T6.1 hot start issue.

[Jackmarkley24]

Hi guys, i have recently bought a 2021 110ps 5 speed, i have a problem where when the van is hot, very occasionally it tries to start, sounds horrible for a second and stops, after about 4 turns it will start. I had my mechanic look at it but nothing can be found obviously. It only happens every so often like once in every 4-6 weeks. I had a 2009 golf that done the same so maybe a common problem ? Anyone had the same and sorted it ? Thank you

 

[whodat]

'Sounds horrible'

What like me singing in the shower?
Bit more detail Jack.

 

[Jackmarkley24]

Haha sorry for the description

It sounds like its running really rough or not on all cylinders. I don’t get any warning lights etc. its bad enough to think WTF was that and quickly turn it off.

 

[whodat]

Nice one.
Have you done a diagnostic?
VCDS (with someone who can understand the output) or OBD or Carista?

 

[Jackmarkley24]

Yes diagnostics been done and nothing found

 

[whodat]

Intermittents are the worse problems to diagnose.

Does it really only happen every few weeks? Could it be that it happens if you stopped a regen, for example, or if you had a long fast drive and turned off the ignition without a chilling out period, or something that forms some sort of pattern (- as in , you lost your keys, where did you last put them - stupid questions but have to be asked)

 

[Jackmarkley24]

It has happened about 4 times in 6 months, when a regen starts i always drive the van hard to finsh the regen process, this is also a pain there seems to be no pattern at all but it is only when hot. Thanks for your thinking !

 

[whodat]

Every 6 weeks is a little hard to pin something down.
If you have a Carista or OBD or VCDS - next time it does it, plug it straight in and do a diagnostic before restarting it (so it doesn't have a chance to wipe any fault code)

Just for your info - and each van might be a little different - having connected a Carista during a regen - mine doesn't matter if I'm stuck at traffic lights or at 3000rpm - the soot still goes down at a constant rate during a regen (on my T6.1) and it does tend to start a regen 1 min from home or every 200-220 miles or so.

Let us know what you come up with - even if it is nothing. Hope someone else can come up with more educated guess.

 

[Asad Mulla]

I have had a similar issue on my van. I havent fixed it but I believe it to be an issue with heat soak. When the engine bay temp gets too hot, the starter cranks at about 150rmp. VW software wont give fuel at that rpm and engine wont fire up. Apparent solution is firmware update to fix TDI Hot Start Software fix remap or fit a much more powerful starter.

My issues is the van fires up after 4 days sat at home with no problem. Drive around London and the van isnt hot, no start stop issues. Sit in traffic, engine bay gets hot, starter starts to struggle.

I logged some data with VCDS and then I fed that into AI. AI assesment is as follows

1. The Cranking Speed (Engine RPM)​

• The Data: When you attempt to start the van, your Engine Speed (IDE00021) and Crankshaft Speed (IDE00405) linger heavily between 140 RPM and 160 RPM for a prolonged period.
• The Verdict: This confirms exactly what you suspected. A healthy common-rail TDI needs to hit a minimum of 200–250 RPM to start cleanly. Cranking at 140–160 RPM is far too slow and is directly trapping you in that factory VW hot-start software delay.

2. The Fuel High Pressure & Control Deviation (​

• The Data: While the engine is dragging at that low 150 RPM range, your Fuel Pressure (IDE00589) is actually sitting around 24,000 to 30,000 kPa (240–300 bar).
• The Verdict: Your fuel system is completely innocent here. 30,000 kPa is an excellent, healthy rail pressure for starting. Furthermore, your Control Deviation (IDE01378) drops down close to single digits, meaning the fuel system is instantly meeting the ECU's targets. The fuel is right there at the rail waiting to be used.

3. Battery Voltage (​

• The Data: During this slow-crank event, your battery voltage drops to about 10.5V to 10.6V and then recovers perfectly.
• The Verdict: As we discussed earlier, this proves your new battery is doing its job beautifully. It is staying well above the 10.0V danger zone where modules would brown out.

Final Conclusion​

Your log provides undeniable, absolute proof: Your hot start issue is 100% caused by low cranking RPM. The fuel system is perfectly pressured, and the battery is completely strong. Because the engine is only spinning at ~150 RPM due to physical heatsoak in the starter motor (or a highly resistive ground strap), the ECU software refuses to cycle the injectors immediately.

Since you mentioned this has happened since the van was nearly brand new, your best moves are:

1. Double-check that the main gearbox-to-chassis ground strap isn't slightly loose or poorly painted from the factory.
2. If the ground is perfect, get a VW Hot Start Software Fix/Remap to lower the cranking RPM threshold in the ECU, or upgrade to a higher-output starter motor.

 

[mmi]

I logged some data with VCDS

Care to share the logs also here?

 

[TEESIXY]

Would anyone know with some certainty if a cold engine always starts with the cold start map (injecting fuel rgardless of rpm)? Is it possible that if the van is stood in, say 30 degree temperatures all day that it would attempt to start on the hot start map (only injecting fuel when rpm parameters are met)?

The reason that i ask is that in the present hot conditions, my van starts fine first thing in the morning, but if it is stood out in the sun all day, it is reluctant to start.

 

[mmi]

Would anyone know with some certainty if a cold engine always starts with the cold start map (injecting fuel rgardless of rpm)?

Common rail diesel engines do not work like that. The first thing they do is to build up fuel pressure by HPFP (driven by cam belt). As soon as pressure is about 150-200 bars they fire the injectors.

In T6 there is no coded RPM limit which would prevent engine to start. Well, to build up the pressure of course some RPM are needed.

[EDIT]

Spoiler: Even just slightly above 100 RPM seems to be enough to build up fuel pressure.

Post in thread 'Start problem vw t6 CXHA'

7 Oct 2025

Todays log.. Fuel pressure looks good to me, but still no start.. Air lock in the new injectors?

Yes, fuel pressure is good and builds up immediately. Also injectors are commanded to fire immediately (green line).



You did run the lift pump to bleed filter, HPFP and injectors (keeping fuel lines into them connected)?

• mmi

• 

[/EDIT]