While the wagon is getting some TLC, I have been driving the SD. Today when coming down a long hill (maybe 15 degrees) As always, I noticed that with my foot removed from the accelerator, the speed held steady. Curious to see what the cause might be, I slipped the trans into neutral and she started speeding up significantly.

That would seem to point to the transmission and or the engine that either by design or problem, is causing engine braking.

Given the high compression both in term of ratio and PSI, is it a normal engine braking effect?

Also noted on the SD as opposed to the TD, you can let your foot off the brake on level ground (after a complete stop) and it will not creep forward. The same holds true for being on an incline (nose up), the car holds steady if you take your foot off the accelerator.

If by moving your foot slowly you remove what feels like free play in the accelerator pedal (maybe 1/4" to 1/2") the car moves forward...release the pedal and you can feel the power to the wheels drop off. To me, this function seems to be designed / engineered into the trans and controlled by vacuum.

Thanks in advance for any input.
Don

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

You are correct, sir. Diesels "suck,".......literally. It's the compression, as you've surmised, giviing braking-action. Don'tcha know!

Enjoy ALL aspects of diesel-ownership! braking-suction, and ALL!

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'06 E320 CDI
'17 Corvette Stingray Vert

This is incorrect. Compression itself is not a loss, as the energy is released on expansion. Any energy consumed by the engine on overrun is caused entirely by pumping loss. This is leakage and restrictions, but not the compression.

Right. Most of the energy absorbed on the compression stroke is released on the rebound (combustion) stroke.

Diesel engines continue to combust with foot off the pedal. I dont notice any engine-braking effect at all. Plus diesels are notoriously tougher on brakes than gasoline powered vehicles. Trucks are equiped with jake brakes that shut down valve activity to assist engine-braking, otherwise they never slow down on engine drag. Its a myth that high compression diesel engines stop dead in their tracks when taking foot off accelerator.

OK... then

Are you saying that coming down a downgrade your speed increases (not the case in either of my MB cars) and what do you think is causing mine cars to hold a steady speed on a downgrade with my foot off?

Thanks
Don

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

That is the Bowden Cable you are feeling. It means that the cable is adjusted right. I noticed that awhile ago in my 300.

__________________
Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

as noted, it's the transmission and the engine displacement relationship to the weight of the vehicle. that gives you engine braking. if your transmission has overdrive, you will get no engine braking. our older diesels have no overdrive, so we get engine braking. also, if you have a model that starts out in 2nd gear, you will get little forward movement on level ground at idle. if your starts in 1st, it will creep forward even on slight inclines.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

In the end...I prefer the braking effect. I live in the Sierra's and almost exclusively drive mountain roads. I never have to downshift and or ride the brakes.

ALSO I am really glad I don't have more items to add to my MB to-do list.

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

Brian explained it right on. Its all in the transmission setup and torque converter. I love how you don't gain speed down most hills in these cars. In our minivan you always have to ride the brakes or it will speed up 20mph on a slope.

The design of the torque converter on American cars probably is also why when you slam the throttle down, they'll break your neck with acceleration, for a few seconds, then the engine is roaring the rest of the time and there isn't much acceleration. I've noticed that on many vehicles, mainly fords though. GM/Chrysler seem to have less.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

They use the same technique with a progressive throttle. It will move the throttle quickly for the first 1/2 travel.

Perception of performance, thats all it is.

On a somewhat side note, this topic is also kicked around quite a bit on the Diesel truck forums. Mainly due to the quest for more engine braking power when towing. Side by side, with identical transmissions, the gasser will provide more braking power because of the vacuum pulled against the throttle plate that is absent in the Diesel.

Various people have suggested things like putting plates in the intakes for more vacuum but most Diesels aren't designed to pull vacuum in the intake and you end up pulling oil out of the valve seals and from the turbo compressor labarinth seal. This is why the aftermarket simply puts a valve on the exhaust to gain the additional braking power.

On my truck and I think on my '98 as well, the injection is completely shut down by the PCM in a coast situation to further aid the braking action.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

How about some mini parachutes?

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1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

I find this discussion, limited as it is to automatic transmission equipped cars, a bit misleading.

I have a standard transmission in my 240D (it is my 3rd 240D, which were all preceded by a 1971 220D, also with a manual transmission-automatics should not be allowed within several hundred feet of one of those engines...). I have also driven many gasoline engine powered cars with manual transmissions, from VW's, to Fiats, Honda's, several 190E 2.3-16's, to our present 1988 300E with a five speed.

My experience, and my wife's experience differs from the generic conclusion reached in the prior posts.

The Diesel engines in the earlier incarnations (220D and 240D in the W114/115 chassis) had a flapper in the intake that closed off when the "idle" throttle position was reached. These cars had very peculiar feeling "coming off idle" performance as a result, and I believe this feature is controlled differently on the W123 240D as they are definitely different coming off idle, especially if the idle speed adjustment knob is turned up. I am not clear it exists on the 617 turbo engines at all.

Manual transmission equipped 220Ds, and 240Ds have pretty serious engine braking capacity, enough so that when you are driving on snow covered or icy roads, it it very ill advised to abruptly yank your foot off the throttle without first depressing the clutch.

I have personally spun these cars out coming down hills in challenging traction situations, and so has my wife. Just by taking my foot off the throttle. No need to touch the brakes. The rear wheels instantly came to their rolling speed at idle, which was less than the speed of the car and the front wheels, meaning the engine braking caused them to lose traction. And that was the beginning of a 360 degree approach to a stop sign in Girdwood, Alaska.

My wife was not so lucky in the 1982 240D, and backed into a guardrail (actually a cable supported by some rotten stumps that just broke off, so no real damage done). That same result couldn't be duplicated, and I tried in second instance noted, with one of the 190E's or in any other winter driving experiences I have had, which spans 4 decades.

I also think the net power of the Diesel engine in these cars at idle injection volumes and engine speeds other than idle speeds, is very low. At idle the freaking car won't even stay warmed up in really cold weather, and it is pretty easy to stall the engine unless the idle knob is turned up. Increasing speed, without increasing fuel flow will increase losses without a commensurate increase in engine output.

I think the braking "advantage" of the Diesel is a feature of its lower idle speed capability and is more apparent at lower speeds. At a trailing throttle, meaning just below the "maintain" speed/load position, the engine braking is not noticeably different than gas cars, which is good because it might otherwise make driving them smoothly a bigger chore.

I have no idea what the interface between the automatic and the throttle might be to make the automatic equipped versions have a weaker braking response. I used to figure it was the fact that there was a fluid connection for transmitting torque that might not be as efficient running "backwards" as it was running "forwards." But there may be other reasons too. All more motive to keep automatics away from my Diesels. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Do MB automatic gearboxes have a one way or overrun clutch in their lower ratios like a Borg Warner 35? If so, in conjunction with the torque convertor would give very little engine braking.

On diesels with manual gearboxes in the UK, it was quite common to see them fitted with a throttle damper which reduced the rate that the fuel lever on the pump moved towards idle.

I agree with the posters who say that you get a lot of the energy in compression back once the piston goes over top dead centre. But for any engine, petrol or diesel once the engine is producing less torque than is required to overcome the various sources of drag the vehicle will slow down.