Hi guys. Sorry about yet another misfire thread but I wanted to present a list of what's been done so far in the hopes of something else to try. So please accept my escalating this to you guys (and the rather long post); I wanted to show I did my homework but am still looking for more.

'08 Sport. 128K miles on odometer.

Here are the symptoms:

* Rough running immediately after cold start, where it abruptly runs smoothly after ~5-6 seconds of runtime.
* CEL flash when engine transitions from rough idle to smooth cold idle. P0304 indicated in RAM.
* Engine and transmission deliver strong power (for a Honda, at least).
* Overall fully warmed up engine idles smoothly. Occasionally stutters when raining/wet/humid.
* By the computer, running anywhere from 27mpg city/33-34mpg highway.
* By the tank (dividing total miles by gallons filled), anywhere from 26-27 mpg.
* I end up needing to fill near the bottom of the tank after about 200-230 miles to satisfy my fuel safety margins (I don't like getting stuck anywhere. Did it too many times when I was younger and stupider.). Given that, I'm usually putting anywhere from 9-10 gallons in. Still paying ~$20 per fill-up so I'm happy with that.
* During light to moderate acceleration there is a light but noticeable clattering noise underhood. Seems to be most prevalent at the 1,800-2,300 RPM range -- goes away if I pull the gas or stomp on it.
* Computer does flash P0304 occasionally under moderate acceleration randomly. I never feel any actual misfire (no stumbling, no stuttering, no audible noise from engine) but I do notice a drop in overall output temporarily before the CEL flashes. Interestingly enough, 0304 is the ONLY code I've seen it ping -- none of the other cylinders appear to be misbehaving. Again, this is a random occurrence -- I can drive it all day, a hundred miles at a time and it won't hold back until it seemingly arbitrarily decides to hold back and ping 0304; then it's back to normal.

What I know that has been reported done to the vehicle prior to my ownership:

* Valve lash adjustment to factory specs.
* Fuel injector replacement for one cylinder.
* Wiring harness to transmission replaced (due to rodent damage).

What I know about the state of the vehicle:

* Does not burn or leak oil.
* Does not appear to burn or leak coolant.
* Am not followed everywhere I go by a big embarrassing cloud screaming "Hey everyone! Look how cheap I am!"
* There IS however a smell exhaust-side like the engine is running with high exhaust gas temperatures and rich of peak. It's noticeable when the engine is fully warmed up and having been driven for a while. It's especially noticeable when I'm reversing with the windows down. However, the last time I was underneath the car I did not notice any discoloration of the exhaust manifold, the downpipe, or the catalytic converter.

What I've done to it so far:

* Replaced all four coilpacks (Hitachi IGC0053)
* Replaced all four plugs with Denso irridiums (5358's IIRC).
* Replaced downstream O2 sensor (Denso 234-4358).
* Top-end cleaning.
* Pulled and cleaned the EGR (passenger side of intake manifold) as best as I could).
* Both driven the piss out of, and babied this vehicle.

Theories:

* Mechanic lied about doing valve lash adjustment, or did adjustment incorrectly?
* Head gasket on the #4 cylinder side leaking into cylinder?
* Another fuel injector needing to be replaced?
* Inadequate grounding?

Why I bought the vehicle in the first place:

* Um . . . I needed a project?

More seriously, all of the above hasn't overtly affected the driveability of the vehicle -- it has more been impacting fuel economy and my level of annoyance. The car is actually very well-built. It gets me around with, ignoring the above, very little fuss.

Anyway. What do you guys suppose I'm missing?

 

Denso 3401 is the plug that should be used.
How the old plugs were looking?
Are the new plugs tight? (pull the coils and re-check)
Where did you purchase your coils?
Which injector has been replaced (perhaps from #4)?

 

K. I'll get 'em replaced as soon as possible. The old set looked worn -- not fouled but just worn and carbonized. I still have them stashed somewhere in my garage and could shoot a pic of both old and new for comparison if it helps.

Verified tight, yes.

Through Rockauto.

