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Idle Speed

Why does a car idle at varying speeds. One day idling 700rpms, the next day 500rpms?

Likely a vacuum leak in the intake pathway. What kina car?

What make, model, year, engine option? If your looking for a list, it could be dirty throttle body, lazy IAC (idle air controller motor), need a tune-up, vacuum leak, dirty injectors, weak fuel pump, bad fuel, dirty air filter, bad CTS (coolant temp sensor), bad AATS (Ambient Air Temp Sensor), weak coils, dragging torque converter, and maybe a few other things.

Vacuum leak,really you must tell us the car type.Maybe a 911?Various induction methods for 911,unless your screen name does not refer to your car type.

98 honda civic ex - had a smoke test done - they could not find the leak. There may be other tests for that, I am just not aware of them.

No it doesn’t - the Portia911 is a knick name. its a honda 1998 civic ex. It seems to occurr more in warm weather

Bad or dirty IAC?

A car’s computer will adjust the idle speed as demand for power from the engine increases and decreases. For example, let’s say the electric radiator fan comes on automatically, or you turn on a few electrical accessories, or your battery is at the end of its life. The computer might increase the idle speed to increase output from the alternator. That would certainly explain why it happens more in warm weather. It probably happens as your air conditioning compressor cycles on and off or the electric radiator fan cycles on and off.

Also, what was the engine temperature at the time you noticed the different idle speeds? Before the engine warms-up, your idle speed might normally be at 700 RPMs, and it might slow to 500 RPMs after the engine is warm.

In my opinion, based on this and the many other threads you have started, you are obsessing over this car. Seek help for your OCD and stop worrying about your car. Either that, or take it to a trusted mechanic for a proper diagnosis.

At first, I didn’t take your comment about OCD seriously, but now I am thoroughly convinced. Let it go.

You could try covering your tachometer with electrical tape or duct tape.