Do diesels run as "hot" as gas engines?
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Senior Member
Joined: Jan 2006
Posts: 253
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From: Temecula, CA
Tesla model S
Do diesels run as "hot" as gas engines?
Okay obviously any machine generates heat as it works but do our cars run "cooler" on average than our gas counterparts? I'm here in California where it gets hot and there is alot of traffic which means sitting in traffic with little to no air-flow over the radiator - so in that scenario does my diesel stay any cooler than any other car? 
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Joined: Oct 2005
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From: Loudoun County, VA
W211 CDI, W203, 03 Dodge CTD
At idle, there is obviously no load on the engine. The exhaust combustion temps (EGT) are low. While I don't monitor the temps on the MB, I do on my Dodge. The EGTs will drop as low as 225* if idling for extended periods in traffic. As a result, I've never had a problem with overheating. The water temp can eventually drop. On the downside is cylinder wash down due to insufficient combustion temps.
Take for example a diesel that has been "cold soaked"...sitting in cold temps overnight. Crank it and let it idle. You will find it will almost never get the water temp gauge to budge. On the other hand, when you drive it (gingerly while its cold), you put enough load on the engine to generate some heat. On my fiance's bimmer (gas), idling for 5 minutes will drive the water temp almost halfway to normal operating temp.
Take for example a diesel that has been "cold soaked"...sitting in cold temps overnight. Crank it and let it idle. You will find it will almost never get the water temp gauge to budge. On the other hand, when you drive it (gingerly while its cold), you put enough load on the engine to generate some heat. On my fiance's bimmer (gas), idling for 5 minutes will drive the water temp almost halfway to normal operating temp.
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Joined: Sep 2004
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From: Virginia
2005 Carlsson CD32 E320 CDI Inline-6
Diesels are happy at idle...
Less heat generated at idle.
As Prometheus said, it can actually be tough to maintain temp at idle.
Not to mention, gas engines must have correct about 14.7 +/- :1 stoichiometry, diesels only need more fuel when more power is called up.
As Prometheus said, it can actually be tough to maintain temp at idle.
Not to mention, gas engines must have correct about 14.7 +/- :1 stoichiometry, diesels only need more fuel when more power is called up.
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From: FL410
'05 E320 CDI, '08 BMW X5 4.8i, '11 Duramax 2500HD
I find that if I let my CDI "warn up" for about 2 min before I drive off, then drive very gentle (1200 rpm or less) for about 2 min, it's nice an warm for normal driving.. if however I just let it idle from cold, it rarely gets to warm in time.. and this is with OAT's of about 70 degrees F.
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From: Portugal
'13 E500 Coupe AMG// '17 E350d // '24 E53 AMG
Normal temp (water) in diesel engines never passes the 90ºC even driving hard, for smooth drive 80ºC while a petrol engine is allways at 80-90ºC and in hard driving it exceeds the 90ºC and reaches 100ºC or more 
I'm talking for all the diesels I had and have...
What I been told is, the perfect working temperature for a diesel engine is 90ºC, thats why all brands diesel engine work at that temperature.
I'm talking for all the diesels I had and have...
What I been told is, the perfect working temperature for a diesel engine is 90ºC, thats why all brands diesel engine work at that temperature.
Last edited by gaiex; Jan 18, 2006 at 07:38 PM.
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Joined: Oct 2005
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From: Loudoun County, VA
W211 CDI, W203, 03 Dodge CTD
Originally Posted by gaiex
What I been told is, the perfect working temperature for a diesel engine is 90ºC, thats why all brands diesel engine work at that temperature.
You can push a diesel to run hotter than 100ºC...put a real load on it when its hot outside.
The perfect ambient air temp for a diesel (at least a turbocharged one) is ~16.4ºC. We've done quantitative engine dyno tests to confirm this.
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From: Portugal
'13 E500 Coupe AMG// '17 E350d // '24 E53 AMG
Originally Posted by prometheus
You can push a diesel to run hotter than 100ºC...put a real load on it when its hot outside.
I never saw a car working at 100ºC, just my old volvo 440 turbo, even my jag don't work that hot, at least in many days of the year :p
