wps diagnosis, this looks wrong.

Ask for and share advice on using the PicoScope kit to fix vehicles here.

wps diagnosis, this looks wrong.

Postby antthecat » Tue Sep 18, 2012 9:27 pm

hi all, i had a stilo in today for a stalling fault but customer was unsure if the cam belt timing was ok as according to google diagnostics the stalling is pretty much caused by cam belt timing out.

im not as yet going into details about the problem with the stilo as this is irrelevant i feel, i was only using the wps to check cam/crank timing quickly to rule it out.

first waveform is running compression against coil pack primary, timing looks spot on to me (will say a great method shown by scannerdanner on a recent video), its the amount of compression pressure that should be noted, all was taken on cylinder 1, should have compared cylinder 2 but had no time as customer left car for a hour and my hour was up. this waveform is called stilo timing set up.

stilo.gif

stilo 1.6 timing set up test.psdata
(944.84 KiB) Downloaded 223 times


alarmed by the low running pressure reading, i done a relative compression test and used the wps as a reference, as you can see the pressure is a lot higher.

scudo.jpg

scudo 1.6 comp test.psdata
(1.32 MiB) Downloaded 201 times


before i ask questions, note the low amp draw with the wps, i done another relative compression test via amp clamp and had level amp draw peaks. also used picodiagnostics and three were 100% and one on 95%, not a lot wrong there (i cant upload pddata, it was the only one saved)

here is my questions, why would a running engine have a lower compression than a cranking one? could this be my problem? have i got a fault with the wps?

cheers

Ant
antthecat
Advanced User
Advanced User
 
Posts: 70
Joined: Fri Dec 23, 2011 8:58 pm

Re: wps diagnosis, this looks wrong.

Postby Alan » Wed Sep 19, 2012 7:26 am

Hi,

I hope you dont mind but I added thumbnail images to your post to save others needing to download.

To sum up on the running compression you are at about 800 rpm and the peak pressure is 80psi
On the cranking compression you are about 230 rpm and the peak pressure is 200psi

My first suggestion would be to try a snap throttle test as shown on page 1 of this case study
topic8016.html

When you do running compression at idle the throttle will be nearly closed so restricting the amount of air that can be sucked in. You can see on the running compression that you have about -10psi vacuum prior to compression, it suggests the cylinder had less air to compress.

On the cranking compression you may have manually opened the throttle (or the ECU does this during cranking) as there is no vacuum this time. As such the cylinder had more air in before compression started.

In summary all looks normal here, but you do need to think about the throttle opening when looking at cylinder pressure waveforms.
Alan
Pico Staff Member
Pico Staff Member
 
Posts: 239
Joined: Thu May 25, 2006 8:02 am

Re: wps diagnosis, this looks wrong.

Postby Autonerdz » Thu Sep 20, 2012 1:57 pm

Hi Ant,

Alan is spot on. Put another way, as shown in this image, intake pressure changes and cylinder pressure changes follow each other exactly.

DCTIvacpressurehot.gif
Intake and cylinder pressure changes follow each other.


As for the lower compression peak in the cylinder with the WPS, that is also normal. You have added volume there with the addition of the compression hose.

When looking at the timing of a cpmpression peak something to keep in mind is that the measured peak pressure is only close to true TDC at idle or cranking on a good sealing cylinder.
Tom Roberts
(The Picotologist)
Autonerdz
User avatar
Autonerdz
Zen Master
Zen Master
 
Posts: 272
Joined: Wed Nov 20, 2002 4:19 pm
Location: Washington State USA

Re: wps diagnosis, this looks wrong.

Postby antthecat » Thu Sep 20, 2012 7:14 pm

Cheers guys I have tried on another car and as said this is normal, cheers for the help.

Ant
antthecat
Advanced User
Advanced User
 
Posts: 70
Joined: Fri Dec 23, 2011 8:58 pm


Return to Diagnostic discussion

Who is online

Users browsing this forum: No registered users and 0 guests