P0300 - Random Multiple Cylinder Misfire

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Merkava_4

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Clovis, CA
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.
 
Originally Posted By: Merkava_4
The catalytic converter was ticking very loudly after I got the car home and shut off the engine.


Remove the oxygen sensor before the cat to see if it makes a difference.
 
Look at every thing you moved before chasing down other possible problems.

Double check plug wires 4 & 6 it seams like number 4 & 6 will be the first to misfire on these motors.
Also check the injector wires going to 2, 4, and 6 injectors.
 
Originally Posted By: Warstud

Remove the oxygen sensor before the cat to see if it makes a difference.


I unplugged the pre-catalytic oxygen sensor from the harness and started the engine up. There is no change in the way it runs, it still runs very rough. One thing I did notice is that the engine is running extremely rich by the way the exhaust smells . Thank you for your help sir.
 
Originally Posted By: beast3300
Does the exhaust smell like coolant? My money is on cracked upper intake.


No it doesn't, but I've been waiting for that to happen for years. I keep an eye on the fluid reservoir all the time and keep the level right on the cold line so it's easy to spot. Upper and lower intake manifold replacement is something that I could to do myself. But what's really outside my comfort zone is anything to do with trouble codes. All I am is a parts changer. Thank you for your help sir.
 
You may have a vacuum leak. Check around the area that you recently worked on (rocker arm) for a disconnected hose.
 
Originally Posted By: Merkava_4
Originally Posted By: Warstud

Remove the oxygen sensor before the cat to see if it makes a difference.


I unplugged the pre-catalytic oxygen sensor from the harness and started the engine up. There is no change in the way it runs, it still runs very rough. One thing I did notice is that the engine is running extremely rich by the way the exhaust smells . Thank you for your help sir.


You need to remove the O2 sensor from the pipe to perform the test.
The idea is to relieve back pressure from a plugged cat, if the engine runs properly with the sensor removed the cat is plugged.
It will be loud but its no problem.

Any black or dark grey smoke when first started? Any codes other than random misfire?

Edit: As Warstud posted look for a vacuum leak. There is a large port on the side of the upper plenum that is where you were working.
Do you have a vacuum gauge?
 
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Quote:
The catalytic converter was ticking very loudly after I got the car home and shut off the engine.
What does this have to do with anything? It is just cooling down. Many many cars make that noise. It is absolutely normal.
 
Originally Posted By: Merkava_4
Originally Posted By: beast3300
Does the exhaust smell like coolant? My money is on cracked upper intake.


No it doesn't, but I've been waiting for that to happen for years. I keep an eye on the fluid reservoir all the time and keep the level right on the cold line so it's easy to spot. Upper and lower intake manifold replacement is something that I could to do myself. But what's really outside my comfort zone is anything to do with trouble codes. All I am is a parts changer. Thank you for your help sir.


Is the coolant low in the radiator?
 
The 3 coils are stacked on the passenger side of the engine? Check each plug wire connection at the coils for corrosion. Pal had similar problem with 01 Regal it was corrosion on the bottom coil at the connectors.
 
Last edited:
Tap on TPS and MAF if it has one with a screwdriver handle while idling and see if it changes. Recheck connections to make sure a misrouted wire or hose didn't get pulled out from engine rocking forward and back while driving.
 
Originally Posted By: Trav

You need to remove the O2 sensor from the pipe to perform the test.
The idea is to relieve back pressure from a plugged cat, if the engine runs properly with the sensor removed the cat is plugged.
It will be loud but its no problem.

Any black or dark grey smoke when first started? Any codes other than random misfire?

Edit: As Warstud posted look for a vacuum leak. There is a large port on the side of the upper plenum that is where you were working.
Do you have a vacuum gauge?


Okay that makes sense about pulling the oxygen sensor.
Now I just need to find me an oxygen sensor wrench.

There's no smoke coming out the exhaust, just real rich fumes.

I don't have a vacuum gauge, but will get one if I need be.

Can I pull the Fuel Pump Relay and check for spark without pumping fuel too the engine?
Seems like I heard somewhere that pulling the Fuel Pump Relay disables the ignition. It
would be nice if I could crank the engine over to test for spark without starting the engine.
 
Originally Posted By: Vikas
What does this have to do with anything? It is just cooling down. Many many cars make that noise. It is absolutely normal.


It normally doesn't tick that loudly at all. This adds to my suspicion that it's getting raw fuel
pumped into the catalytic converter and causing it to over heat. After I find the cause
of the misfire, it wouldn't surprise me at all if the catalytic converter is ruined.
 
Originally Posted By: beast3300

Is the coolant low in the radiator?


The coolant level has been holding steady. I keep a close eye on it.
 
Originally Posted By: cjcride
The 3 coils are stacked on the passenger side of the engine? Check each plug wire connection at the coils for corrosion. Pal had similar problem with 01 Regal it was corrosion on the bottom coil at the connectors.


Yes, the coil packs are stacked together on the passenger side of the engine.
Being that the car is running so rich, I'm suspecting an ignition problem.
 
Just for grins, pull the vacuum line off the fuel pressure regulator and check for fuel in the vacuum line. Likely not your issue, but it both quick and easy to check.
 
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