I'm hoping someone can enlighten me with their knowledge and/or experience. I have an 05 MCS with a little over 100K miles on it. About 5 months ago, I did a tune up and a couple of my first mods when the elastomer on the crank pulley failed. I installed an ATI 2% damper crank pulley, 15% SC pulley, NKG plugs & wires, fuel filter, TSW upper motor mount, and thermostat. Well, a couple of months ago, my exhaust system broke clean in two right after the pre-CAT (I'm thinking because of the added vibration from the TSW motor mount...and can someone tell me what the pre-CAT does?) So I took it in to the muffler shop and they removed the pre-CAT and installed a high flow CAT...the guy there assured me that it would still pass emissions, but it was showing and error code of p0300 even thought the SES light wasn't on (I guess he reset it.) But a few weeks ago, as I was off to work in the morning and a couple of blocks down the road, the engine started running very rough, the SES light and the DSC light came on and I thought it was going to die. So I turned it around and started to go back home, but by the time I got there it was running ok again, so I went ahead and drove it to work and a couple of days later the SES light and DSC light went off. Well, a couple of weeks past and it did it again while taking my daughter to soccer practice with both lights coming on, and again was fine by the time I got there. But while coming back home it started up again, only this time it was really bad with really bad vibrations coming from the engine and almost no power...didn't think I was going to make it home, but it was running better by the time I got there. I took it to O'reilly's to check the error code and sure enough it read p0300. Could the O2 sensors, pre and post CAT going out cause this problem? I think someone who had a similar problem also mentioned in another post where a wire bundle to one of the MAP sensors was chaffed really bad and that was the problem. Any suggestions or ideas would be greatly appreciated.

 

Pre-catalytic 02 sensor monitor how the engine is managing the air/fuel mixture or ratio. Placed between the engine and before the catalytic converter.

Post-catalytic 02 sensor monitors how the catalytic is performing. Placed at the end of the catalytic converter.

P0300 error engine code is a misfire. One of your cylinder is not firing right.
Random misfires that jump around from one cylinder to another. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

Another one, catalytic converter that has gone bad. If you smell a rotten egg smell in the exhaust, your cat converter needs to be replaced.

When I had a misfire, I ended up replacing my wires-plugs-coil and solved my problem.
Others had a faulty fuel injectors. So, P0300 means a lot!!!
.....but look into these areas.

 

So, does the sensors adjust the air/fuel mixture, or just monitor it? If it adjusts it and it isn't reading correctly, couldn't that cause a lean mixture and thus a misfire? Could the sensor have gone bad due to excessive vibration, especially after the exhaust pipe broke in two?

 

Although sensor is variable, Sensor does not adjust the reading, the ecu does... reacts to the sensor voltage. It's an input to the ECU.

What am saying, if the sensor (any sensor) is operating properly, the ecu will conform to the mapping configuration. The ecu will instruct the fuel system to deliver the proper air fuel mixture (no more no less).
Yeah...02 sensor can go bad and you need to follow the manual how to meter them. If you find it's not the culprit, then explore what I mentioned.

P0300 means a lot of things. There is no specific fault. Misfires can be 1-2-3 faults...like chain reaction.
However, what I said is accurate and you should check what I mentioned before.

 

what was the cause of this problem? My 05mc started misfiring today, throwing P0300, and leak diagnostic pump and evaporative emissions leak codes as well. I tuned this car up a few months ago with new plugs, wires, coil etc..and new O2 sensors. My ses light flashes and the dsc light comes on with the misfires also with the engine sputtering at idle. any ideas?

 

Check the front left corner of the engine. That little thing with the tube going in clipped to the corner is part of the emissions that comes loose and goes bad easily. Check for vacuum leak if it is there, it might toss the pump code and run lean.

 

its also throwing one more code i didn't mention, p2097 which is post cat 02 is sensing too rich...would that tube cause all this and these misfires? thanks

 

If you are getting misfires and the car is pulling timing it may be skipping spark after a misfire to try and cool it, but really I don't know.

 

Well, I thought I had fixed the problem. After looking through many posts, I came across one that had mentioned the vacuum tube going to the SC boost gauge coming loose and causing the same problems I was having. My boost gauge hadn't been working since I did the maintenance I mentioned in my first post. Sure enough the rubber had dried and cracked and the tube had come loose under the inter cooler. I replaced the rubber and reattached the tube and it ran great....for a couple of months, and it started doing the same thing but worse. It's been in my garage ever since. I'm tired of working on that thing. I have a 2001 Yukon XL with 200k miles on it, and haven't had near the problems I've had with the MC with half as many miles on it.

HRM, exactly what little thing and which tube are u referring to?

 

I figured out the problem....my new ignition coil that I had just put in a few months ago had failed on cylinder 3. Replaced it and now it runs great.

 

Very good Cooper1! I was going to try that before I thought I had solved the problem by fixing my vacuum leak to the boost gauge. I guess I'll replace my ignition coil also and see if that fixes the problem. Are you still running good since the new coil? Which one did you purchase?

Forecast for here in North Texas tomorrow is mid 70's and sunny. Not bad for January...kinda wish mine was running.

 

The coil was my first thought, but I never thought it would be the coil after only 4 months of use with a total tune up....i have a carquest coil in there now with a long warranty. get a coil with a longer warranty so you know its high quality and stay away from msd coils. they're cheap. yeah i live in the UP of Michigan so this car gets to see a LOT of snow, wish I had 70's. btw the car runs great!

 

Well, I just installed a new Screamin' Demon coil that I bought from Way Motor Works. The SES light was still on after I installed it, so I went to O'reilly's auto parts and used their OBDII reader. I erased the error codes (P0300, P0304, and a new one; P0313 Low Fuel Misfire.) It seems to run good...for now, but I don't think the problem is fixed. It ran good for a couple of months after I fixed the vacuum hose also. Could be the mass flow air sensor. I think it has something to do with the fuel/air mixture.