Manual 2009 Civic Si Doesn't Start (Initially Intermittent)

Question

Problem: About 5 times, I would leave the car for a brief time (~5-15) mins after driving, and when I came back, it wouldn't start. It would randomly start again either immediately or after leaving it alone for 5-10 minutes. Now it won't start at all. If I leave it alone for a bit, the starter "tries" and gets worse, and worse on subsequent retries, then gets a little bit better after the car sits.

There is no CEL or codes.

Attempts to fix:

1. Replaced battery - it's always step 1 and it was time anyway.
2. Changed key battery - someone said it might help due to security chip. I don't get it, but whatever.
3. Tried jump-starting - doesn't help at all.

Theories: Please help me identify the most plausible / test.

• Relay/fuse (diagram) - Ignition Switch Main? Main Fuse? Ignition Coil?
• "Wire thingy that goes from key cylinder to something" (edit: ...I meant ignition switch) - I have no idea what this is called, but there is a wire that usually signals that the key is turned to start or whatever to the ECU. I think I had this problem on an Integra before. What is this called? Could it be it?
• Key FOB chip / whatever reads it. How do I even troubleshoot this?
• Clutch "safe start position" switch (seems unlikely to me)
• Other?

Would appreciate any advice on this.

Answer 1

The problem was the starter. If you end up doing this job, know that you don't have to take the intake manifold off, and can just remove the bracket holding the hood lock mechanism. This job can also be done from the bottom of the car. As of right now, the original JDM starter is $400 on Honda Parts Now. I didn't require any other parts for the job, but you may want to clean the bolts and apply anti-seize to them. Wrenches and sockets needed (not exhaustive list):

8mm, 10mm, 14mm (will need extension if not taking mani off), a hex key - don't know size. Everyone claims both the starter bolts are 14mm. My "near" bolt was 17mm and it seems to have been original.

Take pictures as you progress, there are enough similar bolts to get lost.

Other tips:

• Disconnect the black battery terminal before you do anything else
• When putting the bracket back on, start all the bolts before tightening them or it won't align
• Do NOT close the hood until everything is properly in place - maybe even check that the hood cable still works and has all the clips holding it in place - it's easy to break, and if you shift it and close the hood, your hood won't open. This is a major pain to fix
• There are rubber anti-vibration pieces connecting to the bracket near the hex key bolts - I left them on thinking they won't go anywhere and lost one
• If you do decide to take off the mani, you might want to get the gasket for it
• Breaking off sensor/cable holders will be a real issue here

I broke my hood-lock cable while doing this job somehow, so be careful.