Astra rough idle

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Astra rough idle

Postby BradCobbs » Thu Feb 12, 2015 9:36 pm

I have an 05 Astra with idle problems, it starts up fine but after bringing the revs up it either stalls or hiccups once it returns to the usual idle speed.
Engine is Z18XE.
I've cleaned the throttle body and used Launch to open the throttle wide open and it is operating fine. Checked for air leaks etc...
Tested the MAF and it seems to be working fine. Reaching roughly 4.8V upon acceleration.
After a string of tests it finally produced a fault code; P0170 Fuel trim too rich.
Next step was to check the Lambda sensors, and as somewhat of a newbie to oscilloscopes and diagnostics I believe that my waveform isn't exactly what I should be seeing. I've used the acupuncture probes and gone into the grey wire of both zirconium dioxide sensors and I should be expecting a very clear switching from 0.1-0.9V roughly on pre-cat and a steady 0.1-.0.2 post cat if everything is working correctly. Pre-cat I have a large "block like" pattern at the high end of the 1V scale. I'm not sure if this is simply telling me it's way too rich, or is it in fact a failure in the sensor?
I've put the emissions tester in the exhaust and at idle I have as following:
9.372% CO, 9.5% COc, 8.4% CO2, 744 HC and 0.7 Lambda.
So it is running rich...
I've also tested fuel pressure at the rail at idle and it's just below 4bar which according to Autodata is within tolerance of 3.6-4 bar.
I'm not sure where to go next, any ideas would be much appreciated.
Thanks in advanced, Brad.
Attachments
Astra waveform lambda.jpg
Waveform
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Re: Astra rough idle

Postby volrem » Fri Feb 13, 2015 11:13 am

Looks like just noise. Check your scope ground lead and post psdata file.
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Re: Astra rough idle

Postby BradCobbs » Fri Feb 13, 2015 11:20 am

I had both earth leads straight to the battery and hung accross the engine bay using hooks. My figures for everything else have looked good. Are there any other specifics I should try to adopt when scoping?
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Re: Astra rough idle

Postby volrem » Fri Feb 13, 2015 12:07 pm

Could you please post psdata file so we could have closer look.

And what are your fuel trim numbers?
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Re: Astra rough idle

Postby BradCobbs » Fri Feb 13, 2015 1:38 pm

Haven't had chance today to note the fuel trim numbers, I'll get them when I can.
Here is the psdata file.
I've had chance to test another lambda probe which I know is operating correctly, but I still can't eliminate all noise, my connection to the negative on the battery is secure.
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Vauxhall_Astra_2005_Petrol_20150212-0001.psdata
(1006.3 KiB) Downloaded 104 times
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Re: Astra rough idle

Postby Robski » Fri Feb 13, 2015 3:21 pm

Voltage scale on CHA wants altering (the signals hanging high(rich)) , the interference can be filtered out, there is always a lot of noise on an O2 sensor.

I have a feeling I know what this car wants !

Alter your voltage scale/s & retake the capture & force it rich/lean & let's see if they respond ..........
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Re: Astra rough idle

Postby AuthorMike » Fri Feb 13, 2015 3:42 pm

Can't say I know what is happening as such, but you definitely need to increase your scales. Plus under channel options (the letters with black arrows) enable low pass filtering by clicking "Activate Filtering". Should help tidy the waveform a bit, but only once you have re-captured.

:)
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Re: Astra rough idle

Postby BradCobbs » Sat Feb 14, 2015 1:23 pm

Just managed to get about half an hour performing a few more tests around an abundance of other work, you know how it is!
S/T Fuel trim is at -13 when the lambda is saying rich 0.9V.
Clear the codes and induce an air leak, the voltage then plummets to 0.1V.
So I've done some MAF & Lambda tests and here are the waveforms.
If I completely bypass the MAF it goes really lean but the Short term fuel trim stays at -0
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Re: Astra rough idle

Postby BradCobbs » Sat Feb 14, 2015 8:04 pm

Here are a couple waveforms I've captured. It's not letting me upload them for some reason.
Attachments
Lean - rich astra.png
astra WOT.psdata
(844.68 KiB) Downloaded 77 times
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Re: Astra rough idle

Postby Robski » Sat Feb 14, 2015 9:33 pm

that's not what I asked for in the slightest, disconnecting the MAF proves nothing much & masks faults.I wanted to see the O2's .

You crack on ! :roll: :?
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