Background: 2007 Honda fit. 70000 is miles on it

The simple timeline is this:
About a year ago, I began to feel misfires. I checked the CEL at auto zone and I can't remember what the exact code was but the description was a misfire in cylinder 3.

I replaced all of the spark plugs to be safe. This removed the CEL and the misfires stopped.

That lasted around 5 weeks before the misfire and CEL returned.

I went in again to check the park plugs to see if any of them had become fouled up. They all looked fine.

Eventually the CEL disappeared without having changed anything. The misfires continued but were less consistent than before.

Then two semesters of school hit and I didn't give it the time it deserved. During those two semesters I used some fuel injector cleaner to see if that would be a simple fix. I also got a CEL reader and consistently looked at the codes. It never gave another cylinder 3 misfire code. It eventually started to throw a P0420 code.

Now I'm back looking at it to diagnose the problem with a little more diligence.

The misfires are now intermittent. Some days I drive it for an hour (at high and low speeds) and get no misfires at all. Other days it begins to misfire right from the beginning and doesn't really stop.

It still has not thrown another cylinder misfire code as of yesterday when I checked.

I've been on here looking at other posts specifically about the catalytic converter. Issue is that misfires can cause CAT problems from what I've seen. So I'd like to fix the misfire and then take a look at the converter.

Could be the O2 sensor also.

I have some questions as a result.
1) Any ideas on the misfires? Perhaps changing the coil packs? Some kind of gasket issue inside the cylinder itself?
2) Are there tests to distinguish between the O2 sensors and the CAT for a P0420 code?
3) What can I do to make better posts on this forum as this is my first attempt? Length, content, description, etc.

Thank you much. I do appreciate it.

 

Misfires and poor running can definitely cause a cat code (P0420).

Figuring out the true cause of misfires and poor running is up to you. All the forum can do is suggest parts to throw at it.

A set of 4 Hitachi ignition coils would probably be the first things to replace, if they aren't new already.

The spark plugs should be Denso KJ20DR-M13 or NGK IZFR6K-13
Never Bosch. I know the DangerZone pushes them hard, but most of them are crap.

---

Even a brand new cat can be ruined by a poorly running engine, in a matter of only minutes if it's running badly enough.
So...
Figure out and fix the misfire problem before doing anything about replacing the cat.

Fix the misfires, then erase the codes, and see how long it takes for the cat code to return before deciding about replacing it.


In the future, fix running problems immediately so they don't cause bigger problems.


HTH

 

Change ignition coils. Had the same issue twice. 1st time I bought some cheap coils, after a year they were busted. Thinking that it's not the coils I replaced PCV Valve and hoses, cleaned Throttle Body, cleaned IACV and adjusted the Idle, Adjusted valve tapets, still no go. Bought original hitachi coils, solved the issue!

 

OP must hook up the scanner and monitor the cat temperature; it should be 400 C with engine running if it is 100 C or such the cat is contaminated

 

752F for the C impaired.. My 07 runs up near 900 some of the time, 1100 with a can of seafoam in the tank. (High RPM.. )

 

ahhh jerky fit syndrome I just purchased 4 new Honda coils from Honda I know there hitachis maybe could of gotten em cheaper but I'm not playing around and I get the dealer warranty on the part it makes me rest a little easier at night but it I would also make sure but for me its been a coil issue and I think Honda fixed the coil issue given the new part number

 

Was your car randomly missing (aka jolt) when idling in drive at the traffic light?

 

Always have the battery tested when fighting ignition problems, My fit got quite cranky as the old battery started to die.

I'm also considering a ground cable from the one that goes for the battery to the fender,
I want to add a cable from the fender bolt to the engine block so the car isn't in the equation for grounding the cylinder head The cable is already tiny, can't hurt anyway...

 

Bad battery will be accompanied with dim lights erratic gauges and overall reduced engine performance

 

she was missing at idle and when you try to get out on the highway she sputters and last two times they changed the coils it was corrected

 

I had the same issue a while back. Replaced all my coil packs and it didn't help. The reason was pretty simple in my case. I purchased spark plugs that Auto Zone recommended for my car. Ended up buying the proper ones from Denso (I think...) and fixed the problem.

 

At what mileage?

 

I did spark plug replacement around 110 ~ 115k miles I believe. It worked fine for a while but after a few months, the car started to misfire and threw out CEL that kept going on and off.

I knew the coil packs were a common issue so I purchased 4 new ones which didn't seem to solve anything. In the end, I ended up looking up the correct spark plug and purchasing from Denso (or NGK, I forget) and haven't had an issue since.

 

My 2007 Fit also had a mysterious idle misfire. When it became bad enough to throw codes they indicated a random misfire on all four cylinders. My mechanic thought that the engine computer was bad. He purchased a used one on E-bay which then indicated that the misfire was only on the number one cylinder. Replacing the coil pack and the spark plug on that one cylinder solved the problem.

He charged me for the computer and threw in the coil pack and spark plug for free.

 

In my case, the misfire was all caused by spark plugs. I once had my coil pack actually go bad and it wasn't firing in that cylinder at all.

The engine was running very poorly so what I did was remove the coil pack for that cylinder while the engine was running and it made no difference. I then removed all the coil packs, 1 at a time, to see if the idle got worse or not.

I'm not sure if all the bad coil pack symptoms are the same as mine but it was a fairly simple and effective test for me.

 

 

I noticed my coil pack related misfires were more noticeable on humid or rainy days. I replaced my coil packs long before they ever threw a code as I hated the misfire at idle. I replaced at approx. 75K and right on time again at 150K.

 

Really good thread! I agree with Sea Foam, or MCCC (Mopar Combustion Chamber Cleaner). Worked real good on DSM motors back in my turbo AWD Eagle Talon days. You let it get sucked into the intake and of course it coats the intake valves going in. Makes some vape-esque smoke clouds though...

 

Post contents erased because it was not meant for this thread. Here is my post, now where it belongs (CEL random misfires on all 4 cylinders in 2nd gen Fit):
https://www.fitfreak.net/forums/2nd-...ml#post1434569

Sorry doctor J and everyone here.

 

Plus for posting photos , minus for lack of search!
The so called rust is combustion residue that leaks past the threads of loose plugs
Even when plug is tightened (means effort is needed to break it loose), but seal washer is not fully compressed or is seating on rough surface, the gases will leak and satin the coil
I see such almost on every second Fit on the junkyard
(https://cimg8.ibsrv.net/gimg/www.fit...9579453a32.jpg)
However, yesterday I saw Fit with nice blue coil boot tips (053)