hey guys, lately my 01 cavalier z24 5 speed has been violently lunging and bucking and dumping gas into the exhaust causing it to pop and crackle and get poor gas mileage.

Ive replaced the fuel filter, ignition coils, coil housing, plugs (ac delcos), coil on plug corks, iac, cleaned throttle body, thermostat, map, 02 sensors, fixed an exhaust leak, and the fuel pump was replaced 10,000 ago by previous owner. none of this was done to try and fix this issue, I just did it because I had issues with the coil housing cracking and I also wanted to give the car your average tune up. after all the work was done the car drove and ran great with no issues

I took a break from driving the cavalier and started driving my 86 Trans Am for a week or two and then started driving the cavalier again. a couple days ago after I just finished filling the tank in the cavalier I realized that for the past 2-3 weeks I was putting premium in it instead of 87 since I was used to putting premium in the Trans Am. this issue has been going on for about that long and has slowly gotten worse. the tank was finally half full today and the car was running worse than its ever ran before so I filled it back up with 87 octane and it seems to have almost fixed the problem except for the occasional exhaust crackle and buck.

My theory is that the higher octane fuel was too hard for the cavalier to ignite causing it to stumble and backfire, im assuming that by putting the 87 in it today, it brought the octane down enough for the car to run a lot more stable but still high enough to not run as it should. i know ill have to wait until i can get a full tank of only 87 in it to see if its fixed but I'm impatient.

I completely understand that putting regular gas in a car designed to run on premium will cause the engine to ping but what about running premium in a car designed to run on regular 87? its something I've never put much thought into this and this cavalier is the first small 4 cylinder car I've ever owned.

I highly doubt that putting 87 octane in it would cause it to behave differently. Lower-octane fuel spontaneously (prematurely) combusts at the bottom of the compression stroke (before the spark) of high-compression-ratio engines only. On low-compression-ratio engines neither fuel type will spontaneously combust. But both fuels types will very happily combust with a spark. And both will combust with the same force.

I foolishly wasted my money running high-octane fuel in my low-compression 4-cylinder Olds, until I learned that I was being a fool. When I switched, I couldn't tell the difference. At all.

I'm grasping at straws but...
1) You might want to try replacing the coolant temperature sensor. It's cheap and easy. The computer make decisions about how rich to make the fuel mixture based on the data from that sensor.
2) Maybe replace the MAP sensor vacuum tube (for like less than a dollar)?

Please let us know when you figure it out. I'm sure I'll have the same problem some day!

Thanks for the reply, I was pondering the coolant temp sensor as well and it's cheap enough so I'll most likely do it today after work. I'll change all the vacuum lines today as well, I've been meaning to get around to it but I have a busy work schedule so time is limited. Driving this bucking horse of a car to and from work every day is becoming a huge chore and I really don't want to kill the catalytic converter with all this fuel I'm dumping into it

Based on my unfortunate experience, I'd say you've probably already killed the catalytic converter. It doesn't take long. If so, you'll need to get the old one cut out an a new one welded in.

I had a feeling it was cooked after the first tank, it's time for emissions so I'll try it with this cat and see if it passes or fails, it sucks because I just replaced the pipe running from the cat to the manifold lol.

But the problem was fixed, I pulled the Idi cover off and inspected the coil housings for cracks and then realized my ground strap wasn't making a good connection.