1984 318i idle speed changes from 1400 rpm to 900 rpm

There are 2 idle specs. One is in gear with brake applied. What is that speed?

If you mean the idle speed, once she's warmed up the idle should sit around 900 rpm. Fast-idle maybe 1200 - 1500

700-900 rpm

No spark plugs only produce spark to ignite the fuel mixture and do not control idle speed. Run fault codes to determine if there is a sensor fault as it should be idling around 900 rpm at idle and stay close to 900 in drive as the ECM will adjust the idle rpm to compensate for the extra load. Nissan maxima use special plugs that do not need changing

Maybe a short to the TPS or something shorting it out. There's also the IAC that really controls the idle but usually it will cause the RPM's to go up and down a little erratic if bad. The MAF sensor can cause major problems as well but since you already swapped the TPS and it helped and then stopped I would focus on that sensor and especially it's wiring to and from the ECM. Be careful with the sensor signal wire to the ECM can blow it if more than 5v passes or spikes.

Yes, sounds like the idle speed is too high. If you keep driving like that, you will fry the torque converter / turbine. Since idle is computer controlled, you will have to take it for service tune up. There is one possibility that the rich, high rpm idle used for cold start is not disengaging properly after the engine warms up. Air conditioning will raise the idle speed by a couple hundred rpm, so avoid using it until you have the problem fixed. BTW.....warm engine rpm in idle, and neutral should be around 700-800. Anything over 900-1100 will result in a hard shift into 1st gear and dragging at stop lights.

For this car it shouldn't go over 900, but should have a good idle around 700-800. It sounds like a lot, but my expedition idles at 700-900, depending how fast i want it to heat up.

mcdevito75 here, It could be the air idle sensor or motor, have your shop run a scan or change this air idle motor / sensor.

Sounds like the Oil Gauge sensor or level unit.

Try to disconnect the Air Flow Meter. It is the first plug just before the air filter/air intake box. If this improves your situation you will need to source a new or aftermarket Air Flow Meter.