# Diesel engine heat after shut-down.



## 5SeriesNatsFan (Mar 8, 2010)

Just curious and this is of no importance. Do diesel engines stay hotter longer than gasoline engines after shut-down?


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## mecodoug (Nov 30, 2007)

Since they run hotter, I think the answer is yes.


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## F32Fleet (Jul 14, 2010)

I don't know. Diesels burn cooler than gasoline hence the aux. radiator instead of an aux. oil cooler.


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## Flyingman (Sep 13, 2009)

I don't think anyone can say for sure without conducting some actual tests or studies.

Once you shutdown any engine it holds a certain amount of residual heat. As the diesel engine is usually heavier than a gas engine (i.e. more mass in way of iron, aluminum, steel and oil and possibly other liquids), I would expect the diesel would take longer to dissipate it's radiant heat. In addition I would surmize that the diesel engine compartment is better insulated for noise reasons, so may also help retain heat longer.

So, as a mechanical engineer with no actual test data, I say "Yes" the diesel engine takes longer to cool down, and warm up vs. the comparable gas engine.


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## Snipe656 (Oct 22, 2009)

Also are you comparing a non turbocharged gas car to a turbocharged diesel? The turbos get pretty hot so just those will take time to loose their heat.


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## HIREN (Jul 14, 2006)

BMWTurboDzl said:


> Diesels burn cooler than gasoline hence the aux. radiator instead of an aux. oil cooler.


I don't understand, can you elaborate?


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## mecodoug (Nov 30, 2007)

I am no diesel mechanic, and this is just anecdotal from when I was 10 years old and lived on a sailboat with a 27 HP diesel. That engine ran better when its temp was high - even past the point of 'overheating' many times due to pump issues, and so I was always under the impression that diesels were intended to run hotter than gassers.

Even if it doesn't run at a higher oil temp, the larger thermal mass almost certainly retains its heat longer. 

But alas with no electric water pump there is no heat at rest feature for the 335d - the only feature I wish I still had from my 2006 330i. OTOH that electric water pump failed at around 50K on that early E90 - I hope the mechanical pump on the d will last longer than that.


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## bimmerdiesel (Jul 9, 2010)

mecodoug said:


> But alas with no electric water pump there* is no heat at rest feature for the 335d -* the only feature I wish I still had from my 2006 330i. OTOH that electric water pump failed at around 50K on that early E90 - I hope the mechanical pump on the d will last longer than that.


Do you mean you dont have rest feature on your left temp dial?


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## bigslickak (Jun 29, 2007)

Just an observation on this:

On my 335d I do notice that the cooling system does remain on more often after turning the ignition off, compared to my old 335i. More often than not, I shut the car down, and the fan is still going. This is even after a short drive, in mild temperatures.


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## Snipe656 (Oct 22, 2009)

bimmerdiesel said:


> Do you mean you dont have rest feature on your left temp dial?


The 2010 328i I had for awhile, had no Rest button on it. I think that is the case in the 2011 models as well. I did read a thread on here where people where saying that they based upon the 7 series manual that there is still a way to engage the Rest feature. No idea if they were correct or not. Even down here in Texas I use my Rest feature.


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## AZ335D (Aug 19, 2010)

Rest feature????


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## HIREN (Jul 14, 2006)

bigslickak said:


> Just an observation on this:
> 
> On my 335d I do notice that the cooling system does remain on more often after turning the ignition off, compared to my old 335i. More often than not, I shut the car down, and the fan is still going. This is even after a short drive, in mild temperatures.


This is due to the diesel regeneration cycle, not an indication of running hotter all the time.


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## Snipe656 (Oct 22, 2009)

This and looks like a couple posts into the thread they say how to engage it on the cars that lack the specific button for it:

http://www.e90post.com/forums/showthread.php?t=323116


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## Snipe656 (Oct 22, 2009)

Snipe656 said:


> This and looks like a couple posts into the thread they say how to engage it on the cars that lack the specific button for it:
> 
> http://www.e90post.com/forums/showthread.php?t=323116


Take that back, I read more into the thread, so maybe there is not a way to do it, I did not actually read the entire thread though.


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## mecodoug (Nov 30, 2007)

I don't see how REST could work for more than a minute or 2 without an electric water pump, which we do not have.

I am shocked that the newer 328's don't have that feature though - it still uses the variable speed electric water pump doesn't it?


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## Snipe656 (Oct 22, 2009)

The Rest works just fine in my 335, I use it every time I drive the car this time of the year. I am away from it for far longer than 2 minutes although it typically is still going when I get back to the car. Did you read through the thread I posted? It says that supposedly BMW quit installing the pumps needed for this feature to work and it sounds like that applies to all 3-series. Also sounds like they did this change starting in the 2010 model year which is probably why my car still has it, the 2009 328i I had for awhile also had this feature, the 2010 did not have it.


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## autoJeff (Oct 1, 2009)

2009 335d has rest. 2010 cars did away with rest. Rest works as you would expect in my 2009 335d. 

Fan running at conclusion of a short trip most likely indicates a DPF regen in progress when you shut off the car. You might notice that prior to shutting off the engine you the car was getting worse mileage and holding higher rpms in lower gears under gradual acceleration due to the in-progress regen. 

I was under impression that the diesel motors run cooler than the gasoline motors. I think the BMW mechanic even told me this. Allegedly takes longer to warm up the diesel due to running cooler and maybe in part to larger thermal mass. The gasoline cars don't have the aux ceramic heater that the diesels have for more quickly heating cabin at cold start. 

I've also read (unconfirmed) that the diesel motors use about 1/4 the fuel of a gasoline motor at idle. And that, even more so than with a gasoline motor, a cold diesel heats up slowly at idle. This is partly why you should not start your car and let it idle to warm up. You should start and begin gently driving immediately. It will warm up when the engine is under load. Idling a cold engine, especially one that remains cold for a long time when at idle, puts more wear and tear on the engine.


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## Snipe656 (Oct 22, 2009)

They do take longer to warm up, maybe that is because of a cooler burn like some said it has, I'd not know for certain. As far as the amount used for idling, I'd believed that up until I got this 335d. This 335d I know for a fact guzzles fuel at idle. The times I have been on it and had to wait long periods of time for my wife to get off work, the car has had a substantial decrease in overall fuel economy. Neither of my other vehicles do this at all.


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## mecodoug (Nov 30, 2007)

Thanks for the info and link on the REST feature. I do miss that one,


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## F32Fleet (Jul 14, 2010)

AZ335D said:


> Rest feature????


The passenger side temp dial which now say "ALL" used to say "REST". With the car off pressing this butten would allow the fan to remain on for ~ 15 minutes pushing residual heat into the cabin. I rarely used it with my 330, and TBH I prefer the ALL feature since I think having dual temperature control in a cabin as small as the E90 is about useless.


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