Misfire Mystery (NOT Plugs, Coils, Injectors, Lash, Compression) - What's Left??
 

I'm hoping to get some fresh eyes on a persistent misfire issue I'm having with my 2008 Honda Fit Sport 5spd.

I've done a fair bit of troubleshooting, but no matter what I try, it consistently misfires specifically on Cylinders 2 & 4 (P0302, P0304 & P0300).

Symptoms

• At lower speeds and cruising, the car generally feels fine.
• When accelerating in 3rd and 4th gear at around 4500-5000 RPMs, the engine will stutter and the check engine light flashes.
• If I back off the throttle the engine smooths out. The idle does becomes a bit jumpy after that, but will settle down if I clear the codes.

What's Been Done

1. I swapped ignition coils between cylinders. The misfire did not follow the coils, so spark plugs must be the issue.
2. Replaced all of the spark plugs (NGK 6774), cleared codes, took a test drive, and still got misfires on 2 and 4. Figured it must be injectors.
3. Rebuilt and swapped injectors, cleaned intake manifold, and adjusted the valve lash. Cleared codes, took a drive, same result. Maybe it's worn piston rings?
4. Ran both dry and wet compression tests. No significant variations between the cylinders, so probably not rings.

Now I'm at a loss of what to check next.

If the misfire were random, then I'd consider fuel filter, EGR, PCV, etc., but I don't see how those could make misfires always happen on Cylinders 2 & 4.

Could this be something related to VTEC? The misfires do happen at the RPMs where VTEC normally kicks in. Though I'm curious how something like a partially clogged VTEC solenoid screen could cause misfires on specific cylinders.

Any advice is appreciated!

Leak-down test to check for valve damage. Electrical - wiring and relays associated with the coils, injectors and ECM, swap ECM itself.

Update / Resolution

After a lot of troubleshooting, I finally got this resolved.

The mechanical issue turned out to be just a single bad ignition coil.

The main reason it took so long to pin down was that the bad/cheap OBD2 Bluetooth adapter I was running with Torque Pro wasn’t actually clearing the stored codes. The ECU was just stacking new misfire codes on top of old ones. The only times the codes were actually cleared was when I disconnected the battery to change plugs, swap coils, check compression, etc. That's why there was no rhyme or reason to the misfire codes.

Bottom line, make sure you have a good OBD2 scanner and, just to be safe, always rescan to confirm codes are truly cleared before continuing diagnostics.

Hi
I recently sent my car to the mechanic to read the codes for a check engine light (I don't currently own an obd2 scanner) and also have misfires on cylinders 2 & 4.
I realised I had a bad ignition coil a few month back when I replaced my spark plugs, so I replaced it. It was the back-side coil of cylinder 2 (I have a i-DSi engine).
Because of that my mechanic told me there might be an issue with my coil wiring harness but I find that unlikely.

Which ignition coil was bad in your case ? And how do you figure your first coil diagnosis failed to show the truth ?

If you want to diagnose it yourself, you'll need a scanner of some kind. Decent units can be had for $50 or less that will read most basic codes. For $200 or less you can find good units that will also read & reset ABS, SRS, TPMS, etc.. Just be careful with the cheap $10 Bluetooth adapters, that's what I'd been using. I've had several before and never had an issue until this current one.

The standard way to test the coil is to move it to another cylinder. For instance, if you get a misfire code for cylinder 2, trade that coil with the one on cylinder 1. Clear the codes and wait for the check engine light to come on again. When it does, if the misfire code moved to cylinder 1, then you know the coil you moved is bad. If the code stays on cylinder 2 then the coil was probably fine and you should look at spark plugs, fuel filters, injectors, etc. which are far more common than a failing wiring harness.