my 0.02 jut my 0.02, others will have different 0.02...
This is a fairly polarizing topic that comes up regularly.
my 0.02, and I respect that others may have different 0.02-
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Although I don't have specific examples of why, I tend to agree it's probably not good to allow the TDI to idle for extended periods of time. The above replies explain that. Others will say that's garbage, idle for as long as you want.
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I am a fan of NOT shutting the engine off immediately when you stop- allow it to idle for 30 seconds or so, maybe a bit longer on a hot day. Why? Because it has a turbocharger. Part of the turbocharger sits in the exhaust stream, and gets very hot when the engine is working to produce power. (the engine produces relatively little excess heat while idling). The turbo has spinning parts, which are lubricated by oil fed from the engine. There is a pressurized oil line that goes from the engine to the turbo, and a little drain line that takes the oil out of the turbo and back into the engine oil pan. In addition to lubricating the turbo, the engine oil helps to cool the turbo bearings. So what you're doing when you idle for 30 seconds or so (bit longer on hot days), is allowing some time for the turbo to cool down, so that when shut off, the hot turbo doesn't cook the oil that happens to be left in it at the time. That's one of the reasons these cars call for using synthetic oil- because synthetic is capable of handling slightly higher temperatures than conventional oil without "coking". When I first bought my 02 jetta (new), this was recommended to me by a friend who had been a technical salesman for a big diesel engine manufacturer. I'm just coming up on 300K miles on original turbo. I have seen reports from others about high miles with no problems, and they don't "idle down".
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Still, I agree with the above- your BF should buy you the dinner of your choice.