Hit a stretch of bridge today and stretch the legs on the car a little...

2014 C63 (completely stock)
48k miles
Current oil: Shell T6 (changed about 1k miles ago)

Was driving for about 25 minutes before this happened, engine was plenty warm... Oil temp at the time was 240F coolant was 197F. I'm in Florida so it regularly gets that hot in the climate with spirited driving. Anyways, was driving in "M" mode went through gears 3 and 4. At about 110 Mph, I noticed a large amount of white smoke coming from the rear. No CEL, everything was running as normal. I pulled off at the next exit (about 2 minutes later) and once I parked, the smoke has stopped. Looked under the hood and everything seemed fine. Once I got home I did notice some type of hot plastic type smell. Definitely not oil as I know that smell. Checked oil/coolant levels and everything is normal.

Drove it to get gas after as I was almost completely out. Started normal, car drove as normal. I got the oil temp up to 245F but could not replicate the problem. Car is still under CPO warranty. Any ideas here?!?!?

For reference, this is what mine looked like:

Watch @0:21:

That didn't look like it was coming out of the tailpipe. Does your engine have oil on it looking from under the car? Has it done this again?

Yea it didn't seem to be coming from the exhaust. I was wondering if it was the trans/rear diff, although it seems to let up once I stopped using the throttle. I'll take the covers off the bottom in the AM and look but I don't think so. I did take it out again and got the temp up to 240F again, which is not hard to do here in FL. No white smoke. If I take it into the dealer what/how much investigating will they do with a CPO warranty?

Yea it didn't seem to be coming from the exhaust. I was wondering if it was the trans/rear diff, although it seemed to let up once I stopped using the throttle. I'll take the covers off the bottom in the AM and look but I don't think so. I did take it out again and got the temp up to 240F again, which is not hard to do here in FL. No white smoke. If I take it into the dealer what/how much investigating will they do with a CPO warranty?

White is usually coolant. Blue-ish will be oil related. If its CPO, dont waste your time. Bring it to the dealer.3

Whats CPO?

That does seem more like coolant although not coming out of the exhaust is still puzzling (thinking block, head cracked), is coolant level still the same? Smell would give it away but hard to do from inside the car.

If the inspection of the bottom of the car doesn't show anything out of place or leaking... Are all the coolant or PVC venting or lines still good? Maybe something from a line dripping on exhaust at temp/speed?

CPO = certified pre owned. meaning you have warranty

I had this, it was transmission fluid dripping on the exhuast. If you take off the covers do you have any oil/ fluid there?

We have a winner! Seal appears to be fine. Looks like it vented fluid. What was the outcome with yours? I made an appointment with the dealer to take it in next week.



M156 transmission fluid leak caused white smoke when fluid his the exhaust pipe.


M156 transmission fluid leak caused white smoke when fluid his the exhaust pipe.

Had it done by an indie shop as part of the 40k service. Is it possible my leaking fluid is radiator coolant? It's green like that, I thought the trans fluid was red? From what I can tell the seal looks fine I didn't check the bolts though.

I had this happen on my car after some spirited driving.
Turns out the dealer had over filled the transmission. Their was actually a service bulletin about it.
I think the initial service fill level resulted in this venting.
The service bulletin calls for a lower fill amount.
Since the dealer drained my transmission, and refilled correctly, all has been fine.

Not off the top of my head. I may still have it at the office.
I will only be back on Tuesday, and will have a look.

I'm leaning towards this... Dropping it by the dealer in the AM. The fluid leaking is green like the radiator coolant. Isn't the trans fluid supposed to be red?

Trans fluid should be red or blue depending on the fluid used. Coolant should be clear or blue. Check your coolant reservoir to see what color fluid is in your cooling system. Sounds and looks like it could be coolant to me.

Good to hear you are sorted, and it was nothing major.
But I must say, it gives you one hell of a fright when you are pushing hard, and suddenly you get this huge cloud behind you. I just about had a heart attack thinking something had blown badly on the motor.

Yea, same thing I thought in the heat of the moment. The dealer checked mine and said same as the other guy that it was overfilled. They did ask me to reproduce as they can't have their techs driving customer cars at 110 Mph. Said they book me another appointment in 2 weeks to recheck.

hi, i have the same problem , replace nearly everything on the engine also the turbo, when passing the 40km/h i have the same cloudy white smoke. does youre problem got solved?