Is it possible to have glow plugs on all the time for certain amount of time?

Question

I have Audi A4 B7 2005 2.5TDI. The temperature drops below 0 °C where I live. When it's cold (e.g. starting from 5 °C) my car starts having problems starting up. When I start it in the morning everything is OK. The glow plugs turn on and the car start's fine whatever temperature it is outside. After I stop the car and let it cool for 1-2h and try to start it again it has problems. The glow plugs don't turn on and it takes as while before the engine can crank itself up. I know that my fuel injector's aren't in the best shape. I have had them tested.

1. Should the glow plugs turn on for a few seconds if the car has been idle for 2 hours (with temperatures below 0)? If yes, then can it be that I have a faulty sensor and it only turns the glow plugs on when the car has cooled down a lot?
2. If no then is it possible to change the temperature at which the glow plugs turn on? Or always have them on for certain amount of time.

Thank you for any kind of feedback!

Answer 1

The way the VW (so assuming Audi is similar) glow plugs are controlled on the TDI Jettas of similar vintage, the plugs should come on (what VW calls "pre-glow") whenever the block is cold. You should see this in the instrument cluster as the glow plug light coming on briefly (only briefly, the plugs are very effective). The glow plugs then remain on (or come back on) after the start to ensure that the engine runs smoothly as it is heating up. There is no indication to show that this "post-glow" function is happening.

It sounds like you may have a bad or intermittent temperature sensor. If you disconnect the sensor it will read as cold and that should trigger the glow plugs. On the Jetta the sensor is on the coolant flange on the back of the engine, (right hand side of the car as you face the engine from the front) above the transmission. There are several hoses that come together there and the sensor is on the back side towards the fire wall. If you can find it, try disconnecting it to see if the car starts easily with it disconnected. If it does I'd say you've found your problem.

For whatever it might be worth, a 1.9 L VW TDI engine (BEW) cools down enough in 30° F weather to trigger the glow plugs after an hour and a half of cool down time.