Help me diagnose P Codes (Misfire)

CEL is on in my '03 S2000. Here are the P Codes:

P0304 - Cylinder 4 Misfire
P1399 - Random Cylinder Misfire Detected (looks like a Honda code)

So, seems to pretty obviously to be a misfire. I think the misfire is happening intermittently at idle when cold. Car does not appear to be down on power at speed. I have noticed it sort of stumble a couple times at idle on startup, particularly if there is also load from the A/C. I replaced spark plugs over a week ago but kinda messed up (details below). Car didn't run any differently, but the light stayed off for over a week, then popped up again with another instance of stumble at idle. So, let's go through the possible culprits:

Spark plugs: I replaced these a bit over a week ago. But I kinda screwed up. Was told the wrong gap by the counter guy, and regapped all of them (not out of spec, just to the very widest end of spec) using the iridium tips for leverage. I don't think I cracked or bent an iridium tip, but I may have damaged one and not known it.

Cables: S2000 uses coil-on-plug. Seems unlikely to be the issue.

Fuel? Check fuel filter? Run some concentrated fuel-line filter through the system?

Vacuum Leak? Maybe a problem with the EGR valve?

So... what would you do to address this? I should probably replace the plugs again, given my ham handedness. Then I was going to throw in a bottle of good fuel additive (Techron concentrate) and spend some quality time with VTEC. If that doesn't do it..? Would I look to EGR or fuel filter first? The filter is inside the gas tank, but I gather is accessed from above.

A local racer had misfire cylinder codes in his AP1, turned out to be a bad injector. YMMV

Since it's setting a specific cylinder code, try swapping things around and see where the code goes. Example: switch cylinder 4 plug with cylinder 1, cylinder 4 coil with cylinder 3, and cylinder 4 injector with cylinder 2. If the code moves cylinders you've identified the bad component

Careful running it too long- it will destroy the catalyst- in a manner that you may not like to deal with- melting it.

Run_Away wrote: A local racer had misfire cylinder codes in his AP1, turned out to be a bad injector. YMMV Since it's setting a specific cylinder code, try swapping things around and see where the code goes. Example: switch cylinder 4 plug with cylinder 1, cylinder 4 coil with cylinder 3, and cylinder 4 injector with cylinder 2. If the code moves cylinders you've identified the bad component

Changing all 4 spark plugs resulted in the same code as last time.

I'll try the injectors.

It's also weird because the code is intermittent. It was clear for - I think - two weeks.

Beer Baron wrote: Changing all 4 spark plugs resulted in the same code as last time. I'll try the injectors.

Did it come back as Cyl 4 misfire?

If so change injectors between 3 and 4. Might also change coils between 2 and 4. That way you can test two things at one time.

Hal wrote:

Beer Baron wrote: Changing all 4 spark plugs resulted in the same code as last time. I'll try the injectors.

Did it come back as Cyl 4 misfire? If so change injectors between 3 and 4. Might also change coils between 2 and 4. That way you can test two things at one time.

In general (although possibly not on S2000s) bad coils are more common than bad injectors. I'd pull the plugs again to see if there was any damage from your gapping technique and swap the #4 coil with another. I'd also do a compression test while I had the plugs out just to rule out a mechanical issue.

Well, I'm a moron. Swapped the injector and coil form cyl 4 onto other cylinders to see where the code moved to... but I didn't put any kind of mark to remember whether I put the injector on cylinder 3 and the coil on cylinder 2, or the injector on 2 and the coil on 3. I figured the code would pop up again in a week, and it didn't come back again until after I left the car sitting in the garage for over 2 months for winter.

Code popped up 2 days ago. P0300, P0303, P0304, and P1399. So kinda looks like it sorta jumped to cyl3, but I can't remember if that's the coil or the injector. :-/

Swapped coils from 3->2 and 4->1 to see that does anything in another week or two. Coils are a lot easier to move than injectors.

The code seems to throw only in a very specific situation. Car has to be started, brought to temp and then sit for an hour to allow it to cool a little bit, but not all the way, then started while the engine is warm enough to lower the idle speed but not up to full operating temp (2 temp bars instead of 1 or 3). Then it throws a code immediately on start up. If the car is cold, the raised idle prevents it from throwing the code. If the car is still up to full operating temp, it doesn't throw a code. If the car is already running and idling, it doesn't throw a code.

Hmm. I don't like that it's still showing a #4 misfire as well as the #3. Do you have a scanner that will let you look at the freeze frame data? That may be necessary to really get to the bottom of this problem.

APEowner wrote: Hmm. I don't like that it's still showing a #4 misfire as well as the #3. Do you have a scanner that will let you look at the freeze frame data? That may be necessary to really get to the bottom of this problem.

I do not. It's just a basic code reader.

Showing cylinders 3 and 4 seemed weird to me, too. Firing order is 1-3-4-2. So it's possibly that the problem is on cylinder 3, but it threw the timing off enough to affect the next cylinder in order.

Does this engine need the valves adjusted with shms?

