Why shouldn't a Mercedes Diesel be left to Idle for long periods. - PeachParts Mercedes-Benz Forum

pberku · #1 02-09-2004, 12:39 AM

I can't figure this out.

Why can Diesel busses and trucks idle for long periods of time without any apparent damage, yet idling for extended periods is a no no for Mercedes Diesel cars.

sixto · #2 02-09-2004, 12:43 AM

Who says it's a no no? Besides my neighbors, I mean

Sixto
95 S420
87 300SDL

#3 02-09-2004, 12:51 AM

Whoever told you this is wrong. Nothing bad with leaving a Mercedes diesel to rest at idle for "long periods." Most times, car manuals state not to let a car idle, or warm up for emissions reasons, added to the fact that an idle engine doesn't warm up the transmission, another key part... But anyways, an idling diesel just cloggs up the exhaust system with soot, making it "nice" when you pull away with a big poof of smoke. It's always good to beat up these diesel engines often to keep the "regular" so to speak. But, I don't see any issues with letting them idle.

#4 02-09-2004, 12:58 AM

1-2 hours, mabye overnight if you must is fine. Just remeber, even though they are diesels, MB diesels are nothing like the big american semi engines. They will get carbon build-up, soot and fuel will get past the rings quicker contaminating oil, and oil pressure will be under 2-bar unless you raise the idle rpm. Under 2-bar, the piston cooling jets do not open.

P'bert · #5 02-09-2004, 01:42 AM

But, why is this something you would want to do? Does the clatta help you sleep? Worried about the oil industry profits?

Phil
'84 Euro 300D
'85 300D

whunter · #6 02-09-2004, 08:06 AM

Hello Everyone
pberku has cold start issues in Montreal, Canada.
Personal view; let it idle all night, raise the idle speed to reduce soot and Italian tune every day.

leathermang · #7 02-09-2004, 09:09 AM

I agree with WHunter ( again ).

82-300td,
Do you have some reference for this statement ?--
"Under 2-bar, the piston cooling jets do not open."

R Leo · #8 02-09-2004, 09:16 AM

Quote:

Originally posted by 82-300td
.... oil pressure will be under 2-bar unless you raise the idle rpm. Under 2-bar, the piston cooling jets do not open.

Maybe so but, at idle (ie no boost, no load), the oil jets cooling the piston crowns aren't really necessary.

leathermang · #9 02-09-2004, 10:01 AM

I believe Randy is correct from the fact that at idle a diesel uses only 1/80 th the fuel of a gas engine... just enough to keep it turning over .... and heat comes from the fuel ....
However, I did look this up because I have been told by many MB people that as long as an oil pump is putting out more than 1 bar at idle that it does not need to be changed out...
So I checked the book and it says that the cooling jets start at 1 to 1.5 bars of pressure.

#10 02-09-2004, 10:29 AM

Yes you see big trucks and busses idling all the time. It still is not good for them. It's seen as a necessary evil to keep the driver warmer than having it shut down, or cool during the summer.

Diesels can not maintain their operating temperature at idle. Once the engine has cooled enough, raw fuel will get past the rings, and dilute the oil. The diluted oil will not lube as well, and increased engine wear can result. The fuel can gum up the rings as well, making them stick, and causing vertical scoring of the bores.

dmorrison · #11 02-09-2004, 11:01 AM

Why would you do this??

Northwest Airlines flight attendant, actually stewartess, it was a long time ago and that what they were called then. Did this with her Mercedes diesel. Had an over night turn and it was bitter cold in Minneapois. She new the car would not start in the morning when she returned, so she left it idleing. Locked the door and went on her turn. Returned to a nice warm cozy car 15 hours later.

Another story I heard. Same airport. A guy returned to his car after his trip. 1 foot of fresh snow covered all the cars at the airport parking lot. His was easy to find. It was a Volkswagon Bug. Dug the car out and when he put his car in the lock it woudl not turn. He thought great my locks are frozen. Then he realized that he did not have black interiors!!! He had dug out the wrong car.

Dave

R Leo · #12 02-09-2004, 11:11 AM

I don't know how 'big rig' cooling systems differ from MBZ engines but it seems that if any diesel had a chance of long-term idling at the proper and constant operating temp, it would be an MBZ.

Anecdotally, remember that these engines were/are successfully used extensively in taxis, a vehicle that quite commonly spends much of it's career in low-load, no-load situations (idling).

If I understand correctly, the MBZ's closed circuit design permits complete circulation of coolant through the engine block while the thermostat is in the 'closed' position and only opens when the coolant temperature rises above the opening value for the thermostat. Even under no-load idle conditions, it seems to me as if there would be enough heat generated to maintain operating temp and to distribute it properly with the MBZ design system.

Diesel Power: A couple of questions.
1) Why do 'big rigs' often have 'shutters' on their radiators (doesn't the engine thermostat perform the thep control function)?

2) And, why do big rigs sometimes have 'curtains' covering (or partially covering) their radiator grilles?

leathermang · #13 02-09-2004, 11:14 AM

It does not take much snow for one Beetle to look like another one.... LOL

On an aircooled Beetle this would be a very bad thing... where I am not convinced it would be with our diesels...

Air Cooled VW engines used splash lubrication for the cams.... and under 1700 rpm you could not count on them getting any lube... they warn of this in the manual... and setting the idle to 1700plus.... it would probably run out of fuel before your return flight arrived.... LOL

I have seen pictures of Ski Resorts where you could tell the MB's were locked and running over night....

Lars · #14 02-09-2004, 12:00 PM

Pberku, I also live in the Montreal area, Boucherville actually. Below -15c, I do not have any other choice but to plug in or leave my car idleing over the lunch hour. I do not notice excess smoking or sooting after extended idleing periods, but I never let it idle more that 2-3 hours. My experience is that at -30c, I cannot leave the car not running for more that 3 hrs or else it cools down beyond the starting threshold. However, it's been quite mild these past weeks, so no more headaches.

pberku · #15 02-09-2004, 12:13 PM

Just for the record, although I do live in Montreal Canada, and yes it does get down to -16F in the winter (it did last week), and 95F+ in the summer I don't necessarily leave the car to idle because of the cold/hot weather. My car always starts fine.

The reason I asked that question is that I find myself always waiting for the wife, outside stores, malls etc. sometimes for an hour or so. Therefore, while I wait, I like to let the car idle to get some heat in the winter and AC in the summer .

I figured its better to let it idle for an hour or so than to start and stop the engine every 10 minutes. Starting and stopping the engine will shorten the life of the Glow-Plugs, Starter, Battery, etc. Yet many diesel experts say its not a good idea to let a diesel idle for extended periods of time.