relative compression test

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Re: relative compression test

Post by victor2k » Sun Oct 23, 2016 12:24 pm

New day,new question:someone tested on a engine with the compression issues?
According to the Pico diagnostics I have on Avensis 1ZZ-FE:10,12,13,13 bar compression.
According to the real life I have :10,13,12,13.
My question is:was the firing order included in program for 4 cyl or not?
Tommorow I will test without the spark plug at cyl 2.
Using the data from Avensis SDS all values must be good/green(minimum allowed compression is 10 bar).
SDS Avensis.png

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Re: relative compression test

Post by FioranoCars » Sun Oct 23, 2016 12:40 pm

This reminds me of a feature request I'm sure I've raised before, if not for compression test, then for Cylinder Balance, but I guess it applies to both, and would certainly help confirm Identities ...

:idea: FEATURE REQUEST :idea:
Dear Pico ...
Please can you add to the "options" dialogue 2 new fields
1. Cylinder that Pressure Sensor is mounted in
2. Firing Order

Opening some old files, the current Diagnostics version refuses to open 10 cylinder captures, and on a 4cylinder capture the labels on screen are still cut in half horizontally ... again I'm sure these have been raised before ?

It would be great if the "Results" display (and Print) identified the Sensor cylinder in some small way (not huge).
The identification of the firing order would make the output more "User" friendly for clients to understand and help stressed workshops avoid silly mistakes, when brain fade sets in on a Friday afternoon :oops:


Steve Smith
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Re: relative compression test

Post by Steve Smith » Mon Oct 24, 2016 6:56 am

Hello to all and thank you for the posts. (I apologise in advance for the late response)

You are correct, this is a bug indicating “Red Bar Graphs” when all cylinder pressure values are near equal (they should be green)

The issue has been resolved in the next beta build which is undergoing final testing this coming week. It is a huge beta build with numerous additions and bug fixes, hence the delay. (I have been talking about it for some time with regards to NVH)

There are a number of points to note with the Pico Diagnostic (PD) relative compression test when using the pressure transducer, the first of which is the hose and attachment.
I have experimented with numerous hoses and the Pico hose must be used in conjunction with the pressure transducer.

Internal diameter, wall thickness and “flex” all have an effect on either the pressure waveform when using PicoScope, or the test results when using Pico Diagnostics

Pressure compensation (PD) is also paramount for accurate pressure readings.
For petrol engines this is not such an issue as the default compensation value of 5.00ml is perfect (using the Pico Hose)

However when adding a dummy glow plug (for diesel engines) to the Pico pressure hose, this value can increase to 7.00 ml (minimum) up to 10.00 ml when using “long reach” dummy glow plugs. (Remember to also to include the compression ratio and engine capacity when using the compensation feature)

When using the PD compression test with a pressure transducer attached to a diesel engine, without compensation, the result will most certainly indicate 1 x cylinder as being low on compression (Often a Yellow bar in bar graph) Remember we are adding a minimum of 7.00 ml of volume to the cylinder thanks to the Pico Pressure Hose, Pressure Transducer and Dummy Glow Plug.

You can view the effects of the pressure hose and dummy glow plug when selecting “Display Raw Data” from the Bar graph screen after capturing your data.
Here both the pressure transducer and battery voltage waveforms are displayed. (The peak cylinder pressure aligns with the minimum fall in battery voltage during cranking due to the reduced compression)
No Compensation.jpg
Diesel engine without pressure compensation
Using the compensation feature will correct the measured pressure value and amend the remaining cylinder pressure results
Compensation applied.jpg
Diesel engine with compensation applied
Location of pressure transducer

The PD compression test always assumes the WPS pressure transducer is connected to cylinder 1
The technician (where possible) must use cylinder 1 for the WPS pressure transducer. (I understand this is often very difficult) This will be amended in the future where the user can specify the cylinder location of the pressure transducer.

Firing orders

The firing orders pre-loaded into the PD compression test when using the pressure transducer are 1342 for a 4 cylinder engine and 153624 for a 6 cylinder engine. (Unfortunately this cannot be changed at this stage)

A new feature request has been added to PD development in which the user can specify the firing order and include firing orders for higher cylinder counts (8, 10, 12 cylinder etc.) This does not prevent the testing of 8, 10, and 12 cylinder engines as all compression events are displayed numerically

Assuming the pressure transducer is located into cylinder 1 of a 6 cylinder engine with a firing order of 153624, the bar graph will display the cylinders numerically (123456):

Engine Cyl 1 is number 1 in the PD bar graph display, Engine Cyl 2 is number 2 in the PD bar graph display, Engine Cyl 3 is number 3 in the PD bar graph display, and Engine Cyl 4 is number 4 in the PD bar graph display etc.etc.

I am a very big fan of PD as an introduction into PicoScope and as part of routine maintenance or initial non-intrusive diagnosis during the “Golden Hour” of diagnosis. (I use this regularly)

The Golden hour is what we refer to as the 1 hour labour charge agreed between the technician and the customer to carry out an initial assessment of the vehicle based on the customer complaint

Using the pressure obtained from one cylinder and the associated fall in battery voltage from all cylinders during cranking, PD will calculate and present accurate cylinder pressure values for all cylinders. (Perfect for engines where cylinder access is near impossible without component removal)

Often in this 1 hour, the technician can find the fault using PD or inform the customer the work is going to take a lot longer than 1 hour thanks to the information obtained by PD (low compression for example).

I hope this information is of some help, and rest assured we are working continually to improve all PicoScope and Pico Diagnostic features thanks to feedback like that above

Take care…………Steve

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Re: relative compression test

Post by victor2k » Sun Oct 30, 2016 7:03 am

Will be solved the issue from File/Preferences/Regional and Language/Metric system for the Compression Test?
The results is show in Psi instead of Bar. :cry:
Thank you

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Re: relative compression test

Post by grunf1976 » Mon Nov 21, 2016 5:13 pm

I dont mind fields not turning green , but I cant change the readings from psi to bar. tried changing settings in preferences setup, but as soon as i start it goes back to psi. Can someone please help with this?

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Re: relative compression test

Post by victor2k » Sat Aug 05, 2017 1:14 pm

Some good news about the location of the pressure sensor in Relative compression test?
Thank you.

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