TDI engine doesn't start when cold

Hi.

I am looking for a specialist who is familiar with TDI engines, specifically those with BWN/125kW. I have an article detailing the problem I'm experiencing.
extensively studied.

I'm just wondering now what the difference is if I...

A: -5 degrees Celsius. The engine won't start on the first attempt; it just cranks and doesn't catch. However, it starts on the second try and runs perfectly all day long.

B: -5 degrees Celsius, turned on the pre-heating system 10 minutes prior, engine starts immediately and runs perfectly all day.



The following things were done.

Fuel filter.
Hoses to and from the filter, inlet/outlet, are new.
New tandem pump.
PD elements cleaned / Seal kit.
Timing belt and all pulleys replaced and adjusted.
Fuel temperature sensor, new.
Air mass sensor, new.
Camshaft sensor (new).
No errors found in the control unit.

I can't wrap my head around it.

Best regards,
Jens

Have you checked the glow plugs to see if they are working? At engine temperatures above 0°C (which is the case when using the auxiliary heater, and usually also if the engine has already been running that day), a TDI will start even without preheating - but it won't start at -5°C.

Hello Jens,
You guys have already done quite a few pointless exchange activities.
Apart from the spark plugs that were already mentioned, what about...?
to the coolant temperature sensor?
Simple test:
If it's cold, disconnect the sensor plug and then start the engine.
Upon immediate success -> Re-examine the donor very carefully during the diagnosis.
(it likely doesn't provide the temperature needed for preheating).
or exchange (<<20€ if you are able to do it yourself).
If it still doesn't start, check the glow plugs and their control system.

Best regards,
Micha

Thank you for your suggestions.

I completely forgot that when I first got the car, I immediately replaced the battery and glow plugs; that was essentially the first thing I did. I'll test the coolant sensor tomorrow.
The tandem pump, filter, and PD cleaning were all due to a faulty tandem pump caused by oil in the diesel filter.

I have one more question. How can I check the glow plug function without removing them, using a clamp ammeter?

Clamp ammeters are a way to use an adapter with a...
old 55W headlight bulb (which is then connected in series with the filament being tested).
It also works quite well (and is visible from the inside).
To force the preheating process, removing the temperature sensor can also help.
Then, the MSTG assumes that it is cold and needs to be preheated.

Best regards,
Micha

[edit] I've renamed the topic. This might help the search function return more relevant results. [/edit]

How many amps should I expect? I've read that it's 15A per candle, so it should draw around 60A.

Thank you.

The size is appropriate (I believe the candles had a cold resistance of 0.8 ohms).
If the value is lower (0 to 75%), measure each individual resistor with an ohmmeter or...
Test with a test lamp.

Hi!
I don't know the exact values for the TDI models.
But in the past, with the diesel engines of the Volkswagen Golf 1 and Golf 2, there were 4 glow plugs, each drawing 12 amps.
Initially, using a clamp meter on cold glow plugs showed readings of around 60-64 amps. However, after the glow plugs had been activated for a few seconds, the reading quickly dropped to the expected 48 amps and remained stable thereafter.

Based on the error message I'm seeing, it might be related to the glow plugs in some way.

You could also check if there's a fuse for it. In older cars, this was often a small but common cause of problems.

Hi there,

above all, measure accurately, don't just visually inspect.

(This is a URL pointing to a forum thread.)

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Best regards,
Adrian.

masterjcl wrote:

-5 degrees Celsius. The engine won't start; it doesn't even crank once before stopping,

.
Can you provide a more precise definition of what "it breaks down" means in this context? Does the starter motor fail (e.g., due to a dead battery), or does it continue to turn? Does it turn fast enough?

masterjcl wrote:

When it reaches the 2nd setting, it turns on and runs perfectly all day long.

Ich gehe davon aus, dass der Wagen eine el. Vorförderpumpe hat, die bei jedem Einschalten der Zündung kurz anläuft. Ri"Is that correct?"
Could it possibly help to crank the engine twice and then start it? (To bleed air from the system).

Greetings.
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It refers to a sudden and strong gust of wind.

Check the idle speed with VCDS! I just experienced this myself. It was a 125 kW BMN engine. With the injectors disconnected, my idle speed was around 190 RPM. I installed the 2kW Valeo heater. Now the idle speed is at 330 RPM. Please check that as well.

Sure, here is the translation of the text from German to English:

"Best regards," or "Sincerely,"