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?

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.

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.

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.

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.

Not all of them. I think 90ºC is a tad hot, though certainly not detrimental. One can swap out thermostats to open at a cooler temp. My Dodge runs a steady 82ºC and runs just fine. :v

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.

At least mines never worked at that temp, even after +-2h running at full throttle (+-400km) in summer days with 30-35ºC :v

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