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Discussion Starter · #1 ·
I've noticed every once in a while that I have a light surging feel in specific situations. It always happens going up a hill at moderate to highway speeds. Engine speed is around 2,500 rpm and throttle is about 1/3, in gear 4–6. The tach doesn't bounce, so I've ruled out the transmission, at least the torque converter lockup. No CEL.

This is one of those things that the dealer or any shop would just roll their eyes about, and good luck replicating the behavior, let alone actually diagnosing it. Could it be turbo compressor surge when the throttle is partly closed but generating a decent amount of boost already?? As I hit the gas and load up the engine more in the same gear (higher airflow), it goes away! I would hope MB would have sized the turbo correctly for all operating conditions.

I'm interested if anyone else has had this sensation. It's pretty subtle, but it feels like a sudden short gust of headwind has hit the van. More noticeable if you're in M mode climbing a hill in moderate traffic.
 

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I've experienced that on my diesel GLK on occassion, and very rarely on the Metris. I assume it has something to do with how the computer is adjusting fueling, has never caused me any issues on either vehicle.....
 

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Discussion Starter · #4 ·
This has nothing to do with Crosswind Assist. I was only describing what it feels like: how a sudden headwind gust would slightly slow the speed/acceleration for one second.

If it's turbo surge, then that is a flaw of design for a specific engine operating point. Is this the same turbo as the other MB cars' M274? I'd expect more problems at full-throttle if it was anything else...
 

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Discussion Starter · #6 ·
That's what I assumed. Maybe it's an artifact of the Metris' weight then; where all the other cars powered by M274 very rarely or never operate at that load/speed for more than a fraction of a second. I do think I've noticed it mostly in M/manual mode, such as ascending I70 up to the Eisenhower tunnel where I want a power reserve in a lower gear. These motors seem to like being loaded up with boost in a high gear.
 

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It's pretty subtle, but it feels like a sudden short gust of headwind has hit the van.

Are you getting an increase (surge) or decrease in power?
 

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Discussion Starter · #8 ·
Decrease in power. That would be consistent with compressor surge, I believe, since it would stall and spool rapidly. This is at part throttle, but even then, the power would be decreased by a sudden decrease in pressure upstream the throttle plate (or intake valves).

Other components associated with airflow control could be the throttle plate position, intake valve cam selection (there are actually two cam profiles that are selected by the computer: full lift and reduced lift with early closing), and intake valve timing. Based on some white papers circulating on the M274, there is quite a complex control situation at part-throttle to balance the selection of these components to maintain a smooth driving experience.
 

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It’s probably the Powertrain shuffling from one configuration to another based upon what it thinks would be more efficient or powerful (and thus appropriate for the circumstance)

Let’s remember that you have two cam profiles, a variably active turbo charger, at least two fuel profiles (regular and lean burn), and a transmission that has 14 distinct forward configurations (seven gears, for each of which the torque converter can be locked or unlocked). That means that for a given throttle plate position you have at least 112 potential permutations of activity.

Since this is a direct injection engine fuel flow is independent of throttle plate (which like in a diesel only controls air flow)- and I’m not sure that throttle plate is the correct term anymore anyway- this adds another variable for a given plate position. Add to that that since this is (like all modern cars) drive by wire and therefore the throttle plate and gas pedal position are not directly correlated either.

All of these things are controlled by computers that are trying to interpret your intentions. So it is not surprising that the way you expect it to behave is not always the way it behaves. Keep in mind that (for instance) the computers will automatically increase power delivery to compensate for external variables- like going up hill.

I sort of had this problem with crosswind assistance. I would feel and hear a strong crosswind and automatically compensate for it... just as the computer did the same. Then the car would veer wildly against the wind. It took me some time before I learned to largely trust the CWA system and just keep my car centered without respect to wind.
 

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Discussion Starter · #10 ·
Let’s remember that you have two cam profiles, a variably active turbo charger, at least two fuel profiles (regular and lean burn)
Green, yeah it definitely could be cam switchover jumping back and forth in that very specific load/speed range. As long as it's not a malfunction, I'm ok with it.

Where did you read these US-spec motors run in lean-burn mode? I assumed they didn't strap that NOx storage catalyst on these US vehicles due to cost reasons, and only run them in homogeneous mode to meet EPA requirements. When Audi came out with the FSI technology (direct injection with homogeneous and stratified charge), I was very excited, but then I realized the US cars didn't get the lean-burn modes.
 
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