Hey guys, I gotta say "Wow" what an awesome website, I had no clue there was a Bonneville website until I stumbled upon it last night. I am in desperate need of help with my '99 SE with the non-SC engine, 121,125 miles. This problem all started about 6 months ago with a shifting problem, where the transmission had problems downshifting and upshifting from/to 3rd gear, which also set a DTC light, which was discovered to be P0304. I figured it was time to do a tune-up, since it was over 100K miles, and I hadn't gotten around to changing the spark plugs and wires yet. I replaced the plugs with the AC-Delco Iridium plugs, and AC-Delco wires. I also replaced the fuel filter, air filter, PCV valve, and changed the oil/filter. So, all was well for a few months, the check engine light went away, but then it all started acting up again a month ago. Now, when I drive the car hard, under full acceleration, it is sometimes unable to shift into 3rd gear, and the car will get to 4000 rpms and not go any further. When I let off the gas and slowly accelerate, everything is fine, and it will shift fine on through to OD, and cruise fine. The car now has another DTC code, but I have not had time to check it, but will, in another post. Does this sound like a ignition problem or a transmission problem? Has anyone had this problem?

Oops

I used the AC-Delco Iridium plugs from Autozone - they actually say made by NGK on the side on them!

Sorry about that, I see you stated that in your original post. Get those codes read.

I agree. Get the code read so we can move in the right direction, as there are many things that could cause your problem. Can you double check your original code. I can find no information on P0304 being a valid GM code.

Hey Guys, I just got back from Autozone, and they checked the DTC; it is P0300, which is multiple cylinders are misfiring. The car runs great while driving it easy around town, but if I were to really test the car and drive it hard, then I get the problems where it refuses to downshift to hit the passing gear. If I were to stay on the accelerator, It will stay at 4000 rpms and cause horrible missfires which cause the DTC to come on and flash for about 30 to 45 seconds. Oh, on the other code (P0304), was the code from the Acton code reader that they have at Autozone, he said it meant that Cylinder 4 was misfiring, which seems to not be an issue an longer, since I replaced the spark plugs and wires.
This website has the code descriptions too - them seem to be a generic OBDII code.

http://www.obd-codes.com/trouble_cod...e-detected.php

Yes, code p0304 is the generic code cylinder #4 misfire. However, GM does not use that code. Here in techinfo is the listing of valid codes for your car.
http://www.bonnevilleclub.com/forum/...e=article&k=73

With your current code, considering you have done a tune up, I'd be concerned about the UIM problem. Read about that in techinfo here.
http://www.bonnevilleclub.com/forum/...e=article&k=38

Check you fuel pressure too. You may have a fuel pump that isnt putting out enough pressure to support the demand at the higher rpms.

Hmm, that is some interesting information about the UIM that I was unaware of. I hope that is not my problem. How common is it for the '99 models to have that problem? I haven't had to add coolant to the overflow jug in about 7 to 8 months, and it is always at the "Coolant level - Hot" line on the jug and never moves. It is still a bright red color too - I check the fluid levels in my car about once a week.

It'* not a question of if you will have the problem, it'* a question of when. Continue to keep any eye on your fluids. You may want to do the fix as a preventative measure. As that appears to be OK, then do what TheDude suggested above. If the pressure is low, you may want to try replacing the fuel filter before the pump.