Glow plugs problem?

[paulfoel]

When I start my C250TD I wait for glow plug light to go out and then it starts OK. Trouble is the glow plug light comes back on and stays on for a few mins. This happens every time I start the car.

Also, when the engine is cold, even though it starts it seems to 'miss' for a few minutes until its warm.

I'm wondering whether this is a dodgy glow plug problem? If so, are they as easy to swap as spark plugs in a petrol engine?

Whilst I'm on the subject, how do glow plugs works in a diesel engine? I know its not quite like spark plugs which ignite the fuel in a petrol engine. Does'nt diesel ignite itself or soemthing? Are glow plugs just to get the diesel engine going or are the used all the time during running?

 

[jimmy]

You have at least 1 faulty glow plug out of the five, the light coming on indicates there is a problem, also the rough running at first is a sure sign.

The glow plugs are used to preheat the chambers, incoming air and fuel. They glow cherry red within a second or two and stay on after the car has started for a short time too.

They are not difficult to change themselves but the rear one is particularly difficult to get at to change on the C250TD. You have to remove the whole inlet manifold, whilst you are at it replace the O-rings on the inlet manifold and you may as well change all five glow plugs, one might be faulty now so the others may be on their last legs too.

 

[glojo]

Whilst I'm on the subject, how do glow plugs works in a diesel engine? I know its not quite like spark plugs which ignite the fuel in a petrol engine. Does'nt diesel ignite itself or soemthing? Are glow plugs just to get the diesel engine going or are the used all the time during running?

Hi Paul,
Jimmy has answered most of your questions and Dieselman will correct me if I put my foot in it.

Unlike petrol engines, diesels do not need an electrical spark to ignite the fuel.

The diesel is ignited solely through heat from the compressed air that is in the cylinder. Basically and ignoring the two stroke/4 stroke issue, when the piston goes down it sucks in air, roughly at the bottom of the stroke the valves close. The piston comes up, the air gets squashed, and as it gets squeezed so it gets hot. Just before the top of the stroke fuel is squirted in at extremely high pressure. This fuel is immediately ignited solely by the heat of this compressed air.

If you put your finger over a bicycle pump and try to stop the air from coming out as you pump, you will notice your finger getting hot! This is because you have compressed the air within the pump.

The glow plug helps warm-up the cylinder and hence the air, making it easier to ignite when the engine is cold. Once running the engine quite obviously generates its own heat.

In days of old you could buy 'ether' (smelly volotile chemical) based products that you would squirt through the air filter!!

I laughed when someone mentioned it is more dangerous to put diesel into a petrol engine??? As you can see putting petrol into a diesel engine will\can prove very costly. Putting diesel into a petrol is inconvienant and will need pumping out?

Good luck with getting your plugs replaced,
John

 

[Dieselman]

Hi Paul,
Basically and ignoring the two stroke/4 stroke issue, when the piston goes down it sucks in air, roughly at the bottom of the stroke the valves close.

Why ignore 2 stoke, it works exactly the same it just uses ports not valves.

 

[glojo]

Why ignore 2 stoke, it works exactly the same it just uses ports not valves.

Hi Dieselman,
I simply did not want to get into the confusion of explaining the firing every other stroke on the 4 stroke as opposed to every stroke on the two stroke.

I always remember my instructor:

Suck, squeeze, bang, blow. He believed in keeping it simple for simple students (me)

Beats: Induction, compression, power, exhaust (could never remember that)

Nice to hear from you,
Regards,
John

 

[paulfoel]

The glow plug helps warm-up the cylinder and hence the air, making it easier to ignite when the engine is cold. Once running the engine quite obviously generates its own heat.

So am I right in saying the glow plugs are only used to warm up the cylinder when initially starting?

Once the engine is running on its own, do they just switch off and sit there then?

 

[paulfoel]

You have at least 1 faulty glow plug out of the five, the light coming on indicates there is a problem, also the rough running at first is a sure sign.

The glow plugs are used to preheat the chambers, incoming air and fuel. They glow cherry red within a second or two and stay on after the car has started for a short time too.

They are not difficult to change themselves but the rear one is particularly difficult to get at to change on the C250TD. You have to remove the whole inlet manifold, whilst you are at it replace the O-rings on the inlet manifold and you may as well change all five glow plugs, one might be faulty now so the others may be on their last legs too.

Sorry for the ignorance, but how difficult is it to remove the inlet manifold ?

 

[glojo]

So am I right in saying the glow plugs are only used to warm up the cylinder when initially starting?

Once the engine is running on its own, do they just switch off and sit there then?

Sorry,
EDIT: See Satch's very informative link a couple of messages down.


It sounds like you have a problem that needs sorting.

Regards,
John

 

[jimmy]

This is getting more confusing than need be!

When the glow plug light comes back on after starting, it is a warning that one or more glow plugs is not working. The car knows this by the current that is being drawn from the battery.

The time that the glow plugs operate is determined by the coolant temperature, the colder it is the longer they will operate before you should try and start the engine. A hot engine will require less pre-heating.

On some engines the glow plugs continue to operate for 30 seconds or so after the engine is started even if the light has gone out, I am not sure about MB's!

