Tip:
Highlight text to annotate it
X
Certain engine faults can result in decreased output power,
and sometimes these faults are difficult or impossible to diagnose with a scanner
Often difficult to diagnose problems are a result of incorrect engine mechanical timing
A common way to isolate this kind of problem involves disassembly which can be time consuming and expensive
Let us discuss an alternate method to physical disassembly. The car in our case is Daewoo Nexia with the 1.5i 16V DOHC engine
The car owner complained of lack of power and lower gas mileage
Connecting the oscilloscope to the car is very simple
Connect the USB Autoscope to the computer’s USB port via the USB A/B cable
Connect the ignition adapter to the USB Autoscope via its dedicated cable
Connect the ignition adapter’s power leads to the vehicle battery
It is important to maintain the proper polarity. Red clip is for positive, and the black clip is for ground
Disconnect the ignition cable from cylinder number one's spark plug and connect it to the external spark tester
The black capacitive probe will synchronize the measured data to the spark in the first cylinder,
which allows us to measure the ignition timing advance
Connect the probe to the ignition adapter and clip it to cylinder number one's ignition cable
The pressure transducer will measure the internal pressure in the cylinder, but without the combustion
Connect the signal cable of the transducer to the first input of the USB Autoscope device
Connect the power leads to the vehicle battery, using the same points as the ignition adapter power leads
We have to unscrew the spark plug for the first cylinder and put the transducer in its place
The spark plugs for this engine are fitted deep in the block,
so we must use the pressure extender
Electrically disconnect cylinder number one's fuel injector. During this test no fuel should enter the cylinder
Start the USB Oscilloscope software and choose the "Px" mode
In the plugin user interface choose the sensor type "Px+Longer",
because we are using the pressure extender
Start the engine and let it idle
Start recording data,
and within several seconds gradually accelerate the engine to 3000 RPM
Release the throttle and wait until the engine returns to idle
Abruptly push the gas pedal to the floor,
and when the RPM reaches 3000, release the pedal
Stop recording data
Now let us check the acquired data of the internal pressure in the cylinder
The program automatically identifies the divergences at key points in the chart
Points D and E shifted to the left implies over-advanced exhaust valve timing
Point I shifted to the left also implies over-advanced intake valve timing
Let us go to analyze the acquired data using the script "Px" written by Andrey Shulgin
Choose "Run the script" from the menu
The script outputs several tabs
The "Report" tab contains the text part
The following values are shown here:
Measured compression ratio 10:1
According to the engine specifications, this value is on the upper tolerance limit
If the compression ratio in a particular cylinder is lower when compared with others,
it could mean a bent connecting rod as the result of
ingestion of a substantial amount of liquid into the cylinder
If the compression ratio in a particular cylinder is higher, it could mean
an increased amount of carbon deposits in the cylinder due to internal oil consumption
In the same tab there is also the value 17% for
the "Compression loss in the power stroke"
An engine in good mechanical condition would normally be between 10% and 20%
If the reported value exceeds the limit, it could point to a compression leak in that cylinder,
such as worn out piston rings or damaged valves…
The "Amount" tab shows the chart of the gas amount in the first cylinder
in correlation to the piston position and stroke
during the first five engine cycles
The green portion shows the cylinder filling process during the first intake stroke
The beginning of the blue portion indicates the end of the filling process
The beginning of the horizontal part of the blue portion denotes the end of the intake stroke, at which point the intake valve is closed
The yellow portion corresponds with the piston power stroke
The pressure data is recorded without the fuel combustion,
therefore the horizontal part of the yellow portion
represents, in fact, the gas expansion in the cylinder
Both valves are closed during this stroke,
therefore the amount of the gas in the cylinder is virtually unchanged
There is a negative pressure in the cylinder at the moment of the exhaust valve opening
Hence the gas from the exhaust manifold
is sucked back into the cylinder
The end of the horizontal part of the yellow portion
denotes the beginning of exhaust valve opening
The red portion of the gas amount chart
depicts the process of expelling the gas into the exhaust manifold
The beginning of the horizontal part of the blue portion
and the end of the horizontal part of the yellow portion
should overlap at 145 degrees
on most production engines
In fact the exhaust valve started to open at 130 degrees after Top Dead Center (TDC), which is 15 degrees advanced
Intake valve is 15 degrees advanced, too
The actual shape and position of the red and the green chart portions within the valve overlap section is typical for advanced camshaft timing
The "Timing phases" tab shows the gas diagram as well,
but now in correlation to the crankshaft angle
The green chart portion depicts the gas intake into the cylinder during the first piston stroke
The beginning of the blue portion denotes the end of the filling process
This point in the blue portion shows the intake valve has closed, thus ending the intake stroke
The yellow portion corresponds with the expansion stroke
The end of the yellow portion depicts the backward gas circulation from the exhaust manifold to the cylinder
The end of the yellow portion corresponds with the exhaust valve opening
The red chart portion depicts the expelling of the gas from the cylinder into the exhaust manifold
In the chart we can see
that the intake valve has been closed at 160 degrees before TDC, which is 15 degrees advanced
The exhaust valve started to open at 130 degrees after TDC, which is also 15 degrees advanced
We can then see that
the timing is asymmetrical in this case
The actual shape of the chart during valve overlap is typical for the advanced engine timing
The "Ignition timing" tab shows the chart for ignition timing
according to engine speed and load
The higher the engine load, the warmer the color will be
Ignition advance was 40 degrees at 3000 RPM without load, and 20 degrees on the loaded engine
The blue chart portion depicts the ignition timing advance at the minimal load
in deceleration mode
The "Exhaust" tab shows the amount of engine work used to expel the gas from the cylinder
The red line is the acceptable limit of power loss during exhaust
If the chart line is over the red line,
it indicates a restricted exhaust path
The diagnostic conclusions are:
The engine mechanical timing is incorrect,
the intake camshaft is 15 degrees over advanced,
which is actually one sprocket on the camshaft gear
The exhaust camshaft is 15 degrees over-advanced as well,
the compression ratio is a bit higher,
the cylinder leakage is within the limits,
the ignition timing is properly controlled by the Engine Control Unit,
and the exhaust gas back pressure is within tolerance limits
The mechanical timing fault was confirmed during disassembly
and repair
Let us repeat the timing check with the Motortester after the repair
Open the USB Oscilloscope application and select the "Px" mode
Start and idle the engine
Start recording data
Smoothly increase the engine to 3000 RPM
Release the accelerator, to close the throttle
Abruptly fully open the throttle,
and when the RPM reaches 3000, release the pedal
Stop recording data
All the data in the "Report" tab is the same
Now we can see on the gas diagram that
the beginning of the horizontal part of the blue portion
and the end of the horizontal part of the yellow portion
are both situated at 147 degrees,
which is a common value for most production engines
The chart now has the correct shape in the valve overlap area
It is apparent when you look at the chart that now
the timing is symmetric
The chart now has the correct shape in the valve overlap area
The ignition timing chart shows the correct operation of the ignition timing subsystem
The ignition timing advance is controlled according to the engine RPM and load
The exhaust loss chart is unchanged as well,
and lies below the red line
The car owner drove the vehicle and confirmed that the engine accelerates much better
and after some time he also confirmed that the fuel economy has increased
The owner declined to further investigate the cause of the slightly higher compression ratio