I have a 1998 Bonneville SE with 155,000 miles on it. The car has run like a champ since I bought it 4 years ago with 100,000 miles on it. Anyways, I have a random/multiple cylinder misfire code coming up on the car and symptoms of this are lose of power while going up small hills, and a jerking motion during acceleration. To explain further, while driving on the highway at 70mph if my car starts to go up the slighest incline, and I mean small, the car starts to slow down and I have to push the gas pedal in a little bit and all of a sudden it revs up to 3000rpm until I get to the top of the incline then it goes back down to 2200 rpm. Also the car does not idle smooth, the car fluctuates between 400 and 800rpm during idle and its totally random when it does, sometimes it idles smoothly, other time not. Some things I've recently done on the car are new IAC valve, new TPS, new sparks plugs, new wires, new coil packs, new intake manifold, new PCV, new MAP sensor, new tranny fluid, new fuel filter, oil change, and recently had the injectors removed and cleaned. Any suggestions would be greatly appreciated, I've tried posting this at other automotive forums but have received nothing but dumb answers, one of my friends referred me to this site and said you guys know your Bonnevilles.

Thanks!

Erik

Hi Erik and welcome.

Your problem sounds like an ignition related type of issue and you have ruled out most of that already. That hopefully makes the diagnosis much easier.

First thing I would suggest is checking to see if the #4 or #6 plug wires are touching the O2 sensor. This can cause interference to the sensor/wires and cause poor running.
What brands of parts did you use on the plugs and wires?

Next up is checking for vacuum leaks. Your car has 4-8 vacuum fittings on it that will degrade and crack over time. Disturbing them to change things like the upper manifold you mention having replaced can result in a vacuum leak. Easy way to check for leaks is to start the car and spray throttle body cleaner around the connectors/fittings. If the motor changes it'* idle, check closer for a leak in that area.


You mentioned changing out the intake manifold. Were the lower intake manifold gaskets changed at the same time? These are plastic gaskets originally and all aftermarket are also plastic replacements. Only the last year or two were aluminum frame gaskets available from GM dealerships. The plastic version would deform over short periods of time and can allow vacuum and coolant to leak. This can also cause running issues etc. Usually tougher to find if the coolant is leaking into an intake runner and into a cylinder. Does the car appear to be burning or using any coolant?

O2 sensor and catalytic converter. When an O2 goes bad you will notice fuel mileage drop off and this can clog a catalytic creating a restriction in the exhaust. If the exhaust can't escape there can be a suffocating issue to the motor.

Hey thanks for that post, to answer some of your questions I used all AC Delco parts for replacements, I did also change the upstream O2 sensor, I forgot to mention that in my first post. Yes lower intake manifold gaskets were changed. I do no notice a loss of coolant, Thats why I changed the upper intake manifold because it cracked and coolant was leaking into the cylinder. I did do a vacuum test a few weeks ago and it measured out at 20psi which is normal. The other things I forgot to mention in my first post is that when going up inclines and my car loses power and shoots up to 3000rpm, the check engine light flashes until I go back down to 2200 rpm at highway speeds.

Thanks!

Get the computer codes read and post them. That might help. I'm leaning towards a vacuum leak. Take a can of carb cleaner and with the engine running spray all the vacuum lines and connections. Listen for a change in RPM. One area missed is the EGR tube, spray this as well. Also spray around all the gaskets on the UIM and LIM and see if you find a leak there.

If it is a vac leak the codes will be odd codes that dont seem to make sense, miss fire, lean bank 1, things like that.

Check engine light flashing is defined in the FSM as a possible catalyst damaging condition. Usually from a misfire..other things can cause it though.

Like Dan said check for codes, they could be a big help.

The Code is P0300 random/multiple clinder misfire.

Well I fixed the problem, its was a faulty coil pack. I bought new ones from autozone about 4 monts ago and apparently one of them was bad. I bought some new delco ones and everything runs great now, no check engine light and all my power is restored. The moral of this story is, never buy parts from autozone. Thanks guys for your replies!