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First of all let me say that I already read this question and its answer, so it's clear to me why diesel-powered cars need a warm-up time.

My question is: do modern cars need it too? I have VW Golf VI (year 2009) and I normally leave the system on for a couple of seconds before starting the engine... but am I being too cautious, or does the car really need it?

  • Modern diesels are much quicker now, but they still have that annoying lag haha. – Dan Jul 15 '15 at 7:28
  • Dan could you in case put that as an answer and eventually give some references? – Noldor130884 Jul 15 '15 at 8:30
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    Having driven more diesels than not for some years, the sequence glow plugs, seat belt, start has become automatic, and putting on the seat belt provides enough time. My Transit van lights the glow plugs light for a few seconds every time (unlike my Peugeot with an HDi engine) but actually on a warm day or if it's been run within the last couple of hours you can just turn the key all the way on the Transit as well. – Chris H Jul 15 '15 at 10:43
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I have both a 2004 Bora TDI (Jetta Mk4 in the US) and a 2012 Mk6 TDI Golf. When either of these cars requires it's warm up system, it will automatically switch it on for the time it needs. This is signified by the glow plug light illumination on the dashboard:

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Once it's up to the temperature it requires, the light goes out and the car can be started.

I live in North East England and see temperatures ranging from 25 degrees celsius in the summer to -8 degrees celsius in winter and only ever see the glow plug light very occasionally on the coldest of mornings.

  • So you are basically saying that "normal" outside temperature is enough to get the engine correctly started. I will check on my dashboard. It would be really nice to know an average of the "lag" needed by a random modern car. – Noldor130884 Jul 15 '15 at 9:02
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    For both of my diesel VW's, in normal temperatures they start on the first turn of the key with no warm up time. When it's especially cold, they typically operate their warm up circuits for around 5 or 10 seconds only. – Steve Matthews Jul 15 '15 at 9:16
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    VW use different types of plugs in different regions. My TDI in Europe uses metal plugs that get to temp almost instantly, but my TDI in America uses ceramic plugs that require a few seconds even in summer. – Mr. Mascaro Jul 15 '15 at 13:13
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    In addition to the temperature, glow plug warm-up time depends on the atmospheric pressure. My 2001 Jetta requires little to none during the summer at home (about 2000 feet elevation), but In Utah, around 4500 feet, it needs half a minute or so, even if the weather is just as warm. Otherwise it will stall as soon as I let out the clutch. I assume that this is because diesels use compression for ignition, and the lower air pressure reduces the effective compression. – andybalholm Jul 15 '15 at 16:44
  • Any engine struggles at altitude because there is less oxygen which is required for the actual burn part of the combustion or compression cycle. As stated, waiting until the engine is up to temperature will help but if you had access to a dynomometer (rolling road) at sea level and altitude you would get a lower power reading at altitude. Listen to Pikes Peak Hillclimb cars as they start to near the summit, you hear them starting to struggle and gasp for air as the power starts to fall away. – Steve Matthews Jul 16 '15 at 8:26
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Diesels have never absolutely needed it - even on older diesels, they'd still usually start without waiting for the glow plugs. However it does make it very much easier for the engine, so starting will be easier in adverse conditions (cold/altitude) where otherwise the engine might not catch. Also you'll find the engine runs rough for a few seconds until the combustion process has got everything up to temperature, which isn't good for the engine long-term and produces a lot more emissions.

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