'86 Mustang GT searching

I love my '86 Mustang GT convertible. Currently at 106,000 miles and running strong. The problem is searching at idle. I have replaced every sensor that me and my mechanic can think of. New plugs and wires along with rotor and distributor cap. The engine will start searching, i.e. idling up and down, and at time stall. If I apply slight pressure and idle up to 1,000 RPM the searching stops. Hard to do sometimes as this is a stick. Anyone else with the same problem and perhaps a solution?

You didn’t mention checking for a vacuum leak.
Has anyone done that?

If no vacuum leak, has the throttle body idle adjustment screw been messed with ? The correct way to set the base idle is to disconnect the IAC and if it stalls adjust the idle by turning the screw in a turn or two so it keeps running.

The base idle should be between 750 and 850 rpm. If you can get it to idle normally like this then re-connect the IAC and see what happens…Just a shot in the dark as I have an 89 GT.

Also what about the EGR valve…it can be stuck partially open causing a vacuum leak…this valve should be closed at idle and at WOT. Are you getting a check engine light when this happens ?

You can also have a small vacuum leak in the power brake booster. You need to hook a vacuum gauge on this motor.

Thanks for the great answers. After checking for vacuum leaks and all the other suggestions we checked the throttle body idle adjustment screw and it was not set correctly. Problem solved, a great big thanks to all!

Great Joe. Yes, this is a common problem on cars with idle adjustment screws. If the idle adjustment screw is modified from its nominal position in an attempt to fix an idle problem caused by some other issue, the result is often idle rpm surging. The engine computer assumes the idle screw is set within a narrow range and then the computer modifies the idle speed slightly from that based on operating conditions. If the screw gets set outside this range, it confuses the computer to no end, and it just doesn’t know how to keep the idle speed constant.

What would be a good analogy? Well, say your car’s AC were set to keep the temperature to 70 degrees, but the gadget which measured the passenger compartment temp for the AC was not giving a consistent reading, one time it would read 74 degreees, the next time a few seconds later, 64 degrees, the AC system wouldn’t know whether to make it cooler or warmer, and would go up and down in temperature.