Im getting 4 and 8 cylinder misfires. Newer plugs and wires. Ran a spark test on all cyl on pass side and swapped out plugs on pass sided. Erased codes and still got 4 and 8. Nothing was done to vehicle prior to the codes besides o2 issues. Any ideas?

 

Blown head gasket.
Those are the two most common cylinders to leak.
Do a compression test.

 

I agree. Head gasket is the likely source

 

Those are almost always the ones to go with a HG. Are you eating coolant?

 

Updated: Spoke to soon.... Same codes and rough running again. I will run a compression test tonight but what would have caused a blown gasket other than overheating, not once has it even changed in temp. I was loosing a bit of coolant but had a couple hoses that had small leaks that I thought were the culprit...

 

Overheating does not cause a blown head gasket.
These trucks are notorious for eating headgaskets, many theories as to why.
Just buy a coolant exhaust gas tester and test your coolant, that will tell you.

 

REALLY????? Please explain this one

I am stumped on this because As far as overheating goes when the aluminum expands it crushes the gasket and then it begins to show the symptoms. You would still to find the reason it overheated but the overheating can and does cause head gasket failure.



There are multiple variables when diagnosing head gasket failures. So let's start with the most common causes. The most common cause for your head to fail is usually due to the engine overheating. Then this piggy-backs off several other variables such as: loss of coolant, broken serpentine belt, water pump failure or thermostat or/cooling fan failure. Yeah, lots of failing that is usually hard to pinpoint initially. But a sure tell sign is the cylinder head which can swell so much it will literally crush the head gasket. This mostly occurs between the cylinders due to their thin nature (thinnest point on head gasket). Then the cracked combustion armor showcases a leak path for coolant and may include combustion gases.

Another cause for head gasket failure is the obvious; vehicle age. Which has as much to do with dated technology as it may have to do with deterioration. The reduce freight cost (manufacturers) leaves cooling system far-from-par on these older cars. As a result of this, they lose coolant and are not overall efficient which obviously makes an engine run hot. Other side effects are low coolant levels, dirty cooling systems, a cooling fan that is slow to kick in, or a gooey (sticky) thermostat, even a plugged converter can make an engine an overheating candidate.

 

You need to really overheat it to do that, since the block and heads are both aluminum, if the block was cast iron and the heads aluminum they would expand at different rates.
If you overheat the engine enough to blow the headgasket you have bigger problems.
I will give you that continually overheating the engine could cause a blown head gasket but not a once or twice thing.

 

Update: I did a compression test this morning and all were within 160-180 psi except cyl 5 which was 135. Since I pulled the tips off the spark plug wires as I always seem to do I redid all the wires. I also pulled the upper intake manifold to swap the ignition coils out in case one of them was bad. I triple checked the wires and put everything back together. erased codes and started it. It ran smooth for about 20 secs and then started choking up almost stalling. Shut it down and looked over everything. Restarted it and right away was popping and choking up/cut it off. Checked for codes but there was none. Looked over it again and started it. Now it sounds like a loud metal tapping sound when it starts. Took upper intake mani off again and rechecked everything. Nothing out of place. Noticed a bit of oil around the upper intake gasket toward cyl 8 and 7. Enough to look bad. Any ideas???

 

Don't let it get the best of you just yet. Hang in there. I know it's frustrating, but you'll figure it out.

That loud tapping metal sound you're describing - Could that be stray spark? Do you think a wire or coil is arcing?