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2014 Mercedes-Benz GLK250 Bluetec, 1981 Mercedes-Benz 300D W123
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Discussion Starter · #1 ·
When Mercedes sold these cars, did they every mention to the buyer or hand out a paper that said that the turbo needed time to cool down after it was use? It there anything in the manual that says so? Would going from highway driving to city driving 25-35MPH be considered cool down? or should the engine idle for a certain amount of time when a trip is over?
 

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2003 Volvo V70 5 Speed
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I usually let mine cool at idle for about 30 seconds before I shut down my car. I used to do the same thing on heavy equipment as well..
 

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380SL diesel
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Normally there shouldn't be any real need to do anything special for shutdown, however, if you were to do a hard pull to the top of Loveland Pass (12k ft) and pull off at the top and immediately shutdown to look at the view, that could be a problem. If these cars were pyrometer equipped you could shutdown based on EGT.

Just be prudent and if you've been running hard give it a few moments to cool down. The problem isn't so much with the turbo, but rather with coking the oil that is in the turbo and damaging the bearings.
 

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1984 300D
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The cool down period is good for the Turbo but I doubt if the original owners the other owners since the time I bought the Car gave the Turbo a Cool Down.

I sometimes let it run a bit before I shut down but that is because I listen to the engine a bit before shut down.

However, that cool down is extremely important on a load carrying Truck; and for the Engine also.
In fact when it used to be when a Trucker pulled into a Truck Stop to Eat they left the Engine running.
 

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2004 CLK 500, 1983 300D Turbo Diesel, 2000 BMW M5, 2002 BMW M3, Ram 1500
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We deal with this same issue on turbo charged aircraft engines. Now granted the speeds are slower on the MZB turbo, but the principles are the same. As mentioned above, it is the "coking" of the oil within the turbo shaft bearing that is the issue primarily.

Kind of a rule of thumb I use is after getting off the freeway ( typically @ 70-80), I'll slow down to local traffic speeds (@ 40mph). There's no need to push the car as much and it allows the turbo to "cool down". You can think of it as "spool down" time but in reality it is really "cool down" time, and all about temperatures. From the freeway to my house is about a 10 minute drive. Once at the house, I'll let it idle for a good 30 seconds and then shut the car down. :thumbsup:

Do I need to let it "cool down" from the freeway as long as I do? Maybe not, but it's pretty cheap insurance, and also allows me to transition to the slower speeds.

For that fact, I do the same with the M3 & M5 for cool down as well.

Don't think it will help? Try running at high speeds/temps and then shutting everything down right away. :eek:
 

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ANY engine profits from a short 'cool down' period if you've just trashed it... Especially longer inline engines. If you turn them off when HOT, water flow stops and local heat pockets may boil or vapourise the water and tensions build up (same effect when you put high loads on engine and suddenly need to stop and the coolant temperature rises, stop-n-go traffic p.e. - heat build up due to load - coolant flow decreases and heat rises due to lower rate of heat-transportation).

It's just the matter that those turbo engines are under very high thermal stress and actually do need a cool down when hot, also the turbo shaft gets very hot - if the oil flow suddenly stops the oil burns and the shaft takes damage from heat & coking deposits.

After normal use - I just shut it down. But if I feel it may be hot, I let it idle for a bit. I do the same with my 300CE gasser - it's a looong engine :)
 

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'86 W123 200, OM617 non-turbo, bastard 5-speed; '95 W202 C250 Diesel, OM605 non-turbo, 5-spd man
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You don't have to let the engine idle for too long after start-up. But you should not exceed ±1500rpm for the first 30 seconds or so.

After driving, the engine needs to idle a while to keep the oil supply to the turbo going. What really kills the turbo is not heat (so you're not really letting the turbo "cool down"), but the fact that it may still be spinning at its very high speeds when the oil supply to its bearings is shut off, making it run metal-to-metal. You're thus actually (supposed to be) letting the turbo "spool down".

I never let my dad's turbodiesels idle for less than 1 min after highway driving.
 

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1984 300D
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ANY engine profits from a short 'cool down' period if you've just trashed it... Especially longer inline engines. If you turn them off when HOT, water flow stops and local heat pockets may boil or vapourise the water and tensions build up (same effect when you put high loads on engine and suddenly need to stop and the coolant temperature rises, stop-n-go traffic p.e. - heat build up due to load - coolant flow decreases and heat rises due to lower rate of heat-transportation).

It's just the matter that those turbo engines are under very high thermal stress and actually do need a cool down when hot, also the turbo shaft gets very hot - if the oil flow suddenly stops the oil burns and the shaft takes damage from heat & coking deposits.

After normal use - I just shut it down. But if I feel it may be hot, I let it idle for a bit. I do the same with my 300CE gasser - it's a looong engine :)
When you shut down the Engine the Thermostat remains open as long as the Coolant is hot enough to keep it open.

