So this is a bit long, but I'll start with the current symptoms. Basically when I hit the highway at accelerate above 60, the CEL light starts flashing. The vehicle is also misfiring at lower speeds.

The long story is - My catalytic converter was clogged so I removed it out of desperation to get the vehicle running.

It worked.

A few weeks later, because of the noise from the exhaust, I reattached the pipes to the muffler, but I had only the downstream O2 sensor connected since the upstream O2 sensor was disconnected. 2 days later, the vehicle starts misfiring and running rough. I disconnected the pipes from the muffler again, and removed the muffler. The vehicle still ran rough. I eventually had everything, including a new muffler welded on. I did not install a catalytic converter because of cost. I had both sensors installed and everything welded up nicely.

The vehicle runs a little better but still misfires at low speed and flashes the CEL when running at higher speeds.

Could this problem be exhaust related? Like if I install a new cat will the vehicle run better? Is it possible when I reattached the pipes to the muffler that this damaged my spark plugs?

Thanks. Hope I make sense.

 

Flashing CEL is telling you the PCM knows the engine is misfiring. Clogged cat tends to foul spark plugs, especially on 6 and 8. How old are yours? Cap? Rotor? Wires??

 

plugs, cap, rotor and wires are about 30-40k miles old. Right now, I’m gonna pull my plugs and see if they are fouled and check the wires too. If they seem fouled I’m going to add a new catalytic converter and replace the plugs and whatever else needs replacing.

 

The only time I had a flash CEL is when I totally lost #4 injector

also a clogged cat will do it too

 

If those are copper plugs, it's time to change them. If they are NOT copper plugs, it's still time to change them, and install copper plugs. These engines just don't like the rare-earth fellers..... tends to lead to mystery misfires.