INTERMITTEN fault diagnosis: in 5 steps

Intermittent faults are the bane of our lives. Use these five steps to dramatically improve your chances of a first-time fix

Published:  25 September, 2019

Picture this: You’ve a mildly grumpy client stood at your front desk. We’ll call him Mr Brown.  He has been a happy customer for many years, you’ve serviced his vehicles regularly and all has been good in the world.
Unfortunately, today is Mr Brown’s third visit to your counter for the same problem. He’s understandably becoming irritated by the fact that your technicians aren’t able to identify the reason behind the intermittent loss of power with his VW Golf, and he’s starting to doubt the competency of your business.
It’s a common problem, so your garage should have a robust solution. If not, then you could lose Mr Brown's custom. See this happen too often and it will put a major dent in your bottom line. Nobody wants that.
Is there a solution to the intermittent fault dilemma?  In many instances yes. You just need to apply the right routine. With the right routine, tools and information you’ll dramatically increase your chances of finding the fault in the first go. Apart from keeping Mr Brown happy, your techs will enjoy the buzz of early diagnostic success. The big question is what needs to change in your garage to improve your chances of nailing these elusive faults? A look at Mr Brown’s vehicle will show a pattern that you can use in your garage.
Mr Brown’s Golf has been experiencing a sudden loss of power. It makes him pull over to the kerb. Stopping and starting can make the problem disappear and not occur again the same day.
The vehicle has been scanned and road-tested on both visits. A fault code relating to rail pressure deviation has been recovered in both instances, but no fault was found on the multiple road tests completed. What can you do to get to the root cause of this problematic situation? Just use this routine:

1: Thorough grilling at front desk
We find the following statement usually has a client bending over backwards to help: “Sir – It would be great to speak, if you have a few minutes, so that I can find out some specific details of the fault on your vehicle. We often find that a few minutes of your time now will often help us find the issue and could save you money on the cost of your diagnostic evaluation.”
All you have to do now is ask the right questions to ascertain a point at which something changes on the vehicle and the details around that, as well as some specifics as to when the fault occurs.
In this instance further questioning revealed that the problem normally occurs around a mile or so from Mr Brown’s home, after he’s started his journey in the morning and occasionally on his way home.
Armed with this information, you give Mr Brown a courtesy vehicle so that you can carry out the tests the next day.

2: Where did I put that silver bullet?
We’re already one step closer compared with previous visits, and have a constantly recurring fault code for high-pressure control deviation. Initially, it’s worth making this the focus of our diagnosis. For starters you could take a look in ELSA (where the VAG group keep their silver bullets) for any known issues in their service bulletin archives.

You find one relating to a known mechanical fault for the high-pressure pump wear resulting in constantly low pressure, but nothing for an intermittent fault such as yours. To be on the safe side you inspect the known issues and find the connection between the camshaft and the high-pressure pump to be in good order.

3: Desk diagnostics -Where the magic happens
It’s in stage 3 where the magic begins. You need to make an exhaustive list of the reasons that could raise this fault code:
A Restricted supply to in-tank pump
B Faulty in-tank pump
C Power/ground supply fault to pump
D Faulty low-pressure fuel pump control module
E Restriction between the low-pressure and high-pressure pump
F Faulty high-pressure pump
G Faulty high-pressure control valve
H Power/ground supply fault to high-pressure control valve
I Faulty high-pressure sensor
J Faulty wiring to the high-pressure sensor
K Faulty injector/s
L ECU power or grounds
M ECU software
N ECU hardware

One fault with 14 possible causes? There’s only one thing for it:

4: Testing, testing, testing - It’s all about priorities
There’s a straightforward reason why this vehicle hasn’t been diagnosed on the earlier visits. Quite simply, the 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.

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