Robbie wrote:

I don't doubt that doing it hot is the correct way and most service manuals state this as a requirement. For larger gauge glow plugs this does work and whilst I am sure it can only help with the fine plugs fitted to this engine they are just not strong enough to be reliably removed.

You may be right. On my friend's workshop came preponderantly big equipments with engines over 6000 cmc.

But, tbh, why people bother with changing glow plugs?
Normally CR engines can start OK up until towards near zero temperatures without glow plugs.
Indeed, the engine will not be so "green", but it will start.
And below zero, as we all have it, a blip on the webasto and in a few minutes the engine is again on the easy start zone.
So in a way you don't have to care too much on the glow plugs. Or at best, on a single one, webasto's. Which is much accessible and cheaper to fix.
I didn't make a calculus, but even the final energetic consumption during entire startup process can lean towards usage of webasto.
This for the pragmatic, but for a passionate, a car in tip-top shape could justify the perfect functioning of glow plug system.

I may be wrong, I don't know how a D3/4 works without glow plugs, once started, but FL2 works perfectly without them, again, once started. And up until starting, FBH could be a good ally during cold starts.

Arif wrote:

I understand what you are saying but I think you'd feel the same if someone tried to rip you off for £4000

Gazellio @ Prestige Cars wrote:

Interesting bit in italics:

Some diesel engines contain a glow plug. When a diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire (think of the hot wires you see in a toaster) that heats the combustion chambers and raises the air temperature when the engine is cold so that the engine can start. According to Cley Brotherton, a Journeyman heavy equipment technician:

All functions in a modern engine are controlled by the ECM communicating with an elaborate set of sensors measuring everything from R.P.M. to engine coolant and oil temperatures and even engine position (i.e. T.D.C.). Glow plugs are rarely used today on larger engines. The ECM senses ambient air temperature and retards the timing of the engine in cold weather so the injector sprays the fuel at a later time. The air in the cylinder is compressed more, creating more heat, which aids in starting.

In theory, that is correct.

However, in practice...

In my previous employment, had a non starting D3, when it was cold. 60 seconds of cranking and it would fire up. Also had a slight misfire when cold. They said glow plugs, I said injectors.

As a test, they left the car overnight, and as luck would have it, a TDV6 RRS.

Their car was exactly the same, 60 seconds before it would run.
With the RRS, the car was unlocked, bonnet opened and glow plug wiring looms unplugged, simulating the same conditions as 6 inop glow plugs. It fired up as normal, albeit, a little lumpy.

Unfortunately, they took the car away, without it being repaired, so we wil never know if it was glow plugs or injectors

I tried the same with my D3, which I know needs 6 injectors, and it wouldn't start with the glow plugs disconnected... 6 injectors are much cheaper than snapped glow plugs

Make of that what you will

crwoody wrote:

I always thought that the fuel was sprayed over the heated tip of the glow plug to help atomise/vaporise it from a cold start.

It would take a lot for just the plug tip to heat the whole head or cylinder.

Perhaps I could aid the understanding a little.

The glow plugs on the TDV6 are not just there to aid starting as in almost all conditions the compression is enough to achieve ignition.

To burn at the correct rate and manner the fuel must be atomised and introduced at the right point in the cycle. Unfortunately when cold some of the atomised fuel re-condenses on the cylinder walls which reduces the effective charge. To compensate the engine adds additional fuel and adjusts the timing and profile of the fuel injection; this costs efficiency, increases emissions and produces rough running.

In an effort to increase efficiency the glow plugs provide additional energy to the charge of fuel (rather than the cylinder) which reduces the rate of condensation and extends the period where the fuel charge is willing to burn through the afterglow feature. As a result, less fuel is needed to produce the required charge. It also allows the timing to be set to a point closer to normal which further increases efficiency.

The contribution of the glowplugs (energy and timing) is controlled by the ECU with the prime source of data being taken from the engine coolant sensor. If your coolant is warm and the engine is still cold (due to the use of an aftermarket FBH trigger) or if the sensor is failing it can trick the glowplug system to the point that they provide little or no energy. As a result the engine can be a little rough after start. Of course, with an FBH pre-heat the coolant is warm and so the cylinders will warm up rapidly and normality will return. If the injector profile is incorrect due to worn injectors the ability of the glowplugs to sustain an efficient burn will be compromised. It all has to work together.

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