Rough idle, P0300, Help!

[wiscomick]

First two things I'd look at:
1. PCV operating properly? Not restricted, pulling reasonable air through the engine? Crankcase vent attached to air ducting as intended, so it can't pull "wet" air from road splash? If the PCV doesn't pull enough fresh, clean air through the crankcase, "wet" air/blowby can pass through the distributor bushings so that the inside of the cap gets wet and becomes conductive. This is especially a problem when the distributor bushing get worn; and that in itself is a common problem on Vortec distributors.

2. Missing rubber seal/weatherstrip where the rear of the hood would seal to the cowl? Allows rainwater to drip onto the distributor area.

Both good there, man. Plus, truck doesn't see much weather.

 

[Steve A]

Might take a look at the EGR valve, make sure there isn't a small piece of carbon holding it open slightly.

 

[wiscomick]

Update. Swapped out the old spider for the new one. On opening the upper intake, it looked pretty obvious what was going on. Half the intake was black and sooty, the other wet and greasy. Pretty clear we had an internal fuel leak. Got everything buttoned back up, and the son of a ***** is doing the exact same ****. I'm at wit's end. Seems the only options left are either distributor, or a blown head gasket as far as I can tell. If anyone has a known good distributor laying around for sale, let me know. Pic of inside the manifold below.

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[Erik the Awful]

Does it still look like #3 and #5, or is it random? I'm assuming these aren't the same plugs from the first post.

 

[RawbDidIt]

Update. Swapped out the old spider for the new one. On opening the upper intake, it looked pretty obvious what was going on. Half the intake was black and sooty, the other wet and greasy. Pretty clear we had an internal fuel leak. Got everything buttoned back up, and the son of a ***** is doing the exact same ****. I'm at wit's end. Seems the only options left are either distributor, or a blown head gasket as far as I can tell. If anyone has a known good distributor laying around for sale, let me know. Pic of inside the manifold below.

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Can't really tell from the picture, where is that coolant coming from?

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[wiscomick]

Does it still look like #3 and #5, or is it random? I'm assuming these aren't the same plugs from the first post.

It's still 3 and 5. Injector made absolutely no difference. Plugs were changed when I pulled the ones in the picture out.

Can't really tell from the picture, where is that coolant coming from?

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Was a little dribble from the upper hose. Thought I could get the upper intake off without pulling the upper hose and thermostat housing.

 

[Erik the Awful]

Well, now you're down to distributor cap or plug wires. I'd assumed from your first post you'd changed them, but you didn't specifically call them out.

 

[RawbDidIt]

It's still 3 and 5. Injector made absolutely no difference. Plugs were changed when I pulled the ones in the picture out.



Was a little dribble from the upper hose. Thought I could get the upper intake off without pulling the upper hose and thermostat housing.

Ok, I think I was able to remove mine without touching it, but wanted to make sure we weren't chasing our tails while looking dead at a head gasket or intake gasket leak.

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[wiscomick]

Distributor cap, rotor, wires, and plugs were just changed. All ACDelco stuff. Cap and rotor replaced in March. Plugs and wires replaced Sunday. Only thing I can put my finger on is maybe it's the distributor. When I had it out to do the intake gaskets, I did notice that the gear was a bit more worn than I'd like, but stabbed it back in and she ran like a Swiss watch.

Just going off of what I know, I'd think a bad dizzy gear would result in misfires across the board, not just 3 and 5 exclusively. Only other thing I can think of is head gaskets, seeing as how 3 and 5 are next-door neighbors, but then why would the compression numbers be fine across the board?

Idk man. If this is head gaskets, I'm going to... blow a head gasket.

 

[Erik the Awful]

The compression test leads me away from a blown head gasket. Maybe valve stem seals leaking oil into the chambers?