Running compression test with the Pico Transducer

Ask any non-NVH related questions about using PicoDiagnostics software here.
Post Reply
LexusDitech
Newbie
Posts: 5
Joined: Tue Jun 23, 2020 2:36 am

Running compression test with the Pico Transducer

Post by LexusDitech »

I have recently started using the transducer and have watched many videos about the different uses. I had a 2018 Lexus RX350 with a A/F sensor imbalance code for cylinder .1 I first did a cylinder leak down test which showed a 60% leak down through one of the exhaust valves, so I knew where the problem was but wanted to learn more about the transducer. I did a running compression test with the Pico hoping to see something indicating the problem but the compression test pattern looked normal compared to another cylinder. It may be that I am over looking a small detail in the pattern. Does anyone have any experience with this that could tell me what to look for in the pattern for a leaking exhaust valve.

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 1291
Joined: Sun Aug 25, 2013 7:22 am

Re: Running compression test with the Pico Transducer

Post by Steve Smith »

Hello and welcome to the forum.

Could you post the psdata file of the offending cylinder with 60 % leak down via the exhaust valve?

Would you have a similar capture from an adjacent "Good" cylinder too?

These will help to assist you with pressure analysis techniques

Take care.....Steve

LexusDitech
Newbie
Posts: 5
Joined: Tue Jun 23, 2020 2:36 am

Re: Running compression test with the Pico Transducer

Post by LexusDitech »

Here are my saved wave forms for the leaking exhaust valve. I hope they attached properly.
Attachments
thumbnail_IMG_4018.jpg
Good cylinder running compression and crank signal.psdata
(20.03 MiB) Downloaded 278 times
Running compression test idle.psdata
(11.03 MiB) Downloaded 260 times

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 1291
Joined: Sun Aug 25, 2013 7:22 am

Re: Running compression test with the Pico Transducer

Post by Steve Smith »

Hello and thank you for the feedback

Looking at “Running compression test idle” waveform could you confirm this was the offending cylinder?
The expansion pocket below measures -597 mbar by comparison to “Good cylinder running compression and crank signal” at -758 mbar
Image 1
Image 1
Image 2
Image 2
Unfortunately, whilst this could be a result of a poorly seated valve, we are not comparing apples with apples due to cylinder speeds. Look at the peak compression for “Running compression test idle” (Image 1: 9.712 bar @ 1150 rpm) as opposed to “Good cylinder running compression and crank signal” achieving 5.173 bar @ 693 rpm (Image 2)

The most likely reason here is that “Running compression test idle” was captured at Fast Idle or with raised rpm where more air is drawn in, so achieving a higher compression. Given both pistons will travel the same distance during the expansion stroke (so increasing the cylinder volume) it’s no surprise that the cylinder starting at 9.712 bar at TDC on compression has a smaller intake pocket that one cylinder starting at 5.173 bar at TDC on compression.

I am not sure if you have started the repair on this vehicle, but if not, could we repeat the tests between offending and good cylinders at the same temperature and same speed. Using the 100 ms/div time base for both will help as we are missing considerable data in Image 1 at 20 ms/div

Generally, a leaking valve seat (to the degree of 60%) would result in leaning compression towers accompanied with reduced “depth” intake and exhaust pockets

Below we have the measurement technique used to establish a leaning compression tower
Image 3
Image 3
With our phase ruler denoting 720° and a signal ruler passing through the compression tower at 1.53 bar, we can align our time rulers with the crossing point of the signal ruler with the compression tower either side of our 720° phase ruler.

Using the displayed degree values at the time ruler handles, we can calculate any deviation in pressure either side of the 720° ruler in relation to degrees of crankshaft rotation so revealing pressure loss. (Remember compression towers should be symmetrical)

Image 3 has time ruler 2 at 47.87° ATDC on the expansion stoke where it intersects the compression tower at 1.53 bar

Theoretically, if the compression tower is symmetrical, at 47.87° BTDC on the compression stroke, time ruler 1 should also intersect the compression tower at 1.53 bar. As we can see above time ruler 1 intersects the compression tower at 673.7°ATDC compression or, 720° – 673.7° = 46.3° BTDC on the following compression stoke. (47.87 - 46.3 = 1.57°symmetry error)

Below is another example looking at compression tower symmetry at approx. 17° before and after TDC on compression. (Remember allow for tolerances, speed & temperature etc.) Note: A leaning compression tower is often easily spotted with the naked eye, however, the above technique will help qualify any discrepancies
Image 4
Image 4
From all the above captures we can confirm similar valve timing and durations, accompanied with “defined” valve open/close events which switches the focus to cylinder “sealing” and not timing.

A valuable test would be to now add the pressure transducer to the exhaust tailpipe and monitor exhaust gas pulsations in the fault condition

I hope this helps, take care…….Steve

LexusDitech
Newbie
Posts: 5
Joined: Tue Jun 23, 2020 2:36 am

Re: Running compression test with the Pico Transducer

Post by LexusDitech »

Well it tells me I have a lot to learn about reading these patterns. Do you have suggestions on any training courses or books. The vehicle has already been repaired so I can not get any more data from it.

Steve Smith
Pico Staff Member
Pico Staff Member
Posts: 1291
Joined: Sun Aug 25, 2013 7:22 am

Re: Running compression test with the Pico Transducer

Post by Steve Smith »

Hello and thank you for the continued feedback.

The following forum post will point you to a number of helpful links with further information on the use and application of the WPS

topic22275.html

I hope this helps and please feel free to post for any clarification on the content included in the above link

Take care.....Steve

Post Reply