My mom's 2003 Grand Caravan Sport with 3.3L FFV (Vin 3) has a P0306 misfire. The code doesn't appear as often as the rough idle would suggest. In datastream mode the scan tool does register misfire counts on cylinder #6 but no misfire counts on any other cylinders. Dad changed out the plugs and plug wires searching for the problem, but all that did was take it from a P0300 random misfire code to the current P0306. We've double and triple checked the plug wire connections. I connected an inductive probe on the #6 plug wire and monitored on the oscilloscope. Randomly there is a goofy pulse that has a decaying fall time rather than the more typical quick fall time. We checked cylinders #2 and 4 as well and these two do not exhibit the same goofy pulse. We then swapped plug wires and the goofy pulse remained. Next we swapped plugs, no change to the goofy pulse. I made a new (8mm) plug wire, the goofy pulse remained. The plugs are OEM Champion double platinums.

I also installed a t-harness in series with the fuel injector for injector #6 and monitored it on the scope. The injector pulse looks fine, no indication of a problem with the injector and the injector resistance is within spec at 14 ohms. So, since the injector pulse is fine, I think the CMP sensor is okay.

We pulled the coil and checked the resistance of the wires from the PCM to the coil and they were all 0.1 ohms, so they seem to be clean and solid.

The PCV valve seems fine (rattles, only flows one way) so we didn't replace it.

Next we used SeaFoam to do a cylinder decarb. Don't yet know if when that gets done burning off it will make a difference.

We also replaced the coil. The goofy pulse seems to be gone, and the scan tool wasn't showing any misfire counts on any cylinders yet, but the engine still seems to idle just as rough. I don't yet know if things got better for Dad on his drive home after burning all the SeaFoam out, but I'm doubting it will.

Any other suggestions? I apologize for the really long post, I just didn't want to be accused of leaving out details.

-Rod

Scope plot showing what I presume is the misfire versus injector pulse.
http://i538.photobucket.com/albums/ff342/raschmidt/Automotive/th_Cyl6_plug_wire.jpg (http://s538.photobucket.com/albums/ff342/raschmidt/Automotive/?action=view&current=Cyl6_plug_wire.jpg)

One of the better posts on this subject I've seen in a while. Nothing to appologise for.

http://www.obd-codes.com/p0300
http://www.obd-codes.com/p0306

Is it just at idle or do you notice it across the board? Any indications of a leaking injector like hard starting or raw fuel smell? I'm thinking a scanner will detect a normal pulse from the PCM but not a leaking injector. Yes/No? Concider a compression check if you haven't yet.

It is only at idle that the roughness is noticeable. The van starts fine and the scan tool suggests that the exhaust is lean by giving high O2 sensor readings on bank 2 when idling rough. The fuel trim numbers, both short and long term, suggest the PCM is not trying to compensate either way. However, Dad did say the fuel economy is down.

We did talk about a compression test, but haven't done so yet. Would we need to pull all 6 plugs to do so?

-Rod

Try moving the plug and/or wire to another cylinder and see if the problem follows. I got a bad plug from Auto Zone once. It took me forever to figure it out. It can happed that either of the new wires or plugs are bad.

Try moving the plug and/or wire to another cylinder and see if the problem follows. I got a bad plug from Auto Zone once. It took me forever to figure it out. It can happed that either of the new wires or plugs are bad.

Yep, tried that (last few sentences or the first paragraph of OP), even made up a new 8mm wire and tried one of the old plugs.

-Rod

Yep, tried that (last few sentences or the first paragraph of OP), even made up a new 8mm wire and tried one of the old plugs.

-Rod

Missed that, sorry.

Missed that, sorry.

No problem, I appreciate you taking the time to offer suggestions.

-Rod

You could try verifying the misfire the old fashioned way- pull the plug wire (or injector plug) at the suspect cylinder, little change would confirm the miss. I assume the plug looked similar to all the rest when you pulled it?

I believe high O2 sensor voltage means low oxygen content in the exhaust (rich).

You're correct, the high O2 reading would indicate rich.... I think I need to make myself a cheat sheet, my head constantly tries to convince me that since the sensor is sensing oxygen, a higher voltage would indicate a larger content of oxygen in the exhaust stream. I guess I need to remember that it's backward of what I expect.

We haven't tried disconnecting the plug wire or the injector plug and running the van. We did install a spark test wire in series with the spark plug and didn't see an erratic spark, but could try pulling the wire off and connecting to a spark tester. I don't want to just leave the wire open circuit since I've heard that can be hard on the coil packs. Fortunately injector #6 is the only one that is relatively easy to get to the connector, so we could try disconnecting the injector as well. The misfire counts seem to be erratic enough that if we force a constant misfire, I'm not sure how much it will tell us.

When I talked with Dad Tuesday he said Mom hasn't mentioned anything about her van either way, but she's been driving it. Before the new plugs and wires she was afraid to even drive it. Also, Dad said it has a lot more zip now after the SeaFoam decarb.

-Rod

How is fuel pressure? Have you thought of a sticking injector? Have you tried a different coil? You might have a bad primary circuit in the coil itself which only manifests itself (now) occasionally at idle.

I'm glad you mentioned fuel pressure. That was one of the first things we planned to check. Alldata showed that the test port should be on the fuel injector rail right near injector #6. It wasn't there. We looked as far under the intake manifold from both direction as we could with a bright light, but not evidence of a test port. We followed the rail to where it switched to a flexible line under the intake air tubing, then as far as we could from under the hood. Nothing. We didn't climb under the van to look for a test port though. And, yes, we have experience measuring fuel pressure and know what test ports typically look like. This van has the plastic upper intake manifold and a single FIR runs underneath the center of the manifold. This isn't like the previous generation where there were two FIR and they were out in the open.

As mentioned in my original (long) post, the coil that's on it is new.

Your comment about "only at idle" resonated with me this morning. In the Ford forums that I frequent, if someone had posted that they had a rough idle and plugs and wires didn't have an impact, one of the first things I'd suggest is that they remove the EGR valve and clean the valve and passages. The EGR valve should be closed at idle, but if it's unable to close due to carbon build up on the valve, it provides what is essentially a vacuum leak. Based on our cylinder decarb procedure with the SeaFoam, and the amount of gray exhaust it created, there's a lot of carbon in the engine....

-Rod

I talked with Dad last night, he said that Mom thinks the van runs fine. She's not afraid to drive it anymore and the CEL has not come on again. Dad hasn't driven it recently to see if the funny idle is still there or not following the new coil and cylinder decarb. For now I guess we'll claim the money spent on the new coil was money well spent.

-Rod