VAG 2.0 TDI - Glow plug diagnosis help

[25th QV]

2017 2.0 TDI Skoda Superb 150PS DSG.
45k miles, full dealer history.
Recently had a bad glow plug replaced when in for the major and DSG service.
Working fine for a few months after.
Suddenly it just cranks and cranks for ages before eventually catching, irrespective of how many times I cycle the glow plug light.
I've had diesels for years and I'm familiar with the gradual increase in starting issues as one glow plug after another fail until you're struggling with one or two.
This feels like there are none working at all, which leads me to believe that it's unlikely that all four glow plugs have suddenly expired at exactly the same time and that it's more likely to be a link in the command chain that controls the?
I'm assuming it's not as simple as a bad relay somewhere?
Any ideas on how best to diagnose?
Or could it be something else unrelated that just happens to present exactly like total glow plug failure?
No warning lights, no other running issues.
Thanks in advance.

[paul_c123]

Sounds like low fuel pressure.

[Belle427]

Always beneficial to change them in a set if they are known to be original.
A full code scan may show up some clues.

[25th QV]

paul_c123 said:

Sounds like low fuel pressure.

Would that not present during normal running?

[25th QV]

Belle427 said:

Always beneficial to change them in a set if they are known to be original.
A full code scan may show up some clues.

There are no warning lights, but I'll dig out my old VAG-COM cable and see if it picks anything up.

[paul_c123]

25th QV said:

Would that not present during normal running?

Not necessarily.

[stevieturbo]

Sounds nothing like glow plugs.

Has the ecu been dodgy remapped to hide fault codes ?

Interrogate the ecu to see if there is anything odd.

Crank trigger etc, fuel pressure.

[hellorent]

Assuming your in this country I'd be surprised if it's a glow plug issue in this weather

[Chris32345]

hellorent said:

Assuming your in this country I'd be surprised if it's a glow plug issue in this weather

It shouldn't be a issue almost anywhere in the northern hemisphere ATM
Doubt they are even warming up in the current temps

[stevieturbo]

Chris32345 said:

It shouldn't be a issue almost anywhere in the northern hemisphere ATM
Doubt they are even warming up in the current temps

modern engines, the glow plugs are used very often and not just cold starts. They can run for a few minutes at times at varying power levels. They're as much a part of the emissions BS as any other on the engines.

[25th QV]

Owned the car from new, no modifications.
In the UK.
Cranks for about 10-15 seconds before catching.
Changed overnight.
Previously fired immediately without issue (once bad glow plug was replaced).

[TwinKam]

My money's on low rail pressure...

[Belle427]

May be worth checking the cranking speed is what it should be, battery may be a bit tired too.

[E-bmw]

25th QV said:

Owned the car from new, no modifications.
In the UK.
Cranks for about 10-15 seconds before catching.
Changed overnight.
Previously fired immediately without issue (once bad glow plug was replaced).

Overnight when?

[stevemcs]

It won’t be glowplugs, but you you do need to scan it. There is a chance the timing has jumped on one of these.

[25th QV]

Belle427 said:

May be worth checking the cranking speed is what it should be, battery may be a bit tired too.

Battery is recent and it cranks (by ear) at the same speed as usual. If there is a difference, it's so small I can't tell.

[25th QV]

E-bmw said:

Overnight when?

About a week ago.

[25th QV]

stevemcs said:

It won t be glowplugs, but you you do need to scan it. There is a chance the timing has jumped on one of these.

I only thought it might be GPs from previous experience of them going bad one by one on other diesels I've had. By the time I've been down to the last one working, it wasn't a million mile away from what I have now, hence I just thought if a single point of failure controls them all and had failed, it might present in a similar way.

Just found out VAG-COM doesn't support Mac, so I'll have to see if I can borrow a windows laptop from someone.

I'll report findings once I can plug it in.

Strange there's no warning lights.

[E-bmw]

25th QV said:

stevemcs said:

It won t be glowplugs, but you you do need to scan it. There is a chance the timing has jumped on one of these.

I only thought it might be GPs from previous experience of them going bad one by one on other diesels I've had. By the time I've been down to the last one working, it wasn't a million mile away from what I have now, hence I just thought if a single point of failure controls them all and had failed, it might present in a similar way.

Just found out VAG-COM doesn't support Mac, so I'll have to see if I can borrow a windows laptop from someone.

I'll report findings once I can plug it in.

Strange there's no warning lights.

It isn't strange there are no warning lights at all.

It is far more likely to either be leaking injectors or fuel "fall-back".

When you get VAGCOM up & running check fuel rail pressure when you are trying.

Have you checked your oil level? As if it is injector/s leaking the fuel will get past the piston rings & into the oil.

[stevemcs]

The glowplug failure will only cause an issue in really cold weather or when trying to regen, usually glowplug 1 in these.

If it didn't do it before the service then there is a chance it could be the fuel filter, i belive you can bleed them through vag com, but normally once they are bled thats it. EGR valves can also stop them from starting. If the timing has moved you would normally see crank/cam corrilation and they don't put the engine light on.