September 2019

right tests have not been completed at the right time. That’s all about to change. You need to decide how best to test the possibilities. This is how I would prioritise: A Fit fuel flow tester in low-pressure supply B Scope CH1 WPS 500x pressure transducer pre-fuel filter D Scope CH2 WPS 500x pressure transducer post-fuel filter C Scope CH3 in tank pump 20Khz PWM positive supply from low pressure pump control module D Scope CH4 ground in tank pump ground E Scope CH5 in tank pump current F Scope CH6 high-pressure rail sensor signal G Scope CH7 high-pressure control valve + H Scope CH8 high-pressure control valve – The objective is to carry out as many tests as possible in unison. I’ll also be able to road test the vehicle and stand a VERY good chance of diagnosing this problem the first time the fault rears its head. 5: Diagnostic sniper Diagnosis is all about ruling out what’s good. Do that methodically and the problem will reveal itself like an enormous arrow descending from the sky: “THE FAULT IS HERE.” This is exactly what will happen when you road test this Golf. You’ve set up your tests and set off on your test drive, complete with assistant. You’ve been driving for around 10 minutes and just as Mr Brown predicted the vehicle loses power and you’re forced to pull over. This is great news. Let’s take a look at the clues: Fuel flow dropped dramatically as vehicle lost power Rail pressure dropped at same time Powertrain ECU increased duty cycle of the high-pressure control valve The results pose the question “is the low pressure supply good?” Let’s take a look. Fuel pressure pre-filter reduce Fuel pressure post-filter reduced Duty cycle to low-pressure pump increased Power supply and grounds to low- pressure pump remained good Fuel pump current increased It’s great to see the pieces of our diagnostic puzzle falling into place. The drop in pressure, and the fact that the powertrain ECU has increased duty to command more fuel clearly indicates the ECU is trying to rectify the drought, but what’s causing this? The devil is in the detail here and the final clue is in the fuel pump current. There are very few causes for low- pressure fuel pump current increasing while post-pump pressure reduces. The most obvious would be fuel pump speed reducing due to pump failure. That’s exactly the fault with Mr Brown’s Golf. Bingo! you’ve found it! This five-step system is straightforward and greater than the sum of its parts when executed thoroughly. Miss any individual step though, and you might not be seeing Mr Brown back in at your garage. SEPTEMBER 2019 AFTERMARKET 23 EYEBROW www.aftermarketonline.net If you’d like to learn how to improve your diagnosis skills then call John on 01604 328500. Auto iQ have a complete technician development programme designed to help your technicians be the best they can be. _____________ To join AutoiQ’s online forum go to: autoiq.co.uk/ garageowners