It has been very cold past few weeks here in Finland. Under -20 degrees centigrade and that's pretty much the same in Fahrenheits, too. The obvious answer to my problems is "Don't drive your Q4!", but this is not an option.
When everything is very chilly I don't push the car hard - I drive gently for a long period of time and let the temp go up over 70 and oil over 60.. Evrything works fine. Then I pedal it. The boost-gauge (aftermarket, VDO) jumps to 1,5 bar and something cuts off BIG time, but just for a second or so. Then it starts pulling again. But may do these cut-offs again. This cut-off is very severe. Like the engine halts completetly. But going below 1.0 bar, everything works normal again.
After pushing and pedalling for a while on small twisty snowy country roads (Boy that is fun)
Everything starts to work as in summer. Maximum pressure under any condition is 1.0 bar, while the normal maximum is limited to standard 0,8 bar. Now the temp reads 85 and oil temp in 90ish. Opening the bonnet we find the manifold and turbo red hot.
And everything works perfectly.
Then going to a main road and cruising at 80 kmh for a while. Temp drops to 80 and oil to 65 or so (oil cooler blocked, it used to drop below scale before the blocking.) Pedal it and the phenomenom first described happens again. Overboosting and severe cut-off. Q4 bouncing like a kangaroo in heat.
Then a series of questions:
What the hell is going on? wink
What item(s) control the maximum boost to stay in the normal level (0,8 bar) What circuitry lets the boost rise temporarily to 1.0 bar ? Can they fail in cold?
What causes these cut offs atr high boost? I intend to chip the sucker later on so I wish it would work upto 1,5 bar. Everything is standard at this moment.
I have 3 different possibilities.
a) Plugs foul at high boost..
b) Fuel pressure isn't enough for high boost
c) Some internal protection comes in the play and cuts fuel... ???