After that there will be some coolant flow due to Thermo Siphoning.

I don't know if that is enough flow to prevent what you were speaking of or not; but for practicle purposes this does not seem to be an issue.

The thermo siphon will end when the Termostat closes.
 

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IMO it's by far not enough, escpecially around the very hot exhaust ports.

You'r right, though. And many pre-war engines rely on it as sole cooling. :)
 

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2014 Mercedes-Benz GLK250 Bluetec, 1981 Mercedes-Benz 300D W123
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Discussion Starter · #13 ·
Another reason to consider using a synthetic oil.
Wow, thank you for your response everyone! I did not think that this would really get any interest. I also change the oil every 3000 miles with Shell Rotella T6 5W‑40 Heavy Duty Fully Synthetic Motor Oil. The engine is tight as a drum, does not leak or burn oil ether.
 

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2004 CLK 500, 1983 300D Turbo Diesel, 2000 BMW M5, 2002 BMW M3, Ram 1500
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You don't have to let the engine idle for too long after start-up. But you should not exceed ±1500rpm for the first 30 seconds or so.

After driving, the engine needs to idle a while to keep the oil supply to the turbo going. What really kills the turbo is not heat (so you're not really letting the turbo "cool down"), but the fact that it may still be spinning at its very high speeds when the oil supply to its bearings is shut off, making it run metal-to-metal. You're thus actually (supposed to be) letting the turbo "spool down".

I never let my dad's turbodiesels idle for less than 1 min after highway driving.
Your correct that the turbo needs to keep the oil supply going while at idle, but incorrect that it is not for cool down. It most definitely is for "cool down".

Keep in mind that during normal operating conditions the turbo can be spinning anywheres from 15,000-30,000+ rpm, and at idle still in the neighborhood of 3,000-5,000 rpm, in any event still some pretty high rpm's. Even if you've let the car idle for 10 minutes, shut it down and walk away.... the turbo is still spooling down.

It is the heat transfer that is most detrimental, and to which you do have some control over. If the residual oil within the bearing & supply line is subjected to "high temps" from the turbo, it will crystalize to what is called "coking". It is not the one time, nor the 10th time that by not allowing the turbo to cool down that will give you trouble, but rather the continuous practice of not letting it "cool down" properly, and a little build of of coking today plus some tomorrow and so forth, that over time the oil passages become blocked and restrict proper oil flow to the bearing and then one day it seizes. Think of it as "clogged arteries" for an analogy.

Todays turbo's and oils are not quite affected as technology has improved with design, with cleaner diesel fuels, as well as synthetic oils.

But again, whether you think of it as "cool down" or "spool down" doesn't really make big difference as long as you let it idle for some period of time. :thumbsup:
 

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1981 Mercedes-Benz 300TD
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I've heard of people seeing an increase in leakage but I wouldn't let that stop you. I use synthetic now and have small leaks, but they existed before the switch and did not become noticeably worse.

There has been multiple mentions of the turbo "spooling down" that I feel should be addressed. Pre-shutdown idling doesn't really do anything for "spooling down" a turbo; road vehicle turbo's spool down in seconds, not minutes. A typical turbo will spool down from a full boost condition, which is close to its maximum rpm, to its lowest rpm (e.g. engine at idle) in >10 seconds assuming the engine went into an idle; and the turbo's speed at that point is closer to 4000 rpm for the Garrett T3. While the operational speed, producing boost AND/OR very high engine speed, can surpass 100,000RPM, your never going to shut the engine off with the turbo spinning anywhere close to that speed......well unless you like to floor it onto your driveway. But you still have to stop so the points the same. :)


This gives me an idea. What if I added a manual vacuum switch to my ignition to allow me to prevent engine shut off when the key is removed, then use my key to lock the doors? Its ideal for quick highway rest stops or when running errands where your in a place for only a minute or two. That would prevent me from spending more time cooling my turbo than I do in the rest stop and spare my engine and starter form additional ware and tear. What do you all think?
 

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380SL diesel
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This gives me an idea. What if I added a manual vacuum switch to my ignition to allow me to prevent engine shut off when the key is removed, then use my key to lock the doors? Its ideal for quick highway rest stops or when running errands where your in a place for only a minute or two. That would prevent me from spending more time cooling my turbo than I do in the rest stop and spare my engine and starter form additional ware and tear. What do you all think?
That's exactly the setup I have on my SL Diesel project, though the purpose is not to do what you are suggesting. The SL ignition key system does not have the vacuum shutoff circuit on it so I had to rig a manual shutoff using a vacuum solenoid and a switch on the dash. I also went manual on the glow plugs for simplicity and to eliminate unneeded glow times.
 
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