Thread: P0300 - Random Multiple Cylinder Misfire

2000 Buick LeSabre
P0300 - Random Multiple Cylinder Misfire

I'm starting this thread in case someone is able to help. This just happened today while driving on the freeway. The car had only been driven for about 10 minutes after start up. I was supposed to shut the engine down immediately to avoid catalytic converter damage, but I chickened out and drove the car the rest of the way home - about 1 mile. The engine was running very rough and would barely idle. Once I got it off idle, the engine would smooth out. The catalytic converter was ticking very loudly after I got the car home and shut off the engine. Also, there was a strong burning candle wax smell present in the air. Recent work done was a rear rocker arm cover gasket replacement and the two coolant elbows were replaced. I did the work myself. Today when I got the car home, I checked and double checked to make sure everything is plugged in and hooked up. Everything appears to be well connected.


Thank you for your time and consideration.

Spark plugs were replaced on October 11, 2010. ACDelco 41-101 Iridium.

I also installed new Delphi ignition wires on the same day.

Work was done by me.

Originally Posted by EJOUIE

How many miles on the catalytic converter?

The car has over 112,000 miles on the clock and the catalytic converter is original.

Originally Posted by Scottydoggs

how many miles since that tune up?

does the engine sound like a vacuum with a full bag when you floor it? like louder from the intake but not in a good way. if so the cat could be clogged. how many miles do you have there?

I haven't floored it because of how rough it runs. The one thing I did do is unplug the pre-catalytic oxygen sensor from the harness and started the engine up. Some guy on another forum wanted me to do that. The engine continued to run real rough and I noticed right away that the smell of the exhaust was extremely rich. I shut the engine off right away after that to avoid doing any further damage to the catalytic converter.

Thank you guys for your help. I appreciate it very much.

Originally Posted by Scottydoggs

how many miles when you did the tune up last? it was 3 years ago now.

is the check engine light on?

Yes, the last tune up was 3 years ago at about 88,000 miles. I take very good care of the car. Up until today it's been a very good running and dependable car. The only thing it's ever needed was an EGR valve, A/C compressor, front CV axles, front hubs, front outer tie rod ends, brakes, front transmission mount, right motor mount, MAP sensor, and fluid changes. All work done by myself. Just recently before this latest episode, the idle was starting to get jittery. Not rough, but jittery. So I took the air intake hose off and shined the flashlight down into the throttle body. Everything on this side of the butterfly is spotless. I then took the MAF sensor off to see if its wires were dirty; it was spotless inside. So what I did then is go ahead and clean the Idle Air Control Valve; it was coated with black soot. Idle quality got a little bit better after that, but not much. The check engine light is on now and it was flashing when I was driving it earlier today on the freeway. From what I understand, the only time the check engine light will flash is if the misfire is bad enough to cause catalytic converter damage. Thank you for your help sir.

You can check spark output for each cylinder at the coil pack. Get all 6 wires loose before you start the car, then pull off each wire one at a time with the engine at idle. You are looking for 2 things a good strong blue spark, and the idle to change with every wire you remove. You do not want to grab too close to the end of the wire or the spark will go through you and "bite" you as I say. This will lead you to which cylinder is not contributing.

I ended up having to replace the Ignition Control Module. It wasn't firing cylinders 1 and 4.