has had many tune-ups replacing all these parts and this problem has never gone away. New carb too.... same problem so its gotta be a vacuum leak or some sensor although I find that hard to believe. I'll try the "search" method in a few and let you know what I find!

 

[TimAT]

I'd still look at the plug wires. Not unusual to have a bad one- even in a new set. If it got kinked somewhere in it's live it may have broken the core of the wire. Check the resistance of them- should be around 300 ohms per foot, depending on what wires you have.

Be a real bite to spend hours searching for a vacuum leak and find out it's a bad wire. Been there. It is.

 

[chevyaddict]

I'd still look at the plug wires. Not unusual to have a bad one- even in a new set. If it got kinked somewhere in it's live it may have broken the core of the wire. Check the resistance of them- should be around 300 ohms per foot, depending on what wires you have.

Be a real bite to spend hours searching for a vacuum leak and find out it's a bad wire. Been there. It is.

Well I did find one pretty significant vacuum leak. There is a large threaded 4-t type vacuum thingy threaded in the manifold in front of the distributor... it was finger loose :ugh. So fixed that. That has solved some of the problem but not all. Couldn't find any other vacuum leaks so perhaps I will check out the electrical. It runs SO SMOOTH otherwise its kinda strange!

 

[TimAT]

Start by pulling vacuum hoses and plugging the connection to the manifold. Could be a hose is bad downstream letting extra air in too. A vacuum leak doesn't have to be right on the engine to cause a problem- these Corvettes have all sorts of things that use vacuum and any of them can be a troublemaker. I've even seen the vacuum reserve tank leaking.