I am chasing this problem everyday. After the car warms up good, at a stop sign, at idle, the RPMs will go up and down. RPMs will go to 1200 and fall to 550. Sometimes the drop is so low the car will die. It is a Z28 305 manual 1LE. I have replaced every sensor that I am aware of, Checked fuel pressure, Changed out all the front components including harmonic balancer, Alternator, power steering pump, idler pulley, new injectors, new plugs, new wires, rotor and cap. Today I installed new gromments in the valve covers to seal up things. It must be a vaccum leak. I hooked a vacuum gage up and the needle did move a little when the idle modulated. Can anyone help me solve this issue? Thank you.

 

Make or purchase a cheap smoke machine. Smoke check for a vacuum leak. Theres no good way to check for leaks on a tuned port other than smoke tests. Something like this... https://a.co/d/7jzVnd8 Or take it to a shop that can fo the test with their machine and show you the leaks, if any, which I'm betting there is.

 

You can also check for vac leeeeeeks with carb cleaner. DO NOT use "brake cleaner": it contains chlorine, which if ANY is left in the intake or combustion tract, will RUTHLESSLY corrode the castings. "Carb cleaner" is much like lacquer thinner: methyl ethyl ketone, acetone, methyl isobutyl ketone, etc.; none of which contain chlorine or fluorine, therefore none of which are corrosive. Spray a SMALL amount of it around every known gasket interface, and any other places you might think might leeeeeeek vacuum (throttle shafts for example). See if it makes a difference to any idle properties, either measured or subjective. (speed, vacuum, smoothness, whatever) If so, ... BINGO, ... vac leeeeeeeek. Fix, re-try, let us know what you found.

 

And you MIGHT want to get an extra can of that carb cleaner. Use the first one to clean the throttle body bores, plates (front and back), and IAC air passages. It might even help to remove and dismantle the IAC itself to clean and lubricate the rack gears and pintle. It's only 35 years old, and there probably isn't any varnish, gum, sludge, and contamination buildup in there since the Bush administration, but you never know.

Reserve the second can of carb cleaner for locating vacuum leaks, in case a clean TB and IAC doesn't resolve the problem.