Car starts good when cold outside, but not good when warm out?

[Jaxon]

I have a 2012 Sportwagen TDI that has been giving me intermittent starting issues since I bought it two years and 50,000 k ago. It's gotten progressively worse.

Here's what I've narrowed it down to - when I start the car in my garage in the morning it's +5 celsius - car usually starts right up, does it's usual couple stumbles, and I'm on my way. I park in a heated parkade where at the end of the day and I go to leave it's +22. The car refuses to start, I need to spin the motor for about 30 seconds (usually pause 5 times) then I get a huge cloud of smoke, stumbles for 10 seconds, then away I go. It is even worse in the summer as the car sits outside then where it could be 15 - 30 C. The car starts perfectly once it's been running and only acts up again once the engine has cooled off.

Here's what I know:
- Not intercooler icing, I've checked it twice, once in the summer and once in the winter - only a few drops came out. In the winter the car starts and ends its day in a heated garage, and has a 15 mile commute each way;
- The car was parked for 2 months, it was +10C out, I connected the battery and it started right up, so I don't think fuel could be draining back;
- Fuel filter has been changed every two oil changes, and the condition has never improved after a fresh filter;
- Just swapped to a CP3 pump, didn't help;
- Malone stage 2 delete tune, EGR disabled, did not help;
- Changed the fuel return line & valve on the injectors, didn't help;
- No codes;
- Changed glow plugs with known good parts from my parts car, didn't help;
- I've unplugged the ASV and it didn't help;
- All maintenance is up to date on the car;
- Battery is new, and is the second new one in six months.

So is there something that regulates how the car starts based on ambient temperature? Last winter I parked outside, and even in -30 C it would fire right up (plugged in). Yet all summer it started really badly. And I know that it has to do with ambient temperature due to the temperature differences in the two garages. I do have VCDS. Thoughts? Can someone help me out here? I've searched tonnes of threads but nothing similar!

 

[Jaxon]

Update: last night I took the lower intercooler pipe off, nothing but a few drops of oil. Cleaned the map sensor, it wasn't dirty at all - didn't help.

So what sensor(s) does the ECU use in order to determine the length of time the glow plugs run? It is definitely related to ambient temperature.

 

[roadlust]

I don't think the glow plugs cycle above a certain temperature. They seem to be working when they are needed. Maybe air getting in the fuel line although I don't know how or why that would happen only above certain temps.

 

[Jaxon]

More testing today - ~3 second glow plug light in my 5 degree garage - car starts quickly, stumbles for 15 seconds then runs fine. Garage at work at 22 C, less than 1 second glow plug, car won't start, finally does, stumbles and smokes.

So the glow plugs are working how they should, well coming on that is. So what would cause the engine to have a hard start when the glow plugs don't function? If doesn't start real nice even when they do come on - stumbles, runs rough, but always starts. Timing? Cam/crank sensor?

Roadlust, I don't think it's air in the fuel line. The car has sat for 2 months at a time and still starts right up. This happens if it sits 2-3 hours or longer. But it's fine if it's 5 degrees or colder out.