Idle Issue With 328
I have a US 1987 328GTS that I've owned for 27 years. About two weeks ago it developed an idle issue where it suddenly jumps from 900 rpms to 1400 rpms, once it is warmed up. If you shut it off and restart it, it idles at 900 but within a minute will suddenly jump to 1400. Yesterday I disconnected the O2 sensor. There was very little effect except the idle was constant until it jumped to 1400 from 900. I let it continue running and it suddenly returned to 900 for perhaps 20 seconds. Then it jumped back to 1400 rpms. That never happened with the O2 sensor connected. It has been tested many times with O2 sensor connected.
The AAV is working properly. So is the idle adjustment at the fuel distributor elbow that lets air bypass the throttle. The TPS is working properly. When driving I have noticed that when the idle suddenly changes, the engine has a sudden and momentary loss of power, hiccup. The engine runs well and idles smoothly. It's only apparent problem is the sudden jumping around of the idle speed and the associated hiccup.
One last thing: Several days ago I attached a dial indicator to the diaphragm that measures air flow to the engine. When the idle jump occurs, the diaphragm opened slightly more than 0.001 inch. Admittedly that is a small amount but when you consider the percentage increase in annulus size, it is significant. That could possibly account for the needed extra air to allow it to run faster.
Does anyone have any ideas about what is wrong with it?