Originally Posted by isleofbones
Starts instantaneously in warm weather, runs smooth. Starts fairly quickly down to about mid twenties. Below that, it is a slow start. Thanks for the advice.
Have fixed similar issues with 200/300 TDIs.
There are 4 basic scenarios related to the antifreeze, fuel system, battery, and the glow plug controller.
One thing that happens when the IP gets worn, is that it does not produce enough pressure to pop off the injectors when being spun by the starter, but will otherwise run fine once it starts. In cold weather when the starter is struggling to spin a cold engine, this could be one cause.
The other as already suggested is burned out glow plugs. Even 1 burned out glow plug can render some engines (but not others) unstartable in temps under 20deg F. DO NOT remove the glow plugs to test them, because if they are stuck in the head and working just fine, they will break when you start wrenching on them. You can use an amp gauge or meter, but the easiest way to test them is to unscrew the factory wiring and run a wire from each individual glow plug separately and touch it to the (+) battery terminal while it is still on the battery. If the stripped wire end flashes a spark while being dragged across the terminal, then the glow plug is drawing current indicating it works. If there is no spark, the glow plug needs to be replaced and must be carefully removed without breaking off the tip. Broken tips are hard to remove. The worst case is the head has to be removed to get the broken tips out.
If all the glow plugs are good. Have the battery checked or get a battery with higher cranking amps. A weak battery or one that is underrated will prevent you from a cold start. We also supply an adjustable glow plug controller that lets you adjust a longer glow duration in the winter and a shorter in the summer by twisting an adjustable knob to set the time anywhere from 2 - 120 seconds.
Check the antifreeze. If it is too diluted, then It could be partially freezing causing the water pump to drag down the rotation as it spins in a slurpee like liquid.
Installing some type of engine heater may only mask one of the above issues.
When it went down to about 12 deg F, started a 200TDI without any glow at all, so yours should start with all the right out of the box features unless your engine has other issues like those above or something else like low compression or a slight overnight bleed down in the fuel system that lets air in that is not noticed once the engine starts and runs after spinning it over and over again to bleed out the air. A 12V fuel pump and sedimenter will go a long way to eliminate this problem.
Hope this helps.