Misfire Mystery

[8thTon]

We had another round or torrential rains and flooding here in PA, and the drive home was rough. I went through one particular deep spot not far from home, and although I went slowly after I got out I could feel a misfire. Shortly after I had a flashing CEL and it barely ran (probably in limp-home mode). I pulled over as soon as I could find a spot, and it had a P0303 (#3 misfire).

There was no obvious water in the engine compartment - everything looked dry. I limped it very slowly another mile to a parking lot, and tried to see what was up. The misfire was persistent, and as the weather was horrid I limped it the 2mil to home on back roads. On the way down hill it smoothed out and the CEL light went off, but when I loaded it again it came back.

After I dragged our lane I pulled the plug on #3, which looked fine, cleaned it and put it back. Sprayed the coil pack and wires with brake clean. Fired it up and got a P0303 and a P0316 (misfire in first 1000 revs). If I reset them and let it idle it's OK and the CEL is off with no pending codes. If I take it out and load it the misfire comes right back with the P0303.

Any ideas? I thought of water getting in the fuel but only #3 is misfiring. I'm worried the head got shock cooled and cracked, but the engine compartment was dry. I'm sure water could have splashed up from below, but it seems like it should take more than that.

 

[Ruff b2]

Is the problem still occuring? How long has it been happening? I've seen water in coil pack connectors causing misfires.

 

[Dirtman]

Switch the waste spark coils and see if it continues.

 

[8thTon]

Is the problem still occuring? How long has it been happening? I've seen water in coil pack connectors causing misfires.

I pulled the plug wires off the #3 and the one that shares a coil. There was no sign of water but I blew them out with compressed air and cleaned them with brake clean. After all that and time some time drying it may be marginally better, but mostly still the same.

 

[8thTon]

Switch the waste spark coils and see if it continues.

Now that's a good idea - wish I had thought of it, and before it got dark, but I'll try it tomorrow.

The waste spark has me wondering why it doesn't misfire on the other cylinder it's paired with, but I suppose there could be a breakdown in the coil pack.

 

[Ruff b2]

It is odd only 3 is missing.. they should both fire at the same time.
Did you check the pigtail connector too?
If the other side is firing then cyl 3 is losing it's power somewhere. Have you checked for spark on cyl. 3 wire?

I pulled the plug wires off the #3 and the one that shares a coil. There was no sign of water but I blew them out with compressed air and cleaned them with brake clean. After all that and time some time drying it may be marginally better, but mostly still the same.

 

[8thTon]

It is odd only 3 is missing.. they should both fire at the same time.
Did you check the pigtail connector too?
If the other side is firing then cyl 3 is losing it's power somewhere. Have you checked for spark on cyl. 3 wire?

I haven't had time to do much diagnosis yet. It fires OK when it's at idle or not under load. I'm going to swap the wires between #3 and whatever it's paired with as suggested above. If it moves to another cylinder it's the coil, if not it's the wire, plug or something else entirely. It will be a good first shot at parsing the problem.

Increasing load means the spark must build to a higher voltage before firing, so it's more likely to break down somewhere else (as in a bad coil). I hope that's it as ignition will be far cheaper and easier to fix than anything else.

There must be some connection to going through that puddle, but I don't know what it is yet.

 

[Ruff b2]

Good luck. I hope to hear results!

I haven't had time to do much diagnosis yet. It fires OK when it's at idle or not under load. I'm going to swap the wires between #3 and whatever it's paired with as suggested above. If it moves to another cylinder it's the coil, if not it's the wire, plug or something else entirely. It will be a good first shot at parsing the problem.

Increasing load means the spark must build to a higher voltage before firing, so it's more likely to break down somewhere else (as in a bad coil). I hope that's it as ignition will be far cheaper and easier to fix than anything else.

There must be some connection to going through that puddle, but I don't know what it is yet.

 

[alwaysFlOoReD]

Swap spark plug to a different hole?

 

[8thTon]

Well the mystery continues. I took the truck out to do a baseline test and verify how it behaved before doing any testing - and it ran perfectly. No codes and no issues. But it still has some vulnerability it shouldn't, and I need to find it.

Something must have been wet although everything seemed dry, and it sat while hot for an hour after I got home. The coil pack doesn't really make sense as it would have to arc to ground on the #3 side, or the other plug would not fire. Possible but not too likely.

 

[91stranger]

Have you checked you spark plug wires for bare spots? we had one where it was running poorly and found that one of the plug wires was resting on top of the manifold and it was arcing out. zip tied it up and problem solved. How deep was the puddle you drove through? Have you checked/cleaned your MAF sensor? Checked you IAC? Have crawled underneath to look for anything obvious? I learned that you can tell a lot about a vehicle just by looking underneath it.

 

[8thTon]

The plug wires are 8mm blue Belden wires, clearly not original. They look good and don't really lay across the engine anywhere - the closest they come is at the loom attachment points on the extended valve cover screws. I relocated those a bit to make the wires dress better. If this is ignition related, there are few places it could be.

The wasted spark coil secondary winding is floating, connecting from one wire post to the other - it does not intentionally connect to ground at all. It fires from one post, out that wire, across that plug to the block/ground, from ground across to the other plug and back that wire to the opposite coil post. To get it to misfire on one plug means it must be arcing to ground from that post before the plug electrode.

1. There isn't even a ground in the molded coil at the spout connection, so it seems unlikely to be the coil pack - still possible but not likely.
2. The wires look good. They got close to the stud at the loom attachments, but that's it.
3. It could arc across the outside of the plug to ground, or if the insulator is cracked. I may have to try another plug but since it isn't doing it anymore....

 

[Uncle Gump]

Wait til nightfall... open the hood and start the engine... take a spray bottle with water and spray the secondary ignition wires and the coil packs. If there is a leak... you will probably see it.