Slight occasional miss, first actual misfire today

[Doober]

FYI this is on my truck (383, TBI, Vortec heads).
Over the past few weeks I've been getting a slight miss at light/part throttle acceleration somewhere between 1600-2000. It's very intermittent so I haven't let it get to me too much, but today I had a flat out backfire. Immediately afterward it continued as normal. I've checked all but 3 plugs so far, all look normal; cap and rotor are fine. I haven't checked plug wire resistance, but I'm wondering if it's possible for the spark to jump plug wires. Ignition is stock besides an MSD coil, plug wires barely have 13,000 miles on them.

Any other suggestions?

 

[t5montecarlo]

have a look at night to see if you can see sparks coming from the wires. I saw it once and was amazed; the wires looked ok, but they leaked. After replacing the wires, the change was dramatic.

 

[Doober]

I'm not sure if t's the cause (wouldn't surprise me) but a little something was caked on one of the plug grounds. Made sure the rest were all clean, and away I go. I have a pic just gotta upload it.

 

[SSedan64]

X2^^ about the Light Show. LOL!
Raise the Idle speed, set the Parking Brake & place in Gear. A little load will help show an arcing wire.

 

[Doober]

There was another with a tiny bit on it, but it wasn't this bad. I'm REALLY hoping it's valve seals, the engine only has about 13,000 miles on it.

This is what the rest looked like:

Aaaaaaaand I gave the underside of the hood a much-needed scrubbing.

 

[LS6 Tommy]

It actually looks pretty normal for a production car plug with some miles on it. The thing I'm concerned about is the fact that they're not all similar.

Tommy

 

[Doober]

Me too. If they all looked like that I wouldn't be as concerned. I don't think it's rings because I've been there before... aka 1qt. every 100 miles with another engine :shock: I don't know the actual mileage on the heads, but I may have also fudged the seal reaplacement a little.

 

[Brown_Gbody]

Could weak ignition cause that deposit on the plug?? Maybe check the resistance of the wires as compared to the others? Look for damage to the wire along the conduit. I had one grounding out on my header (heat had melted the casing on the wire)

Just a thought....

 

[Doober]

Sounds feasible... I drove it quite a bit last night and it didn't miss anymore, but I've driven it that much before and it didn't miss... it's that intermittent. It has an MSD coil and I've been thinking of getting a high performance module to take advantage of the coil.

 

[Damon]

It's not valve seals. You can take that off the list. THose aren't oil dposits and the plug threads aren't wet with oil.

Replace just that plug (might have a bad one) and see if the problem goes away- temporarily or permanently. Definitely check the wire that goes to that plug.