I haven't gotten my SES light scanned yet as it just happened, but yesterday I was driving on the freeway when I started to go over what I thought was a rough patch in the road and the car was vibrating a bit. Then it got a bit worse, so I went to change lanes to see if it was just the lane I was in (freeway was under construction). It was at this point the SES light started blinking in my car. I immediately exited the freeway and pulled into a parking lot to do a quick "once over" on the vehicle. The SES light was solid at this point. Turned it off, car fired right back up, once again light is solid still now and doesn't go away. I have not gotten it back up to freeway speeds as the vibration was fairly bad, however it did appear to go away as I was driving to the next exit, even at 60+. Any preliminary ideas? I will be getting it scanned ASAP.

 

Can a major vibration cause false knock? Just throwing it out there. Or maybe a connection vibrated loose. What kind of vibration is it? What is it related to? Engine speed (rpm), transmission speed (vibrate with clutch in and out), rear end/tire speed (speedometer), etc.? When I have a vibration, I always run those things through my head to eliminate things.

 

It was there, got worse, then before I really slowed down much (maybe 74 down to 65) the vibration kinda stopped, and then the SES light came on. The closest vibration I could relate it to would be transmission vibration based on where I was feeling it in the car, but that is without any certainty... I drove it home 3-4 miles without any additional vibration besides the regular driving vibes I always get, surface streets only though. Didn't act up under engine load as I shifted many times varying between 2500-3300 rpms before shifting to play with it, and the vibration didn't appear to come/fade with clutch (although it wasn't really there at all after the initial). I started it back up at home and revved it, the issue didn't occur when revving the engine. I did not drive it again as the next place I'd like to drive it to would be to get it scanned, or get a friend to scan it for me.

 

You may have simply thrown a weight off of one of the wheels or off of the driveshaft. It sounds like that type of problem. Something that gets faster as the car gets faster is out of balance. That eliminates the engine and transmission. As for the SES, get it scanned and hopefully just something came loose.

 

Go by Autozone and have the code pulled. Be sure to take note of the code number. They can print out a diagnosis chart if you ask, and if the employee knows how to do it.

 

got the car scanned --

P300 - Multiple Cylinder Random Misfire
I believe that was the exact code/wording.

I didn't really ask if they deciphered the codes but I'm taking that to mean plugs/wires?

I don't know why this would have caused the vibration on the highway to throw an SES light though? Could it have been misfiring that bad?

The drive to Autozone it seemed to run fine. Something else worth noting - the past couple months when driving from 1.5k to around 2000 RPMS under load, it seems like the transmission has a lot of vibration as well, it goes away after 2000 RPMs and the car seems to run fine. Wonder if that's related or 2 seperate issues. I was thinking seperate issues.

 

I would think there would be different feel for a multiple cylinder misfire. I guess it would feel like a vibration, but you should be able to tell that it is down on power and running like **** vs. vibrating. Multiple cylinders would be opti, coil, or some sort of ignition problem. Probably not plugs and/or wires.

 

opti....

 

The PCM determines misfires by looking at the output signal from the carnk position sensor. It is very possible for an extremely rough road to affect the signals from the CKP sensor, causing the P0300 code and a flashing SES light.

Or, severe drivetrain vibration in an M6 car can also cause irregularities in the crank rotation speed, that the PCM might interpret as misfires based on the CKP sensor signal.

The fact that the engine continued to "vibrate", and that the SES light went solid red tells me you may have had a severe misfire, so there's no clearcut answer.

 

Hrmm.. not *quite* the answer I was looking for. lol!

The M6 is definitely having some vibrational issues. I will definitely put a new tranny mount on it, however the SES light is not on anymore. It was on after he pulled the code, so it wasn't cleared at the time it was read. But now, no longer on. Don't know if this helps at all, but it's strange that this happened like this. As you said, no clearcut answer means no clearcut solution. I'll fix the stuff I know needs fixing, and maybe pull the plugs to check them as it's not too much of a hassle with the MAC midlengths.

Anything else I can check without throwing parts at it?

 

Blinking SES light is an immediate indication of a bad misfire ("catalyst damaging misfire"). In other words, the engine is missing enough that the extra fuel/air mixture being dumped down the exhaust can do damage to your cat. That is why the SES blinks above a certain misfire threshold.

The P0300 code will set (and turn on the light, as with any emissions code) for random misfire, but the light will only blink if the misfire is above a certain, higher threshold (i.e. bad enough to potentially damage the cat).

My car threw a 300 code last summer. It then went away, but I can still feel the car miss now and then (not a severe miss like it is running on 7 cylinders or anything). For now I'm going to assume it is my (original! ) plugs and wires. When I change those here shortly, I'm hoping the slight miss will go away.

 

Mine went away on the way home from getting it scanned that day. This weekend I changed the Transmission mount, as I was definitely feeling excessive vibration there as of late. This may have actually been part of the problem, the shaking is gone now and I'm not getting vibration at the 1.5k-2k RPM range at this time. I didn't really get much into it on Sunday, as it was around 109 here and I was BEAT, but besides that all is well. I'll be replacing my plugs/wires next week as it is needed anyway and I'll be taking it rom there. Just wanted to keep this updated.