Why wait to turn a diesel on

obi_waynne

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Moderator
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1,157
Location
Deal, Kent UK
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A3 1.4 TFSI 150 COD
Why is it so important to wait for the coil light to go out when you turn on a diesel engine?

What would happen if you just started it like a petrol?
 

HDi fun

TC ModFather
Points
637
Location
Buckinghamshire UK
Car
Passat 2.0 TDi
My Peugeot was the same. The HDi engines don't need glowplugs to start unless it exceptionally cold (below minus ten degrees C kind of cold).

But the plugs do play a role in DPF regeneration
 

SonofKrypton

Wrench Pro
Points
48
Location
Derbyshire
Car
VW Golf Mk5 1.9TDi
On the cold mornings lately, I have noticed the coil light on my Golf (1.9 PD engine) takes a few seconds to go out. I also find the engine starts more smoothly on cold mornings if I turn the ignition off then back on again so the coil goes out for the 2nd time before I start the engine.
 

gladrags

Torque Junkie
Points
202
Location
UK
Car
Focus 2.0
I've heard one or two claims about modern diesels which start better if you turn on the key until the light goes off then turn the key off then back on again. Do that a couple of times and the engine starts more easily, presumably due to the heater plugs working better.

Thing that's always puzzled me about modern glow plugs is whether they just heat up the air in the combustion chamber to improve starting or whether the injectors spray [some] diesel over the red hot heater plug element to ignite the fuel.
 

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