Truck decides to misfire the day my kid is born.

Good news is, wife went into labor this morning and we drove to the hospital just fine, had a healthy baby, Momma is healthy, and after a few hours I decided to ride back home to check on the kids and Grandma. Truck immediately went into misfire mode.

OBDII bluetooth scanner says misfire Cyl 5.

On the way back to the house, I pulled the #5 plug. Had a lot of carbon on it, but not "lumps", just black coating that was a little wet. Looked like fuel to me but could have been oil.

Gave it a new plug (NGK single plat). No change.

Bought some new wires, gave it a new wire. No change.


So, help a guy out here on what to check next. I'm going to pull the Dizzy cap tomorrow and inspect it.

Other than that, what do I check next? Fuel pressure?

Can I check the fuel injector without pulling the upper intake apart? Is there a way to see if #5 Injector is just plugged?

Recent repair history--

New intake gasket, plugs (ACDelco coppers), new wires, new dist cap and rotor--- 10 months ago.
Oil has less than 2K miles on it.

Congratulations, the next year will be the fastest year of your life(if it's your first).

Wish I had some experience with that setup but it sounds like you are on the right track. I would start with the fuel pressure and leak down test. So far you've ruled out just about all of the ignition.

Possibly distributor cap, faulty injector, or a defective rocker arm .

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Keepinitoldskool said:

Possibly distributor cap, faulty injector, or a defective rocker arm .

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I had not thought of that. Wouldn't be too hard to check. Would that also explain some oil getting into the chamber? The plug had a little buildup on it but I assumed it was poorly burned fuel soot.

redfishsc said:

I had not thought of that. Wouldn't be too hard to check. Would that also explain some oil getting into the chamber? The plug had a little buildup on it but I assumed it was poorly burned fuel soot.

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Not very likely. Its more likely to be a bad valve stem seal if it is oil fouling the plug. I would put a new plug in it and run it for a hundred miles or so then take it out and insoect it again to see if it is oil or fuel getting it wet.

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Keepinitoldskool said:

Not very likely. Its more likely to be a bad valve stem seal if it is oil fouling the plug. I would put a new plug in it and run it for a hundred miles or so then take it out and insoect it again to see if it is oil or fuel getting it wet.

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OK that makes sense, I understand now.

I'm going to probably pull the dist cap today to see. Fun fun fun lol

OK it was the dist cap/rotor. The caps posts were surprisingly carbon built up, as was the rotor electrode.

What would cause that after only 8 months?

This was an Auto craft gold, had all brass electrodes.


Either way, swapped it for a Delco, now it idles slick as glass.

Unfortunately the Delco is an aluminum version, which is typically considered inferior. Hopefully it will last.

Congratulations on the new edition to your family.

I'm glad you found the problem about the miss fire. I want to just add a quick tip for yourself, and others when trouble shooting such a miss fire.

If such a problem was caused by component(s) of the valve train, like a broken rocker arm, bent push rod, stuck valve, worn cam, the easiest way to make such a diagnosis is by hooking up a vacuum gauge to manifold vacuum and watch the needle as the engine runs. The needle on the gauge will flicker every time that cylinder miss fires from such a mechanical problem.

The vacuum gauge is one of the best diagnostic tools in the box.

Note too it is the easiest way to check for a clogged exhaust....

If it makes you feel any better, mine developed a nasty torque converter shudder when my 1st was born.

redfishsc said:

OK it was the dist cap/rotor. The caps posts were surprisingly carbon built up, as was the rotor electrode.

What would cause that after only 8 months?

This was an Auto craft gold, had all brass electrodes.


Either way, swapped it for a Delco, now it idles slick as glass.

Unfortunately the Delco is an aluminum version, which is typically considered inferior. Hopefully it will last.

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On all GM vehicles always use GM acdelco ignition parts because after market parts are of lesser quality. I have worked on GM vehicles where aftermarket parts were a problem.