My brother who lives in OH has a '13 Cayenne Diesel. The computer says he gets approx 29-33 mpg on the way to/from work. With the recent brutal cold temps at 0*f or lower, he said the computer is claiming his MPG is 21-24. His oil and water temp come up to normal within 5 miles of his 20 miles one way drive.

I'm guessing that its taking longer to warm up and using more diesel during the warmup.

Thanks for any thoughts!

 

It's normal to see diesel fuel mpg drop in cold weather, but that's a rather significant drop. The usual reason is the fuel rather than the vehicle. Without getting too complicated, diesel fuel refined for winter use delivers fewer mpg than does the summer formula.

Reason #2 is tire pressure. Unless he's replaced the air in his tires with nitrogen, the tire pressure will have dropped. The lower the pressure, the higher the rolling resistance. Might be a good idea for him to top up his tire pressures.

Reason #3 is because of the clean diesel technology. There's a diesel particulate filter (DPF) in his exhaust system, and it requires diesel fuel to work. It works on two cycles; automatic, and regenerative. Warm summer driving promotes more automatic filter cleaning - because the exhaust system simply runs hotter. Hotter exhaust helps keep the particulates from building up in the filter. Regenerative cleaning happens when the automatic cleaning can't keep up with particulate accumulation (ie: lots of short trips and/or city driving). Regenerative cleaning draws diesel fuel from the tank, mixes it with the AdBlue fluid under his spare tire, and gets injected into the filter to "burn" the filter clean. During summer the exhaust system gets up to temp faster, and runs comparatively hotter, requiring less diesel fuel from the tank to burn off excess particulates. Come winter, the exhaust system - being down by the cold road surface - naturally runs colder. Colder exhaust, more regenerative cleaning. Add to that the short (20 mile) trips, and you have a lot of diesel fuel going towards DPF cleaning, that would ordinarily go to the engine in the summer time.

//greg//

 

Thank you very much for your reply. I will pass this on to him.