PROBLEM JOBS
Head scratching nuisances
Owner wastes thousands on parts only to find out it was a relay fault Stephen Kasapis tells GW how ‘a bit of detective’ work fixes a fault where others had failed.
Job SHEET Vehicle: 2010 Audi A4 Problem: Bosch GDI system low pressure History: Vehicle has already had a new high-pressure pump, new low-pressure pump and a new fuel rail sensor.
I WAS asked to see why the engine check light was coming on. This car had already had thousands spent on it but the engine light kept coming on with low power for the first five minutes. I scanned the system with a scan tool and it came up with low pressure system. With the freeze, I saw that the control system volts were only 11.97 volts. Ok, now don't forget this car has already had lots of parts put onto it by another workshop and it was still not fixed. So I set up the scan tool and went for a drive. The fuel pressure was 580 PSI at idle and went up to about 1160 PSI, yes these Bosch GDI systems have very high pressure. Now for more testing, so I set up my PicoScope Lab Scope to see why the car was coming up with
low-pressure codes. I found that when you first start the car, for the first few seconds, we only had 70 PSI. This is just the low pressure working so I set up the scope on the fuel rail. The rail pressure sometimes had good pressure and other times it was bad. This is good for me because I have found something that’s not right with this system. I then set up the scope on the high-pressure pump valve. When I first started the car for only the first few seconds, I lost power to the valve. With no power supply, the high pressure will dump the fuel back to the tank and we will only have low 70 PSI. Now I know why this car had low fuel pressure codes. I now need to find why we are losing power to the high-pressure frequency valve. The power and the control for the valve comes from an ECU so I looked at the wiring diagrams and saw that it had a few power supplies going to the ECU coming from a few relays. I removed the covers under the wipers and found one of the relays for the power to the ECU was bad. It was the reason why we were losing the high pressure, this car did not need thousands spending on it. I put a new relay on and tested the system again. Thanks to a bit of detective work and the big clue from the freeze frame of low volts and knowing how to set up the PicoScope, I fixed this car.
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