The inlet manifold is the big plastic thing on the RH side of the engine bay with 10 pipes going to the engine block, held on with Torq screws. The glow plugs are under this, the front 2 or 3 are accessible without removing the manifold, the back 2 are not easy to get to. The whole job is easier to remove the manifold and replace all 5 glow plugs and the 10 O-rings too.

This has been covered before, do a search. The job is easy to do, just time consuming. It is covered in the Haynes manual.

 

[tim.100]

So am I right in saying the glow plugs are only used to warm up the cylinder when initially starting?

Once the engine is running on its own, do they just switch off and sit there then?

You are right - once the engine is up and running the glow plugs are not needed and all power is cut.

There is a minor exception - on some diesels (even my Land Rover 200 Tdi) the glow plug controller keeps power going to the plugs for up to 30 seconds after start-up just to help things along.

I think Jimmy has photographed the inlet manifold removal; results are somewhere on this forum. Having had a look at mine, it looks like a lengthy process, but not complex nor requiring special tools.

Glow plugs seems to be a known weak point on this engine, with a life limited to 50k average.

Regards,

Tim

 

[glojo]

I totally endorse everything that jimmy has said and if I have caused confusion then I apologise, blame my fuzzy brain.

John

 

[Satch]

There is a minor exception - on some diesels (even my Land Rover 200 Tdi) the glow plug controller keeps power going to the plugs for up to 30 seconds after start-up just to help things along.

On certain EU4 compliant engines the plugs run on for up to 15 minutes! Bosch & others have developed entire new ranges of glow plugs to deal with this but I see the glowplug rapidly becoming a routine service item.

 

[glojo]

On certain EU4 compliant engines the plugs run on for up to 15 minutes! Bosch & others have developed entire new ranges of glow plugs to deal with this but I see the glowplug rapidly becoming a routine service item.

Hi Satch,
I am terribly self conscious of saying the wrong thing, could you please direct me towards this information so I could 're-program' my 'fuzzy' bonce.

I was under the impression that the warning light was on the same circuit as the power to the glow plug.

Thanks very much,
John the uneducated

 

[Satch]

The new style plugs are linked to an electronic management system which runs the plug at above the nominal rated voltage until it (very quickly) reaches the desired temperature, at which point it steps down to nominal and presumably extinguishes the warning light to make you feel better.

http://www.boschautoparts.co.uk/pcGlow13.asp?c=2&d=1

My Land Cruiser has an EU4 engine and is fitted with these things, which is one reason why it has two immense batteries as standard

 

[glojo]

The new style plugs are linked to an electronic management system which runs the plug at above the nominal rated voltage until it (very quickly) reaches the desired temperature, at which point it steps down to nominal and presumably extinguishes the warning light to make you feel better.

http://www.boschautoparts.co.uk/pcGlow13.asp?c=2&d=1

My Land Cruiser has an EU4 engine and is fitted with these things, which is one reason why it has two immense batteries as standard

Hi Satch,
Thank you very much indeed for posting a most informative link. I had been searching high and low for this type of information. It looks like these particular glow plugs heat up to a very high temperature 1300 c, then drops 'just' 200 c, and can remain on for up to 15 minutes. Do you think that when it is at its optimum temperature the light remains on, then the light goes out, but the glow plugs will remain working at the more 'reasonable' temperature.

I am going to have to find out what 1300 c relates to , in regard to heat from a type of source.

Thanks again,
John the Inquisitive

 

[GazCaff]

When I start my C250TD I wait for glow plug light to go out and then it starts OK. Trouble is the glow plug light comes back on and stays on for a few mins. This happens every time I start the car.

Also, when the engine is cold, even though it starts it seems to 'miss' for a few minutes until its warm.

I'm wondering whether this is a dodgy glow plug problem? If so, are they as easy to swap as spark plugs in a petrol engine?

Whilst I'm on the subject, how do glow plugs works in a diesel engine? I know its not quite like spark plugs which ignite the fuel in a petrol engine. Does'nt diesel ignite itself or soemthing? Are glow plugs just to get the diesel engine going or are the used all the time during running?

Thread necromancy I know, but......




Guess what mine started doing today?

 

[Dieselman]

Thread necromancy I know, but......




Guess what mine started doing today?

That's because they have a "it's cold and 'orrible sensor" so the plugs know just when is the worst time to blow.

 

[GazCaff]

That's because they have a "it's cold and 'orrible sensor" so the plugs know just when is the worst time to blow.

Ah, that'll be the "Murphy fuse", otherwise referred to as the "sods switch".

 

[fredfloggle]

Why ignore 2 stoke, it works exactly the same it just uses ports not valves.

Not necessarily - many two stroke diesels use valves and sometimes a combination of supercharger for starting and providing scavenging air , and a turbo charger for high speed running .

 

[Dieselman]

Not necessarily - many two stroke diesels use valves and sometimes a combination of supercharger for starting and providing scavenging air , and a turbo charger for high speed running .

I know all about 2 stroke diesels and how to charge the cylinder, and that they only use exhaust valves and ports for inlet, but wasn't the original post about how the fuel was ignited.
The topic being discussed was the heat generated by compression as the igniter as opposed to a spark plug.

Question: How did 2 stoke diesels without an external charger charge the cylinders?