What's the consequence of starting a diesel engine before the lights go out?

Not that I do, just wondering why and what if you don’t?

Over time you will damage the glow plugs and they will need replacing.

Once in a while, not really an issue.

Just took my car for a wash & the polish guy went to move it & I was ugh ugh ugh it’s a diesel, ugh too late.

A big white smoke screen when it does fire up.

Mine takes care of the delay its self, turn the key all the way and if it needs preheating, it pauses to do so before firing. 11 year old Audi, assumed they all did so these days??

Nothing – absolutely nothing. I have never waited for the light to go out in any diesel car i’ve had and never had any problems.

More pressing is the engine and coil light flashing on my Passat. It’s only a matter of time until the pound sign lights up. VW reassuringly expensive 🙁

As grum showed – certain nuclear holocaust accompanied by clowns. Though more likely is a sooty build up somewhere vague in your engine. However, if it’s a german diesel be prepared for a stern telling off on your third attempt at a premature start.

The pre-heat only helps it to start.. on very cold days it is necessary otherwise it will take a few rotations to get going.. Modern diesels are very clever engines… You do more damage to a diesel by not letting it tick over for 30 odd seconds when you have finished your journey, then switching it off.

the cool down only applies if it is a turbo diesel

i mentioned this on here before and folk tried to debunk it .

posh turbo diesels have auto cool down after you take the key out.

ill continue to let mine cool down , much as i let it warm up before i boot it on the turbo

Could wait all day for glow plugs on my Transit, wouldn’t do any good as the fusible link in the wire blew about 5 years ago 🙂

Never stopped it starting in even the coldest weather.

For any start typically above about -5degC, assuming the engine is in good condition (i.e. you have decent compression, and injectors that are not competely knackered), then other than some slight combustion roughness/instability and an increased level of particulate (unburnt fuel / smoke) emissions there is no result of not allowing the full pre-heat time to expire before cranking the engine.

Old skool mechanically injected engines that have much poorer control of their fuel injection system need the pre-heating to get good combustion events when the combustion chamber is cold. A DI engine ignites it’s fuel purely by the heat resulting from the compression of the air, and with a cold chamber, more of this heat is lost to the chamber walls, and so the CC temperature at the point of injection is significantly lower, hence the slower/dirtier burn.

No mechical damage or long term effects are caused to your engine however by not allowing the full pre heat cycle to occur.

Under ~-5degC, you will start to need the pre-heat to get the engine to fire, by the time you’re down to ~25degC you’ll need there input 100% (and winter grade diesel!). Obviously, in the UK, that isn’t a very common starting temperature!

If your engine is worn (low compression, worn injector nozzles) then you may require the services of the heater plugs at higher temps to get a “clean” start, as the end of compression temp will be lower and the fuel mass less well atomised (and hence more difficult to ingnite)

Modern, common rail diesels, manage all this for you, and some, that use keyless or push button start will pre-heat the engine before you actually go to press the start button so avoid any wait time on behalf of the operator etc

what he said
Not much really but it is better to wait for them to go off but it will start unless very cold

the cool down only applies if it is a turbo diesel
i mentioned this on here before and folk tried to debunk it .
posh turbo diesels have auto cool down after you take the key out.
ill continue to let mine cool down , much as i let it warm up before i boot it on the turbo

I didn’t just “try” to debunk it 😉

I just showed why it was a modern myth.

And you chose to ignore it and carry on as you were 😀

Modern ?

Ok

You won’t damage glow plugs, as they are often still powered even after the light has gone off in the vehicle.

It’s interesting with diagnostics as some vehicles actually let you see under live data how long the glow plugs are going to be on for, the best for this is the VAG motors imho.

Another thing I see regularly is renaults with the silly thin glow plugs, it’s not uncommon to find someone has previously snapped them off (they are very prone to it so just tease them out gently) and it will be running fine, its better if it’s cylinder 2 or 3 as the heat soak from the neighbours on either side soon warms it to get it running right.

Modern ?
Ok

Turbos have not needed cool down time for over 30 years.

That’s what I meant when I said “modern”

I once had a 1996 Polo Diesel that needed the Glow Plugs changing and never again will I attempt any job that may look fiddly.

I’m sure they built the engine around them glow plugs. Many hours of dropping things and grazed knuckles.

My 11 plate Fiesta I don’t even notice the light no matter how cold it is.