Vladds

I have a Xentry Connect C5 , 09/2106.
I am trying to perform the compression test. I put on the parking brake, click on the green arrow to start the test, step on both brake and accelerator to the floor.
Says to actuate the starter until the bar elapses approx. 6 seconds.


What should I expect? I wait for this bar to show up and a timer elapses.


Will this be a software starter actuation like in the video of the STAR? or do I have to crank it with the key?


There is something to connect a 30 amp charger to the battery. Maybe because I didn't connect one, it won't let me perform the test?


I expected that at least it will try to peform the test and maybe fail me because of no charger, but I get no software starter click button (other than the test start green arrow) and no timer bar.


The coolant temperature is over 80 Celsius and the batter is at 12 volts.

ItalianJoe1

You need to start the test, then attempt to start the car. It will crank but not start, as the bar goes across. After a couple seconds you can stop, or run it until the end. It will then show the rpm difference across cylinders.

ItalianJoe1

Also you should connect a charger as battery voltage will drop during the test, resulting in slower cranking as time goes on. It won't really affect cylinder balance but still can change overall speed.

Why are you running a compression test?

Vladds

Thanks.
So to be clear:
Click the green arrow, then key crank the starter, like I was starting the car. As I do this, the timer bar shows up and I keep cranking till the bar elapses?


I don't have symptoms requiring a compression test, but I own the xentry rig and am just checking/learning misc things andi did buy the car used and think it can't hurt to know what's going on.


So I have to get a big charger that can do 30 amps then.

ItalianJoe1

Yeah you should see the result as soon as you start to crank the car.

You should be fine without the charger, Xentry says to connect one anytime you are using it on the car. Technically it warns you if the voltage is below 12.5, which is impossible with the ign on and no charger.

Vladds

Hey, would you answer one more question?


As far as the test of the cam adjusters:
I get to where they conditions are met, temp over 80 Celsius, hold the rev between 2700 and 3500 and for intake cams it shows the angle in a green number, then I click "actuate" and more or less nothing happens, stays green.


Now for the exhausts:
On the left one, stays green at -15 degrees. I click actuate, I get a momentary red of 5 degrees, while the range I think is between -10 and -17 degrees, then it goes back to -15 green


On the right I have a tick. I do the same, before actuating its green at -15, then it drops to red 26 degrees! then goes back to -15 green


Then I get codes for all for cams of unexpected angle, which I think is normal for the testing, I erase them and then I have to do a tech-in, which goes successfully.


But that right cam adjuster is not ok, even if it eventually passes the test, right?

ItalianJoe1

I'd have to see what it is actually doing. If it passes the test it's probably ok, the faults setting during testing are normal. We do see a lot of defective cam adjusters though, so it's possible it is not 100%.

My advice, you are more likely to do harm than good by running all the tests. I'd only search for things you already had faults for or issues with. Some of the tests don't run that well and the results can be inconclusive at best.

Vladds

I finally succeeded with this compression test.


Two things:
You're not really supposed to crank in Xentry, meaning you momentarily turn the ignition key to crank, then let go and it continues to crank.
Only at this point does the duration bar show up and test goes through.
If you keep the key turned on crank, the test won't go through.


The second thing is it helped me to have the batteries connected to the mux, makes it more stable.


Now the test results are allover the place.


I have to connect... something, to support the battery voltage.
I don't have a 30 Amp charger.


Do you guys think it's a bad idea to connect a booster box to the battery? (a Jump box)

biker349

good info. thanks.

Vladds

The booster box stabilized the results.
You should at least run the car for 5 minutes, shouldn't do this dead cold, although there is no temperature requirement.
You know you stabilized it when you get 3 consecutive results with the same configuration of bars, for instance 5 has the highest rpm.
Previously, using just the battery, even after the oil was warmed up, I got all kinds of test results.


So mine passed...

Vladds

At the time I did the compression test, I could not understand the science behind this test (I'm an engineer). The engine is a mechanism. The pistons a rigidized
rotation-wise by the crank to each other, to where no differences of several RPM are possible between them.
The starter cannot selectively crank a piston at the time, the throttle is not per cylinder, (to where you open the intake of a piston at the time and note the overall speed), the gas is whittled for all cylinders at the same time.
So then I figure it out, then some time passes and I bump into this again and by now forgot what the explanation was and have to figure it again from scratch.
So I will save myself the aggravation and post the explanation here.
The ECU does know when each cylinder has its compression stroke, it knows in what position the engine stops, engines tend to stop always in the same position. Although this is a V8, two compressions don't occur at the same time (I wonder if this can happen in a V12).
The engine, as an assembly, slows down its rotation under the starter cranking during each compression.
The duration of time of since a piston is at bottom dead center and until it arrives at top dead center can be timed by the ECU.
The set of engine circumstances of the other cylinders will probably be different for every compression

By example: As cylinder 1 piston begins its compression stroke and is being timed by the ECU for speed of travel, cylinder 2 has an exhaust stroke, 3 has an intake stroke, etc.

So to rephrase in statistic terms,
while cylinder one is timed for compression, there are:
3 exhaust strokes, in other 3 cylinders
2 intake strokes, in other 2 cylinders
2 are around TDC and about to change strokes in the remaining cylinder

However,
while cylinder two is being timed for compression, there could be:
3 are around BDC and about to change strokes
3 intake strokes
1 exhaust stroke

Now since the activity of the starter of turning the engine is affected by what's happening in ALL cylinders and the rotational speed of the engine is measured for ALL cylinders, the would be an equation:
While a compression slows the engine down from turning under starter power, maybe an exhaust stroke where at a portion of this stroke all valves are open, speeds it up.
So ALL events from all cylinders will have a factor, some add, some subtract.
So as we measure Cylinder 1 and determine that it took 3 milliseconds for its piston to complete the compression stroke, we then apply a correction factor based on ... cylinder 3 at the same time was helping because was in exhaust, cylinder 4 was neutral, 5 was helping, etc, with the purpose of being able in the end to accurately compare the traveling time of cylinder 1 under compression with 2 and the rest..


Now the part that is still slightly confusing to me is:
Let's say that in the end, the measurement reads 3 milliseconds for Cylinder 1. How do you take a timed measurement and then present it in rotations per minute?
these can't be starter rotations per minute, I don't think. They have to be engine rotations per minute.
The reality is that it takes maybe a fraction of a rotation of the engine for a compression. So if the circle is divided in 8 and it takes two rotations of the crank for the 4 strokes, then 360/16= 22 degrees, so 1/16 of a rotation.
So the compression of a cylinder is not something continuous that is happening, in terms of the engine rotation.
So taking 3miliseconds/sixteenth of engine rotation and converting measurement units into rotations per minute is incorrect.
16 compressions of cylinder 1 did not happen in one complete rotation. they were discrete events.

If the measurement of each cylinder is converted to more "understandable units", for the purpose of displaying the difference in understandable units my confusion is:
Are these maybe RPM reduction, or RPM?