Car has really low idle, about 200-500 RPM

Hi and thanks for using 2CarPros.

As long as you didn't cross the negative and positive it's normal to see a very small spark because there are components which always draw power.

Now, if this is something you just did and the vehicle is still safe to drive, put some miles on it. The computer may need to complete a relearn process. Basically, since the battery was disconnected, learned programming needs relearned.

If it doesn't change after fifty miles of use, let me know.

Joe

Welcome back

The idle speed isn't adjustable. So, with that being said, is there anything you disconnected or could have bumped when you were working on it?

There is a diagnostic flow chart for idle speed issues, but it requires a live data scanner. I will attach those directions at the end of this post.

Since nothing should have happened to the idle air control system, check to make sure nothing was accidentally disconnected or damaged as far as wiring around the battery. Next, confirm that there are no engine vacuum leaks and the air intake tube wasn't bumped and allowing unmetered air into the engine.

Here is a link that shows how to check for vacuum leaks:
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Here are the directions for diagnosing idle speed problems. Again, I'm adding them just to see what is required. If you have access to a live data scanner, you can follow it.

Idle Speed

INSPECTION
1. Before checking the idle speed, check the following:
1. Ensure the air cleaner element is free from clogging, ignition timing is correct, spark plugs are in good condition, and hoses are connected properly.
2. Ensure the malfunction indicator light does not illuminate.
2. Idle the engine.
3. Stop the engine, and turn the ignition switch to OFF.
4. When using Subaru Select Monitor, refer to the following.
1. Insert the cartridge to the Subaru Select Monitor.
2. Connect the Subaru Select Monitor to data link connector.

Picture 1

3. Turn the ignition switch to ON, and Subaru Select Monitor power switch to ON.
4. Select (Each System Check) in the Main Menu.
5. Select (Engine) in the Selection Menu.
6. Select (Current Data Display & Save) in the Engine Control System Diagnosis.
7. Select (Data Display) in the Data Display Menu.
8. Start the engine, and read engine idle speed.
5. When using the tachometer (Secondary pick-up type):
1. Attach the pick-up clip to No. 1 cylinder spark plug cord.
2. Start the engine, and read engine idle speed.

Picture 2

NOTE: This ignition system provides simultaneous ignition for #1 and #2 plugs. It must be noted that some tachometers may register twice that of actual engine speed.

6. Check the idle speed when unloaded. (With headlights, heater fan, rear defroster, radiator fan, A/C, etc. OFF)
Idle speed [No load and gears in "N" or "P" range].
MT model 650 ± 100 rpm
AT model 700 ± 100 rpm
7. Check the idle speed when loaded. (Turn the A/ C switch to "ON" and operate the compressor for at least one minute before measurement.)
Idle speed [A/C "ON" and gears in "N" or "P" range]. 850 ± 100 rpm

NOTE: Idle speed cannot be adjusted manually, because the idle speed is automatically adjusted. If the idle speed is out of specifications, refer to General Diagnosis Table under "Engine Control System".

Let me know if this helps or if you have other questions,

Take care,
Joe