Could be the first signs of some valves getting tight.

Misfire after a hot soak could be head gasket. But let's pretend it's not and concentrate on ignition coils or injectors. Moving them (and marking them ) is the cheapest way to diagnose.

NEALSMO wrote: Misfire after a hot soak could be head gasket. But let's pretend it's not and concentrate on ignition coils or injectors. Moving them (and marking them ) is the cheapest way to diagnose.

That's about half of why I posted this here: to air my foolishness in not marking them, and to be certain I had a record of what got moved from where to where.

Bad head gasket is something I have not heard as a diagnosis for these error codes. How would a bad head gasket cause an intermittent misfire and no other issues?

Beer Baron wrote: Bad head gasket is something I have not heard as a diagnosis for these error codes. How would a bad head gasket cause an intermittent misfire and no other issues?

Depending on how bad, coolant can leak into the combustion chamber,and could cause the miss when restarted. But you would likely have a lot of smoke, and rough running. But depends how bad.

Where are you getting fuel from? Unlikely, but some stations can draw more condensation in fuel and cause some hiccups on cold starts.

My number one go to is always to swap the coils cylinder.

A head gasket failure will dampen the plug and short stuff out until its had enough air pulled through to dry it. Really, the most accurate way to test is to stick your nose down by the tailpipe when its missing and see whether you get the sweet smell of coolant.

Honda V6 have had some misfires due to valve adjustment, but I have no idea whether that is common on any 4's. Might be worth running the lash.

for what its worth (likely nothing), i chased a misfire code on my wifes odyssey for 6 months. it was valves out of adjustment (too tight, not closing exhaust valves).

a compression test at your 'magic temperature' would be a good idea.

-J0N

also dont discredit the boot on the ignition coil being bad, i have seen them fail and cause misfires

jmthunderbirdturbo wrote: for what its worth (likely nothing), i chased a misfire code on my wifes odyssey for 6 months. it was valves out of adjustment (too tight, not closing exhaust valves). a compression test at your 'magic temperature' would be a good idea.

Would a compression test reveal valves out of adjustment? I can imagine if there's always some pressure on a valve that that cylinder would be just a tad low on compression.

I'm also hoping to see if swapping a coil around makes the error code move. I would expect valves needing adjustment to remain on the same cylinders.

edizzle89 wrote: also dont discredit the boot on the ignition coil being bad, i have seen them fail and cause misfires

Boot? I'm not sure exactly what you mean by 'boot'. This is a coil-on-plug system.

Okay. It finally threw codes again. P0300, P0302, P0304, P1399. So... it would appear the misfire followed the coil from cylinder 3 to cylinder 2.

But that doesn't explain the cylinder 4 misfire staying constant. Which implies possibly some other issue with cylinder 4? May have a bad coil and need to adjust the valves?

Adjust the valves, it only costs you a valve cover gasket and they're probably due anyways.

Argh! The saga continues.

Replaced the coil on Cyl#2. Adjusted the valves (they needed it). Car idles smoother and I'm getting better fuel mileage.

And.... the CEL pops up again.

This time: P0301, P0304, P0300, P1399. In that order. Cyl #1 seems to be the one having the issue.

What the heck now? I'm guessing injectors. Not sure which one, since the issue seems to be migrating. Don't really feel like spending $80x4 for all new injectors if I don't have to.

Kinda thinking of punting this to a shop. Car is due for annual maintenance anyway. Kinda thinking I'll ask them to diagnose the issue then most likely do the work myself.

Beer Baron wrote:

edizzle89 wrote: also dont discredit the boot on the ignition coil being bad, i have seen them fail and cause misfires

Boot? I'm not sure exactly what you mean by 'boot'. This is a coil-on-plug system.

the boot being the rubber sleeve between the coil itself and the spark plug, ive seen these go bad every now and again.

In reply to edizzle89:

All the boots looked fine.

Trying to decide if I keep replacing things and trying to diagnose the issue, or just punt to a shop to diagnose, even if I do the repairs myself.

Beer Baron wrote: In reply to edizzle89: All the boots looked fine. Trying to decide if I keep replacing things and trying to diagnose the issue, or just punt to a shop to diagnose, even if I do the repairs myself.

i've seen a miata COP boot that looked fine, except for the pinhole about halfway down where the spark was shorting through to ground.

Okay. Bringing this back up. It continues to elude me. P codes are on all 4 cylinders.

Dealership said it was spark plugs. Replaced them with OEM as directed, but that didn't solve it.

I sent the injectors to Marren and got them cleaned. One was flowing way too high. Definitely something that could (would) cause rough running at idle. Replaced it. Other three were cleaned. Car seems to idle a bit smoother. P-codes popped right back up again.

Really not sure what to look at next as likely causes or how to diagnose that. Crankshaft Position Sensor? Bad alternator not putting out enough amps at idle? PCV valve? Grimy MAP sensor and/or IACV?

A bent valve could do it, but I would expect other symptoms. That also seems unlikely to have started with only one misfire code and spread to like... all the others.