Correct diagnostic sequencing?

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Correct diagnostic sequencing?

Postby Basil » Mon Nov 28, 2011 11:43 pm

New to using an oscilloscope for automotive diagnostics, so please bear with me. Case is a Nissan Stagea with six cylinder petrol engine with a single turbo, single throttle body, and a hot wire AFM. Coil on plug sequential ignition and 6 injector sequential injection. Symptom is quite noticeable, irregular very short duration miss at idle and low throttle cruise. Fuel consumption is maybe a little high, nothing drastic. Under boost the car runs fine. Hot and cold starts are good. Last time it did this cleaning the AFM hot wire fixed it. I had no scope then, just went with the flow as it's a common problem, allegedly... It's now doing it again and I want to use the scope to see what's occurring here.

Now, if I look at the voltage waveform from the AFM it is erratic, but of course, I have to assume the miss itself, even if caused by say an ignition issue, would show an AFM voltage change as the air consumption is also changing when it misses? So how would one organise a correct campaign to diagnose such a miss without being fooled by changing signals that are the result of the miss, and not (necessarily) the cause of it please?
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Re: Correct diagnostic sequencing?

Postby Robski » Tue Nov 29, 2011 8:39 am

I would start with a code read to ID the cylinder,if this is not possible then FLS/WPS in the tail pipe with a sync to ID the missing cylinder.

Once ID'd would then pull the coil if possible and use a COP extension lead then use a secondary pick-up probe & have a look whats going on whether its a misfire or lean misfire etc.

You could also to start with check the integrity of the COP by primary & current,but if its the same as before when you cleaned the AFM i would expect a fueling misfire :?:
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Re: Correct diagnostic sequencing?

Postby Basil » Sat Dec 03, 2011 7:12 pm

Thank you very much for the detailed reply. The car is pre OBDII, so I can't read any meaningful codes, unfortunately. I don't have a Pico pressure transducer, it sounds useful, but the price is too much for the occasional road car work I do. I have a wide range of pressure transducers for race car after market ECU's, for use as MAP sensors, boot pressure sensors, fuel pressure sensors, etcetera. Not wanting to turn this into a DIY forum, or offend anyone, would it be possible to feed a suitably ranged one 5V and ground and read the signal wire via the Picoscope, with it connected to a smallish bore tube shoved up the tailpipe? Yes, I know, I should just go and buy the nicely encased, proper tool, but for the amount of use it would get, it is probably make something temporary, or work without one altogether :)

Again, thanks for your time Robski.
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