Disco 2 Function of glow plugs

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Bobdog

Bobdog

Well-Known Member
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Location
Horsham West Sussex
What is the function of glow plugs beyond cold starting? Obviously they are to pre-heat the diesel to aid starting in cold weather. Other than that, i.e. in summer for example, do they have any effect on the general running of the engine? I only ask as whilst poking about in the engine bay, I noticed that at least 2 glow plug leads have perished/cracked and have fallen off. I've done a Google search and apparently on some engines, it can result in engine management issues and poor running in general. From what I've previously read, the general consensus is that a td5 doesn't really need them unless it's really cold.
Any thoughts?...
 
Bobdog said:
From what I've previously read, the general consensus is that a td5 doesn't really need them unless it's really cold.
Any thoughts?...
Click to expand...
IMO You better read the workshop manual rather than believe the general consensus of some guys who think they know but never studied deeply the subject... on the Td5 they are important and the temp limit when they are not needed at all is +20*C cos the post heat operation stops when the ECT(engine coolant temp) sensor input is there, it's not written in any document, i made various tests to acknowledge this result, beside the cold start they will protect the engine from carbon deposits due to the post -heat operation which continues after the engine was started, this can last up to 5 minutes when it's cold but it happens even when it's +10*C untill the ECT input gets to 20*C.... here's the relevant part from the WSM(that should be the general consensus IMO):


Td5 - ENGINE MANAGEMENT - 18-1-30 DESCRIPTION AND OPERATION
....................
The 4 glow plugs are located in the engine block on the inlet side, in cylinder 1 to 4. Cylinder 5 has no glow plug. The

glow plugs are a vital part of the engine starting strategy.

The purpose of the glow plugs is:

l Assist cold engine start.

l Reduce exhaust emissions at low engine load/speed.
.............
Pre-heat is the length of time the glow plugs operate prior to engine cranking. The ECM controls the pre-heat time of

the glow plugs based on battery voltage and coolant temperature information via the glow plug relay.

Post-heat is the length of time the glow plugs operate after the engine starts. The ECM controls the post-heat time

based on ECT information. If the ECT fails the ECM will operate pre/ post-heat time strategies with default values

from its memory. The engine will be difficult to start.
.............
In the event of a glow plug failure any of the following symptoms may be observed:

l Difficult starting.

l Excessive smoke emissions after engine start.
 
Last edited:
sierrafery said:
IMO You better read the workshop manual rather than believe the general consensus of some guys who think they know but never studied deeply the subject... on the Td5 they are important and the temp limit when they are not needed at all is +20*C cos the post heat operation stops when the ECT(engine coolant temp) sensor input is there, it's not written in any document, i made various tests to acknowledge this result, beside the cold start they will protect the engine from carbon deposits due to the post -heat operation which continues after the engine was started, this can last up to 5 minutes when it's cold but it happens even when it's +10*C untill the ECT input gets to 20*C.... here's the relevant part from the WSM(that should be the general consensus IMO):


Td5 - ENGINE MANAGEMENT - 18-1-30 DESCRIPTION AND OPERATION
....................
The 4 glow plugs are located in the engine block on the inlet side, in cylinder 1 to 4. Cylinder 5 has no glow plug. The

glow plugs are a vital part of the engine starting strategy.

The purpose of the glow plugs is:

l Assist cold engine start.

l Reduce exhaust emissions at low engine load/speed.
.............
Pre-heat is the length of time the glow plugs operate prior to engine cranking. The ECM controls the pre-heat time of

the glow plugs based on battery voltage and coolant temperature information via the glow plug relay.

Post-heat is the length of time the glow plugs operate after the engine starts. The ECM controls the post-heat time

based on ECT information. If the ECT fails the ECM will operate pre/ post-heat time strategies with default values

from its memory. The engine will be difficult to start.
.............
In the event of a glow plug failure any of the following symptoms may be observed:

l Difficult starting.

l Excessive smoke emissions after engine start.
Click to expand...
Well, even with dead glow plugs, mine starts first turn of the key. Emissions in the first 5 minutes? Pfft... it's not even warmed up....
My real question is, does it affect the running of the engine once it's at operating temperature?
 
