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Knocking and violent shaking has occurred in the engine after I replaced the starter a few days ago. And when the accelerator reaches over 20 mph, the service engine light starts flashing. Please help! . :”(

First of all, yes I'm still driving my 01 STS! After years of tire rebalancing and several small fixes including new front hub assemblies, all of which seemed to make a difference on the severity of that dreaded 65mph + shimmy or vibration through the car, each never eliminated the problem. But first, let me bore you how I got to it. A couple months ago I had the unlucky task of replacing the water crossover gaskets. That was a crap job of all crap jobs. Mind you, I'm not a professional mechanic, nor would I claim to be, however this stuff doesn't scare me from trying it. That being said, I decided I was not paying someone half the cars value to repair that crossover pipe that was spewing antifreeze out as fast as I could pour it in. After a week with the hood open, and every possible thing to replace was replaced, water pump, thermostat and housing, all hoses, gaskets and clamps. I even cleaned the fuel injectors and replaced the O-rings since I had to pull the fuel rail. All I can say is... Never Again! I was nervous to start the old girl when I was done out of fear something wasn't going to be right with all the work I just committed my life to. My fears were calmed by the fact it even started first try! Yay me! Having my pride crushed after a good road test wasn't a good feeling as the smell of hot antifreeze could be found within a 10 foot radius around the car. I couldn't find leaks so that was a good sign. Topping off the expansion tank and repeated road tests revealed that I must've been smelling the last of the antifreeze burning off the engine and tranny. I replace the expansion tank cap which may or may not have helped, don't know, don't care... I stopped losing coolant which I'm sure was just air working out and the smell was eventually gone! Again, Yay me!! What has that to do with the vibration? Absolutely Nothing! What a crap job that was!! However that lead me to really paying attention to the engine, as I searched for coolant leaks, I noticed my belt tensioner (serpentine belt on the passenger side) was jumping a lot... as I scoured around looking for an out of round culprit, there it was, holy mother of grails... the harmonic balancer looked like it was a spinning egg! Though I hadn't quite related that to the highway vibration at this point, I just knew I had to change it. So the deed was done just before I had to make a 400 mile trip and with the coolant issue behind me I felt safe to make the journey in my Seville. I have now logged over a thousand miles and that car is smoother than it ever was in the previous 8 yrs I've owned it. Sorry I put you through "the story" to get to my Title but every time I work on this car, its a story! I'll save the sunroof drains for another day! lol