Help Diagnosing P0304 and P0300

[MostlyRust]

Hey all, hoping somebody out there can help me diagnose a constant misfire on my '97. First appeared driving on the highway and has been running very rough ever since. It never has problems starting up, but it's running real rough.

I've replaced spark plugs and injectors. I checked the other day and cylinder 4 has spark. When I did injectors, I took off the upper intake manifold, and it seemed like all the vacuum tubes and hoses were intact. I've also looked around for a vacuum leak and haven't found one. Did a compression test and had good compression. My next thought was that it could be fuel pump related, except the car smells strongly of gas which makes me think it's running rich and not lean. Wondering if bad O2 sensors could be the culprit, but before I pull the trigger on ordering new ones, I was reading about the VSV valve and coolant temp sensors, and it sounds like there could be a lot more potential causes.

Any help or links to other threads would be appreciated. I'm a piss poor mechanic but I'm trying to avoid paying someone else to fix this.

 

[Rusty Marlin]

Did you change the plug wires when you changed the plugs?
When you changed the plugs, did you nice any oil around the drive hex?

 

[MostlyRust]

Did you change the plug wires when you changed the plugs?
When you changed the plugs, did you nice any oil around the drive hex?

I swapped in a new distributor line at cylinder 4 and still had P0304. There might have been a little oil around the plugs, but I’ll have to take one out again and check

 

[Rusty Marlin]

New plug wire, that's good.
Oil around the plugs can cause a short and a misfire.
It comes from old cam cover gaskets. There are doughnuts around each plug well and one big one around the perimeter.

When did you change the cap and rotor last, or is it new-to-you?

Its doubtful the O2 censors would cause a misfire, run rich or lean with comms to the ECU, but not a misfire.

When you removed the intake manifold did you happen to notice the condition of the injector wire harness? The harness can get cooked where it passes the EGR. This will cause the injectors to misfire.

 

[MostlyRust]

New plug wire, that's good.
Oil around the plugs can cause a short and a misfire.
It comes from old cam cover gaskets. There are doughnuts around each plug well and one big one around the perimeter.

When did you change the cap and rotor last, or is it new-to-you?

Its doubtful the O2 censors would cause a misfire, run rich or lean with comms to the ECU, but not a misfire.

When you removed the intake manifold did you happen to notice the condition of the injector wire harness? The harness can get cooked where it passes the EGR. This will cause the injectors to misfire.

I looked at the harness around the EGR when the manifold was off and didn’t see any issues. I did change the dizzy O-ring recently and also the spark plug gaskets when I first had this issue, but not the valve cover itself because it looked alright. Maybe that was a mistake.

Just pulled a spark plug. These are newer plugs, but I’ve run the car a couple times since they’ve been installed just trying to diagnose the issue. Does this amount of oil indicate that the plug gaskets I put in aren’t working?

Good to know about the O2 sensors. Could a failure in the fuel pump cause rich conditions?