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Fictitious fault with a vehicle - HARD TO DIAGNOSE

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Hi, I'm writing an assignment and I'm looking for vehicle fault that's difficult to diagnose (now I want something really difficult to diagnose) - perhaps some sort of an electrical fault, an intermittent fault, a sporadic problem... but something hard to diagnose. What would be the customer complaint, and what's the best way to approach that problem? Briefly suggest different checks that I should carry out in the diagnostic process to find the fault (in the correct order) and I will elaborate on them. I'm ONLY looking for quality, be inspired.




the hardest faults to find are intermittent ones and electrical intermittent faults

one of the trickiest I had was the car would start to misfire on 1 or 2 cylinders under hard acceleration and only when it was hot

so I started thinking was a faulty spark plugs - replaced these no difference

then measured fuel pressure at the injectors to check no fuel delivery issues -this was fine

then checked the state of the injector connections and coil connections these too were fine and a noid light on the injectors worked well ( admittedly at idle)

A compression test across all cylinder showed no mechanical fault either

Eventually I found it by checking all the connections to the engine wiring loom in the engine bay and found that the major loom connection under the fusebox was pulled slightly tight ( due to some previous work not putting the loom back correctly) and this was putting strain on the connector block

as the engine was revved it moved on its mounts and pulled on the connector block and broke the connection for the injector driver earths into the ECU and it only did it when warm due to the wiring connector
Customer: replied 2 years ago.

Yeah, that sounds like a good example. What make and model was the car, and what was the customer complain? Did you get any codes when you done the diagnostics? I have to do the diagnostics on paper in a logical sequence (I can't say that I start replacing parts) LOL. I need more detail about previous checks, P Codes, perhaps what live parameters would you expect and what would you look at, and some more details about the problem will help me elaborate on the rest and pull it together. Thanks!


it was an Audi s4 so the high powered v8 one and was generating the misfire codes:
P0300, P0302 P0303

there was no access to the dealer level tool to view live parameters ( which would have been nice) as this work wasn't done at a dealership but a small independent setup

customer came in complaining of a ' shuddering ' on trying to overtake and that the car was fine at all other times

it had a full service history up to 80K miles and then nothing after that and was on 102K miles at the time of the issue so god knows what had been done to it in the intervening time as the wiring had been pulled about to fit an alarm and underbody neons of all things.......
Customer: replied 2 years ago.
That's what I'm looking for. But if I had that problem, could you recommend a few checks that I should carry out in order to find that problem? We know that it was a bad earth for the injector into the block connector, but say if you wouldn't know that, what would be the best approach to take to find that problem? (other than start replacing LOL) -- I'm only looking for a fictional scenario that I have to put down on paper, but the diagnostic process is what's important in this example.
Hello Daniel


what you know is a misfire occurring under load

therefore it could be a lack of fuel or spark breaking down

so without replacing parts
1st measure fuel pressure - if low then check pump connections and relay for corrosion / damage and consider replacing filter
If pressure is OK then check the condition of injector connections and harness
if thats Ok then use a 'noid' light to confirm injector operation
If thats OK then replace spark plugs

IF this still misfires then check over wiring loom inspecting for loose ground / earth points / corroded connections and wiring / chafing damage
Customer: replied 2 years ago.

Great! One last thing. You said it was an Audi S4, with a V8 engine. Do you remember the year and when did the fault first occurred? was it after the customer serviced it at 80k miles or some time after that?


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