I'd need to verify that unfortunately -- I didn't get that information from the last time I talked to the last mechanic who worked on this car. Is it possible there could have been multiple injectors going bad, and the worst one was replaced? I suppose I could take some time and swap #3 and #4 and see if the code follows.

Thanks for the quick reply!

 

I'd get in there and verify the valve lash.

If good, it's time for compression and leakdown tests.

 

Likely causes:

1. Plugs - Replace plugs
2. Coil - Swap Coil 3 and 4, see if it the misfire moves to cyc 3.
3. Intake gasket - poor/spray water over the intake and listen for a change in RPMs. If there is no change in RPMs it is not the intake.
4. Head gasket

 

if original harness was damaged by rodent it may be a good idea to double check it again , especially if miss will stay in cylinder #4 after 3 to 4 coil swap

 

Alright guys. I had some time tonight so I swapped #3 and #4 coils and I pulled the plugs for #3 and #4. Turns out according to the manufacturer codes I did actually buy the correct plugs last time around, but they're in pretty ratty shape:




#4 plug.



#3 plug.

At least I know they're still on the to-do list. I'll let you guys know more as I know more.

 

They don't look THAT ratty to me. 'Course the gap might be kinda large.

If you check the gap be careful, see the small electrode and little "puck" on the ground leg? Those are easy to break or damage. (Which is why gapping Iridium plugs isn't recommended.)

 

Plug #3 looks normal. Perhaps a little lean mixture. Can you post picture of #4 plug insulator?

 

Ok, folks. I just did a 10 mile drive in moderate to heavy traffic. Before I cleared codes, I saw a new code entered: P0172 which after clearing codes and filling up did not recur. The results of the drive:

* CEL did not ping solid until I was almost home (about 9 miles in).
* Code stored was P0304.

This was after I swapped the coils for #3 and #4. I didn't really notice any change in operating characteristics however while there was still some light stuttering, I did NOT see a flash CEL indicating misfire immediately after startup.

Sure. I'll need some time to pull #4 again but I do remember that the insulators for both #3 and #4 looked very similar.

I'm monitoring things via torque and an OBD2 dongle; is there anything I can watch out for, data-wise, that might help?

 

Looks like wiring testing from coil #4 is in order (providing that injector #4 electrical connector is properly hooked up.

With coil removed and ignition off plug the test lamp 12v (3.4 watt) between black and black/wht wires into the coil connector (use backprobe method for best result; I use male terminals from old Toyota igniter which seems to fit) with ighnition on(engine off the lamp should be on. Nest move wire harness including splitter connector under air cleaner housing (it has rubber boot); the light should not blink
next inspect wht/blue wire between ECU and coil connector.
I cannot rule out valve issue on cylinder 4 so you could check the valve lash on this cylinder only, however the tight valves cause misfire at cold engine (for Fit, at least)

 

Ok. This is where I demonstrate my rather wanting knowledge of electrical theory -- while I don't think I have a test lamp, I do have a multimeter. Would it be a waste of time to probe the pins on the coil connector corresponding to the wires as specified, to test for voltage and continuity (positive to lead, and ground to chassis/engine)? Would doing that generate anything useful or is it time to hit the auto parts store?

I think I'm on board with you on the valve lash theory. I paid attention the last time I was out, and the sound I've been hearing is like a soft metallic chattering most noticeable between 2,600-3,200 RPM under light throttle. That's about when VTEC is supposed to engage IIRC; I wonder if this is symptomatic of a different issue.

I'm also wondering about the EGR, come to think of it. The last time I pulled it, I cleaned it the best I could but I don't remember if the internals were moving as they were supposed to. I also remember reading somewhere in this forum if the EGR valve is stuck it can mess with the computer and the fuel trim.

I dunno. Maybe I'm just grasping at things and need to actually enjoy my Friday tomorrow.

 

test lamp (or LED) is better for spotting intermittent connection. just for kicks swap the spark plugs 3 and 4. Based on the information that harness has been replaced, I would check it next

 

Alright. Will do. I'll let you know.