Pierre-CT

Today, for the first time in the three years that I have owned my 2012 GL450 (62k miles) I decided to try using the paddle shifters for more than just a short burst. I had the family in the car and drove very cautiously, rarely exceeding 3000 rpm, never exceeding 4000.

About 45 minutes into the drive I was doing 65 on the highway, up a very mild incline, at 3000rpm in 5th gear, when grey smoke suddenly starts coming out the tailpipes. I pulled over and turned the car off; after 10 minutes I started it up again and cautiously continued (in automatic!). I was able to complete my journey without further problems and there are no warning messages or apparent ill effects.

What happened? Did I do something wrong? What are the chances that something got damaged?

I have noticed that, in automatic, the engine rarely exceeds 2500rpm, even when I floor it, which I think might be the cause of the one gripe that I have with the car - a lack of power, most evident in an inability to maintain cruise control speed on mild inclines on the highway. When I have asked the dealer about it (the car has never missed a dealer service) they basically blow it off and say the car is just fine.

Is this normal, or has my car been "tuned" for low rpm? Is one supposed to use the paddle shifters, or is this essentially a gimmick? I certainly would be very hesitant to use them again.

Any guidance greatly appreciated!

alx

Using paddle shifters should be safe as you can't overrev the motor anyway. But I have never felt the need to use them unless sudden passing was required in which cas two downshifts are all that is needed.

Btw, the more careful you drive the slower and lazier your truck will feel as it adjusts to your driving patterns.

Nobody here will be able to tell you why you puffed smoke for a short period of time...

Pierre-CT

Thanks for the insights!

Yes, I drive the truck smoothly and rarely have need for sudden acceleration; the dealer also says that the transmission adjusts to my driving style - it's a little frustrating because, although I'm a defensive driver, when I need the V8 power I really would like it to be there for me. I had an '07 before the '12, and it actually felt like a V8; it was much more pleasurable to drive (although the '12 is fabulous in all other respects).

I'm no mechanic, but I would have to guess that the smoke indicated some kind of excessive engine strain. I've never seen it before and, although it may be coincidental, I'm betting that it has something to do with using the paddle shifters and running the engine outside its "comfort zone". But 45 minutes at 1,500-3,500rpm - geez, surely that should fall within an MB comfort zone?!?

eric_in_sd

I'm with alx on this one; you didn't hurt the motor, and if something bad happened, it is completely unrelated to that driving. The engines are very solid, and are perfectly able to handle sustained higher rpm use. In fact, I advised one guy, who was looking at "chipping" his engine to get a few more horsepower while towing on long hills to simply force a downshift. You're better off running part throttle higher rpm than full throttle lower rpm. In the former, the water pump and oil pump move fluids faster, so the engine is actually happier.

Some people recommend occasional full throttle high rpm bursts to momentarily raise the combustion chamber temperature. This may dislodge carbon buildup.

If you have a sedate driving style, it's possible you have deposits of junk here or there in the engine, and the sustained higher rpm driving suddenly burned it off. If the truck is running normally now, try a few full throttle accelerations, letting the rpms get up. 5-6k rpm won't kill it, honestly. Do it on a nice sunny day and keep an eye out in the rear view mirror, and you just might be treated to some dramatic smoke behind you. But the engine will be happier in the long run.

The smoke you'd be honestly concerned about is white and persistent. That's oil. Skywriting airplanes inject oil into the hot exhaust to make the white smoke.

Engines really don't mind spirited driving - that is, bursts of speed. Prolonged full throttle moderate rpm is the hardest on the engine.

Pierre-CT

Great analysis, thanks!

I took the truck to the dealer today and the service adviser basically echoed your sentiments exactly - The grey smoke was carbon deposits burning up, and I did the engine a favor.

Also, as you say, one needs to make a point of revving the engine periodically to prevent carbon build up – the newer X166 models have Sport Mode, which basically runs the engine two gears down from Comfort Mode, but my car doesn’t have this (don’t know if any X164 450s do?), and is essentially stuck in Comfort Mode all the time. Carbon build up is apparently a fairly common issue in X164s with the 7 gear transmission, and it does seem that the transmission gravitates towards low RPM when left to its own devices (probably for emissions reductions, the same force that has driven the move to V6 engines in the new GLs).

The service adviser was obviously quite cautious about his exact choice of words, but I came away with the understanding that using the paddle shifters is not only safe, but can be good for the engine when used judiciously.

This is the theory anyway, and I’m going to try it out – fingers crossed I don’t end up back here reporting engine damage!

eric_in_sd

In fact, I'll tell you a story about the paddle shifters: I am having the all-too-common tranny failure, and I used the paddle shifters to skip gears that seemed to be having trouble, as the trouble is undoubtedly hard on the trans. For example, I'd put it in D2, rev it up, and then real quick, paddle to D4 to skip 3rd. I messed up one time and revved it up in D1, and I thought, oh fuq, but I watched on the dash as the transmission upshifted itself. It went D1- D2 - D3, etc. So those paddle shifters are pretty much idiot proof.

There are things that will kill those engines, but occasional hard driving is definitely not one of them.

alx

Carbon buildup is common for all engines in vehicles driven in America where said vehicles are designed to run at wide open throttle for somewhat long periods of time. Been to Germany on the autobahn at 2am?