compression test confusion

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compression test confusion

Postby careys » Tue Feb 12, 2013 9:00 pm

VW Golf 1.4 petrol, tested with wps 500 & 4224

Compression test each cyl tested separately reads 13.5 on Picoscope Automotive mode
If I select PicoDiagnostics absolute compression no 1 cyl is down to 8.5
No 2 reads 10.5, no 3 cyl, 12.5, no4 cyl 8.5
I have ticked compensation, compression ratio & engine cc’s
Any clues as to why the variation
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Re: compression test confusion

Postby ecosse » Tue Feb 12, 2013 9:32 pm

I'm not an expert by any means but one thing i would say is, dont use pico diagnostics or pre sets, get into the habbit of setting it up your self for all tests, i tried with pico diag too but never got very good results so just set it up and switch cylinders, see what you get.

post up the psdata files i#m sure someone will come along and help :D
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Re: compression test confusion

Postby Lee » Tue Feb 12, 2013 9:59 pm

Hi Careys

One major factor is that the Software uses both the WPS data as calibration data (Pressure), and the alternator Ripple Signal (for all Cylinders). It then 'tries' to align with the WPS and Alternator data therefore if the alternator Signal is poor, weak or the engine rotational speed is 'erratic' then this can have a huge effect on the way the data is displayed.

Like when doing a manual test, its good practise to make sure the battery is fully charged, the engine cant start, and that the throttle is held wide open. Also make sure the WPS has good battery life i.e. its not about to go flat.

Probably a good idea to save the files if you havent already when using the WPS AND Pico Diagnostics and send them to Pico Technology so they can see the raw data and give as much detail as you can about the vehicle. Alternatively just attach them to this thread and Pico will pick them up here.

For better and more accurate results id advise to use the Standard Compression Test within Pico Diagnostics as this gives pretty good conclusive results and its super quick oppose to taking out every plug (Petrol) or Injector, Glow Plug (Diesel).

There is a video on how to do the 'Relative Compression Test' :D

Alternatively If you want 'Pressure Readings' then id recommend for now using a Manual Gauge, or you could use the WPS500, Alternator Ripple Signal, and do the Test in PicoScope Automotive, this would show you the raw data oppose to nice bar graphs and you would have to do it for every cylinder, which is the benefit of doing it in Pico Diagnostics, you only need to do it in one cylinder.

Then you could reference waveform 3 cylinders and overlay the last one, take measurements and compare on screen but this would take a while to do, so at this point its far easier to do it either with the Manual Guage or just the Standard Compression Test in Pico Diagnostics.

Hopefully this should be more improved in future releases of the Software so keep checking for new releases and check the release notes as this will indicate fixes, features and enhancements.

Hope this helps answer your question

Kind Regards

Lee :D
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Re: compression test confusion

Postby careys » Wed Feb 13, 2013 7:55 pm

Hi Lee
Thanks for your swift reply,it is my first time posting a forum, I will attempt sending files,apologies if I mess it up
so don't take it to serious, if you get the files I would like your opinion, the cranking current is fuzzy do I have a starter problem,
Kind Regards Careys
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Re: compression test confusion

Postby Robski » Wed Feb 13, 2013 8:49 pm

If i were you i would :-

A) do as mentioned & carry out a conventional compression test with a gauge to make sure.
B) WPS in cyl #1 with cyl #1 secondary as a sync because looking at the relative comp wave the ign is firing AFTD !

Are you disabling the fuel system to avoid bore wash on repeated tests ?

What's the problem with the car in the first place ?
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