[Sli4]

My 2001 g20 has 29k miles on it (yep!) and the past few months it's been running a bit off.
I took it in for an inspection and the service engine light went on as it pulled into the lot (seriously). They replaced one of my o2 sensors and changed the oil (without asking me first...)

About a week later - first real drive since the inspection - the car was maxing at ~40mph trying to merge onto the thruway. It started smoking (white/light smoke), lights on the dash started blinking. The car was pulled over and when it all stopped the car made it home driving at a slow pace.
This nightmare has not happened since that one time but the car has been acting weird when accelerating ever since.

OBDII scan shows p0301/p0300 misfire on cylinder 1. I've scanned a few times since but that's the only code that has shown up and it doesn't seem to have happened since.

I'm new to working on the car myself but so far I've changed the spark plugs, cabin filters (unrelated), and ran a can of seafoam through the tank with about a quarter of a tank left. I also poured seafoam fuel injector cleaner in the tank when I last filled up.
Starting the car it idles in park at about 1500-2000 RPM until it warms up. Driving seems okay until you hit 60mph. Somewhere between 60-65mph the car hesitates when every time - rpm probably around 3500/3800ish at that point. The car starts slowing and no matter how much I step on the gas it's like I'm not stepping on the gas at all for a few seconds. Then it comes back and it doesn't happen again unless i slow down and try for 60 again.

I picked up a new fuel filter but was going to wait until more of the injector cleaner runs through before attempting to change it - though I'm doubting that matters anymore since I'm not hearing any misfires happening. It's been some time since the car has had a proper tune up so I also imagine the air filter could use a change as well.

Most of the posts that seem similar to this point to the fuel filter - though I'd imagine it would continue to struggle after 60mph if that were the case?
Any ideas what may be going on?

I'd like to attempt to get it running smooth again myself but I'm probably not the right person to do anything that could accidentally cause more issues.

Thanks for any help!

 

[ds3dt3]

If it's just the one cylinder then probably not the fuel filter. Seems like it would be something unique to that cylinder like the fuel injector.

You might pull the plugs and see if they look different. Color and smell. The misfiring cylinder in particular compared to the others.

Simple things like checking the fuel pressure to see if the pump, filter, regulator are working right.

Look at the air filter. Can you see sunlight through it?

Check the vacuum hoses. Anything cracked, loose?

Visually check the ignition system. Cracks in the distributor cap, damaged plug wire, plug boots are on right... Check the plug wire resistance.

Check the fluids. Is the coolant low?

Check the cylinder compression. Are all the pressures close? Particularly the misfiring one.

You can borrow the tools to check compression, fuel pressure from auto zone for only a deposit. They'll also check the codes for free.

 

[Sli4]

if there was an issue with the injector or another issue with the first cylinder would it only be an issue at 6”mph and then go away?
If the misfire was still happening I’d hear/feel it right? And it would throw a code?
there’s no audible misfire happening and the code hasn’t triggered since that first crazy issue. I also changed all the spark plugs and it still hesitates/feels like I’m not stepping on the gas at 60mph.
Air filter looks clean.
now that it’s cold it’s idling around 1500rpm when in park. In gear it idles normal.
I’ll make a trip to advanced auto to check out a fuel pressure gauge - but if it’s not an issue until I hit 60mph would the issue even show up sitting in a parking lot?

I’m thinking the car shifts at that 60mph area and it’s during that shift that it hesitates and then comes back?

 

[ds3dt3]

Thought you were still getting the code.

Does it just stumble for a second and then go on to 70 without issue?

 

[Sli4]

Thought you were still getting the code.

Does it just stumble for a second and then go on to 70 without issue?

yea. It’s more like a few seconds where it acts like I’m taking my foot off the pedal and then it continues on fine.