Audi TT 2.0 TFSI
Problem: Car wouldnt start, run for about 1 second and then cut out.
After starting with a full autoscan and checking for faults expecting to see immobilizer faults, no faults were found relating to the immobilizer just a low fuel pressure one. Now we needed to check if the fuel pressure was the problem or was that a code that has been caused in the past? Checking the measuring block for low fuel pressure showed a healthy low fuel pressure with the engine off, upon starting the engine the low fuel pressure quickly tailed off. A quick cycle off the ignition and the fuel pressure was back to normal – restarting again and the fuel pressure quickly drop off with the car cutting out.
To double check it was fuel an external fuel source was added through the induction system with this in place the car carried on running.
Points were noted:
- Fuel pump must be working as the pressure returned with a cycle off the ignition
- Being a high pressure FSI system there must be some form off low pressure fuel control
Checking the wiring diagram showed a fuel pump control unit fitted just above the fuel pump with connections to engine ECU this looked like a good place to start. After removing the trim and gaining access to top of the petrol tank the control unit could be seen, checking the basic power and earth’s were correct we set the scope up on the feed from the ECU to the control unit and the feed out the control unit to the fuel pump. This can be seen below in the scope pattern with the Blue=Feed from engine ECU and Red=feed to fuel pump.
The blue pluse is from the engine ECU and stops after 20 seconds or so after the ignition has been switched off, The big red block starts once the ignition has been switched on and last for a 0.5 second and then tails off. This shows us why we have fuel pressure with ignition but no pressure with the engine running. We fitted a new fuel pump control unit and this solved the starting problem. Due to waiting time for the part to come I didn’t get a chance to scope the new one – but guessing I would say the fuel pump is PWM (Pulse width modulation) to alter the low fuel pressure into the high pressure fuel pump. This PWM modulation is over ridden by the ignition to provide maximum fuel pressure for start up.