Compression Test: Diagnosis and Results

Hello,

More recent TDI engine control units offer corresponding measurement values for a compression test, which can likely be triggered via diagnostics.
Has anyone used or analyzed this before?

Best regards, Rainer.

Mercedes has already integrated such a function into the EDC system for its CDI engines for over 10 years.
In my experience, it's completely unreliable. Do it ten times on the same engine and you'll get ten different results...

I didn't know that this was also happening at VAG now.

TDI-GTI-4-Motion wrote:

In my experience, it's NOT very reliable at all. Do it 10 times on the same engine, and you'll get 10 different results...

Unfortunately, I had expected that one could identify cylinders with no compression in this way. Absolute compression values in bar are unlikely to be obtained without the appropriate sensors, but sometimes it's helpful to know whether a cylinder has no compression or if there's simply an issue with its injection.

According to the , such a function is already present in all diesel engines with internal glow plugs. The function is activated through error-driven diagnostics.
The delta is likely determined by measuring the ignition timing.

Personally, I was thinking more along the lines of a cylinder deactivation test and a speed drop test with the idle stabilization system disabled.

Unfortunately, our conversation was interrupted by a customer... I will follow up with him again.

Real values can only be obtained with corresponding sensor technology.

All information is provided without guarantee .

That happened before, back then. They analyzed the current draw of the starter motor.
If the laser line ratio (LLR) is not reliable, what other methods are available besides direct pressure measurement?
hg
Herbert.

Quote:

If the LLR is not informative, what else is there besides direct pressure measurement?

From the timing of the ignition for each cylinder (during startup and without fuel injection), one could actually derive at least a delta p... I think. Especially, this would rule out any issues with the fuel injection system.

Slow combustion, a specific ignition timing, and improved pressure increase due to the start of combustion can only be achieved with fuel injection.
In spark plugs with an integrated combustion chamber pressure sensor, an electrical measurement value for the cylinder pressure is inherently available. In injection systems equipped in this way, the entire pressure profile is included in the electronic control unit (ECU) regulation.
However, the measurement range extends to approximately 200 bar, with corresponding resolution.
When taking measurements without injection, i.e., only cranking the engine with the starter, you get correspondingly inaccurate values. However, it should still be possible to make a qualitative assessment similar to what can be achieved with a load lambda measurement. Of course, in this case, errors from the fuel injectors are eliminated.
hg
Herbert.

Quote:

Slowly, a spark advance, better pressure increase due to the beginning of combustion, can only be achieved with injection.

Of course .

Gemeint war aber der zeitliche Abstand, bzw. Delta, der einzelnen Zylinder untereinander beim erreichen des OT vor dem Arbeitsspiel, bzw. am Ende des 2. Beats.
I thought that was self-explanatory .

If you want to determine the actual compression pressure, you must measure it.
If I've correctly understood Rainer's post, the main point is to be able to distinguish whether there's a problem with the injection system or if the engine is suffering from compression loss... rather than focusing on the actual pressure itself.

Sure.