Engine Revs up and down at idle

Thank you so much for an answer. Yes, I did replace all the gaskets; but that base idle set just may be the problem. I didn't think about reinstalling it to the original setting; because that mark got removed. I just screwed the linkage back to the body and hooked up the throttle cable. I'm going to try and reset it per the factory manual instructions. I'll post the results here. Thanks again for the help.

I reset all the adjustments of the throttle positioner and the idle stop set to the factory specs. Engine still wants to rev up and down at idle. I tested the vacuum sensors and the throttle positioner and they test good. The engine will hold steady at around 2500 rpm's; but when you let back on the throttle to idle, that's when it starts to rev up and down by itself. I'm without a clue now. Can someone please help?

I cleaned the air valve, the throttle body, the EGR valve body which had a little carbon build up, installed new gaskets; but engine still won't idle smooth. I checked the vacuum sensors and the vacuum switches, they all check out OK. I checked all the vacuum hoses for leaks and all were good. Engine still revs up and down all on it's own. 800rpm to 2700rpm then back to 800rpm. When I disconnect the vacuum switch for the fuel rail pressure reg the dash pot for the throttle positioner will start to pulse the throttle up and down. When I disconnect the MAP sensor the engine will idle rough and then die. It also will register a trouble code for the MAP sensor; but will clear out when I reset the ECU. So I think the MAP sensor is good. I actually replaced it a few days ago. I'm really stumped on this one. Can someone please help. Thank you.

I've tried all the recommendations and no go. Engines still revs up and down all by itself. Could the problem be in the setting of the throttle positioner? When I unplug it, engine will rev up to about 2800rpms and hold steady. When I plug it back in, engine starts to rev up and down. Still stumped and I have donated. Thanks for any help.

Yes, I set the TPS per the factory specs and it also tested good. I even swapped out the throttle body and TPS from another running motor of the same and didn't change any settings. It did the same thing. Someone mentioned a possible vacuum leak. Can a vacuum leak actually cause the engine to rev up and then die down, over and over? I've had vacuum leak problems in the past with other repairs and they seem to only increase the rpms and not make the engine die down and then rev up again like this problem. Thanks for the reply; but I'm still stumped.

Found a leak at the manifold gasket. Boy was I stumped about how a vacuum leak at the manifold could rev the engine up and down. I'll let you know how it goes when I get it back together.

I found the intake manifold gasket was bad. Apparently it was misaligned when originally installed and had finally burned out to a major leak. I replaced the gasket and all is fine. This vacuum leak also caused some error in the ECU not registering the O2 sensor malfunctioning when I would disconnect it. The engine never ran better. At least I now have all new sensors, a replaced ECU and a rebuilt throttle body that I really didn't need. Thanks for all the help and keep up the good work here. I'll definately will recommend this site to my friends.

Check the air injection at the exhaust and then the EGR gasket. If the inlet hose is blocked or leaking the idle will bounce. If blocked it will blow the EGR gasket.