98 grand prix gt - p0300 code

I'm new to this so sorry if i do something wrong. I have 203k and am the orig owner. I failed inspection with p0300 - Random/Multiple Cyl. misfire detected and p0420 - cat. sys. efficiency below threshold bank I. A mechanic revved the car and it seems to struggle as it approaches 4k rpm. He said it needed a cat, because it was clogged. I replaced cat (& resonator with failed patch job), post o2 sensor and I'm driving to let the car codes to reset. However, when I rev the car it still struggles as it approaches 4k rpm. I have also noticed that once the car warms up, it has a sort of whining sound when stepping on the gas, I think more at higher speeds, and seems to be a bit sluggish. Other than that (and nonworking AC and ABS/traction control), it's running fine. Plugs & cables are a bit over 1 year old. Any suggestions? I have about a week left on my 10-day inspection extension. Any help would be appreciated. I have read many posts, but don't seem to find any with my exact problem. Thanks!!

1. Did you put a universal weld-in converter on it or an OEM type converter? The weld-in type rarely function correctly, and trip various converter related codes.

2. What brand/type plugs and wires did you have installed last year?

3. On the whining noise, does it sound like a trans issue (changes pitch as the gears change) or a hub bearing issue (changes pitch as speed increases regardless of gear)?

Thanks Richtazz! The cat is a direct fit with California emissions (obdII), which is what the car originally had, though it's not a dealer part. The plugs are oem ACDelco, though I got the iridium ones for my car, figuring it would be an upgrade. I wasn't sure what the cables were so I just checked the receipt which says DEL 756S Wire Kit, S, which I'm pretty sure are also ACDelco. I'll try to check tomorrow. The whining noise comes directly from stepping on the gas. While on the highway and gasing, if I suddenly let up on the gas completely, there is a subtle lag for the whining to drop too. Resuming to hit the gas resumes the whine. The stronger the gasing, the more intense the whine. I was thinking it might be the fuel pump or something in that direction. Does the fuel pump have bearings that would sound like this if they were starting to go? I'm pretty sure it's not the hubs (whines according to gasing not car speed) or tranny (shifts smooth and quiet), especially the way it responds to the gas pedal. I really appreciate your help!

Have the codes come back since changing the Cat? Did you have the codes erased when you had the work done?

Ok, the plugs and wires should be fine, as they are Delco. I was asking in case you put some off brand crap on it.

The whining noise sounds like either a trans issue (a torque converter will whine before they fail but the trans will still shift fine, or a PCS valve will cause high line pressure and a whine) or maybe your power steering pump is low on fluid or bad. If the whine is coming from the passenger side of the vehicle, I'd look into the power steering pump. If it's coming from the driver's side, I'd say its a torque converter or PCS valve in the trans. I doubt the whining is a fuel pump by your symptoms. They will not change pitch with engine RPM. The fuel pump runs constantly as long as the key is on and the engine is running.

Ok - I passed inspection! codes had been reset & I put 270 miles before returning for inspection (no codes returned). After passing I mentioned to the mechanic about the whine and how it struggles when revving to 4k rpm. He said there's a limiter that prevents the car from revving that high - so i guess I'm not so concerned about that. One Major thing that i forgot to mention is that around the same time that I started to notice the whining sound, my mileage went from about 22 mpg to a little over 17 mpg. This is my greatest concern. I had checked power steering fluid level when i first noticed the symptoms and it's full. How can I check if it's bad? Though I don't think the problem is related to PS. My best guess is that the whine comes from neither left or right, but more from the center. The car seems to have power if i gas hard enough, but there seems to be some pep missing. I've put one of those Chevron Techron fuel injector cleaners in, figuring it can only help. would clogged FI's give this sort of behavior? Thanks!

If you're free-revving it in park/neutral, then the rev limiter will kick in at 4k rpm. If it can't get past 4k while driving in gear, you've still got a problem.

The mileage issue could be from the trans problem. If the trans isn't shifting properly, or isn't going into overdrive, this will affect mileage.

Wow! - I think you may have just nailed it on the head! I think that totally makes sense. So the whining sound really isn't whining, but just the soiund of high rpm. Not going into overdrive is keeping the rpm's high, thus high fuel consumption. Any suggestions? I hear that Lucas (I think that's what it's called) makes a great product for trannies. Really, thanks for all your help in this!!!

Lucas, Trans-X or Seafoam are the three best. Another thing to try if it's still whining and/or shifting hard is to disconnect the battery and leave it overnight. We're not sure why this sometimes works, but many forum members have tried it and the trans starts acting normal again. For some it's temporary, for others it fixed it for good.