PeachParts Mercedes-Benz Forum - View Single Post - Blowout/non-full size spare prompted Hypermile run in the CDI

TimFreeh · #24 09-08-2013, 09:30 AM

"Got a couple Qs:
I'm noticing that when I let up on the foot-feed quickly after accelerationg up to 30 mph or so, I hear a quick hissing and the short whistle of the turbo spool doing a quick wind-down. Windows open with a subdivision street curb out to the left of the car to reflect the noise up to the driver's open window...
The CDI's noises I hear are more evident than the 99 E300's, so, perhaps I have a leak in the turbo under the hood somewhere, or, my hearing is sensitive to something not being right, under the hood? RE: (poor fuel mpg)"

I've not really tried the above scenario, I'll give it a shot today and see if I can hear anything. If you've got your foot down and the engine is under boost it seems totally reasonable that you'd hear some sort of sound when you abruptly chopped the throttle. The CDI's have a much higher upper boost pressure range than the earlier engines, IIRC its somewhere in the 20-30 psi figure. From what I know of whats going on under the hood what you are experiencing seems totally normal to me. With regard to your 'maybe somethings wrong with the turbo plumbing...' line of thinking I guess that could be possible but I'm sure boost level is a monitored data item and if it wasn't at the level the ECU was expected to see you'd have a CE light on.

"Another thing - the brakes seem to be 'set' whenever beginning backing, or when shifting to D from reverse after backing. It feels like the disc pads are 'set' if you will when first giving the foot feed pressure."

I have noticed this but not on a regular basis, I just assumed it was the SBC system acting in an I'm-smarter-than-you-are basis when it detected that I'd shifted from R->D and the car was still moving at like 1 mph. I just assumed that the trans computer told the SBC to stop the car before the shift members engaged. Seems like a reasonable course of action but I'm speculating - not sure if its set-up this way.

One feature of SBC vehicles that I've read about is called a traffic-jam stop feature. You put the car in this mode and the SBC assumes that you're in a stop and go traffic jam and the SBC will hold the car in position with your foot off the brake. When you touch the gas the SBC releases the brakes and the car moves forward until you release the gas and at that point the SBC system will smoothly brake until the car stops and will then hold position until the next time you touch the accelerator. Unfortunately it appears that USA spec cars don't have this feature, I could see this being really for todays road conditions.

"Another oddity about the car; the car feels as if it's slowing because of being in the wrong gear approaching 10 mph when coasting toward a stop, before it then feels as if the gear disengages and the car is left to coast freely. Again, perhaps I'm too sensitive to what I feel the car doing, but it's almost as if the brakes are being applied or 'set' by the SBC system. Any symptoms like these experienced by you guys with CDIs?"

No idea on this one, coasting to a stop my car behaves like all others I've driven. I can feel it downshifting as the speeds drop but feels pretty normal to me. Never noticed the SBC doing anything here unless I put my foot on the brake.

"This car has gone through one set of rear rotors already, and disc pads all the way around, as reflected in it's VMI records. That didn't sound right to me, but as I said, maybe it's the SBC system doing the disc pad engagements too much. Hence, perhaps this is affecting the mpg....."

Doubtful, I'm sure it was just the dealer trying to sell pads and rotors. I'm constantly amazed at how much garages charge for pad/rotor changes - at dealer retail prices MB should be installing the pads/rotors for free. When I picked up my car from Tyler the pads were about 50% worn - I just replaced them at 201K and while the rotors were still in spec I also replaced them. I got about 32k on a half set of pads - which is about my average over the years. I don't think the SBC system is any harder on pads/rotors than a conventional system. I also don't think its likely that SBC drag is making any difference on your mileage.

You didn't answer my earlier question, have you ever reset the cluster mileage computer and taken the car for a rural 50-100 mile drive at constant speeds of 55-60 MPH? That's where Tyler and I are getting the 45+ MPG figures and I'm wondering if your car could do it in those conditions.