Bobdog said:
My real question is, does it affect the running of the engine once it's at operating temperature?
Click to expand...
Did you read what you quoted? :confused: ... cos it's obvious that the glow plugs have nothing to do above 20*C while the operating temp is around 90 ... if you'll have the chance to watch inside some engines compare a Td5 which was used without functional glow plugs with one which had them working all the time... or see the difference between the 5'th cylinder and the others on one with working glow plugs then you'll understand... if you insist to fill your engine with deposits it's your choice, if you want to really know rather than suppose what they do read this https://www.techtips.ie/BERU/all-about-glow-plugs.pdf but if it's too boring see a difference in the pic at page 8 what post-heat operation can do ... i care about what my engine does untill it reaches the operating temp too :cool:
 
sierrafery said:
Did you read what you quoted? :confused: ... cos it's obvious that the glow plugs have nothing to do above 20*C while the operating temp is around 90 ... if you'll have the chance to watch inside some engines compare a Td5 which was used without functional glow plugs with one which had them working all the time... or see the difference between the 5'th cylinder and the others on one with working glow plugs then you'll understand... if you insist to fill your engine with deposits it's your choice, if you want to really know rather than suppose what they do read this https://www.techtips.ie/BERU/all-about-glow-plugs.pdf but if it's too boring see a difference in the pic at page 8 what post-heat operation can do ... i care about what my engine does untill it reaches the operating temp too :cool:
Click to expand...

Chill out mate...:cool::rolleyes:
It was just a casual question, I will fix them.
 
Mine has always started in all weathers without using the glow plugs at all. And it runs as sweet as a nut after 170,000 miles..... No visible smoke on starting and as long as I leave it running for 20 seconds to get the oil round the auto box none on take-off either.

Just my experience not any sort of controlled trial.....
 
Bobdog said:
What is the function of glow plugs beyond cold starting? Obviously they are to pre-heat the diesel to aid starting in cold weather. Other than that, i.e. in summer for example, do they have any effect on the general running of the engine? I only ask as whilst poking about in the engine bay, I noticed that at least 2 glow plug leads have perished/cracked and have fallen off. I've done a Google search and apparently on some engines, it can result in engine management issues and poor running in general. From what I've previously read, the general consensus is that a td5 doesn't really need them unless it's really cold.
Any thoughts?...
Click to expand...

Good job you found the problem before it became one ... in the winter it's the last thing you need ...
 
neilly said:
I would guess, he gets in and turns the key to start the engine straight away without waiting for the glow plug light to extinguish.

Cheers
Click to expand...
Which means that only the pre-heat operation is not used, and that's less important... IMO the really important thing is the post-heat operation(which in most cases is ignored) and that happens regardless of "using" them or not so if the plugs are in good working order even neglecting the warning lamp the post-heat will have it's effect hence the lack of smoke... the effect of post-heat opertion is clearely visible and explained at page 8 of that document i linked
 
neilly said:
I would guess, he gets in and turns the key to start the engine straight away without waiting for the glow plug light to extinguish.

Cheers
Click to expand...

Yes indeedy. I assume the post-start operation is as Fery describes. I can't find that description in RAVE - could you provide a link Fery?
 
I watched the glow plug operation on my nanocom last winter on a cold start, they were showing on for a good few minutes before the management switched them off
 
I might try that some time. Presumably it only tells you they're switched on, not that they're actually doing anything?
 
Last edited:
Thanks Fery, I'd seen that but it's a fairly superficial explanation (on p424 of my copy of RAVE BTW!). The document you linked to earlier was a generic one describing all sorts of glowplugs and I wasn't sure which type we have fitted or how applicable it was.
 
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