AFN: Welcher Motorschaden liegt vor ??? | Posts 16+

Deleted as requested.

diesel.gustav wrote:

I find the term "measurement" to be an exaggeration when used to describe an uncontrolled, non-reproducible descent or a manual rotation performed using a handwheel.

Hi Gustav,
Okay, you're probably right about the term "measurement."
But it is definitely reproducible - with the same car, under the same load, on the same hill... what exactly do you mean by "uncontrolled" here?

Quote:

Neither of the methods nor its results is useful because it is too far removed from what is actually relevant in real-world engine operation.

Given that the "revolutions" (likely referring to engine speed) used for a standard compression test differ by a factor of approximately 20 to 30, the results are certainly only partially comparable to those obtained with a running engine.
But I cannot accept this absolute statement that "there is nothing to be done."
If a TDI, for example, is only "running on 3.5 cylinders" due to a slightly leaky fuel injection line that was not noticed , the test can quickly and almost free of charge provide an indication of whether there is a serious compression problem or not.

This tip is intended for situations like these and similar ones.

Quote:

A true compression or pressure loss test through the glow plug or injector holes provides clarity.

Logically, that's why it's located right behind the test.

I've actually just made some changes to the text to hopefully make the idea and the process clearer.

Deleted as requested.

diesel.gustav wrote:

The process is not suitable. It may be that you will not accept that. But I'm also not a missionary who wants to convert you. Then it just is what it is! Bitte gib den Text an, den du übersetzt haben möchtest. Because the compression and combustion pressure plays a significant role in sealing the piston rings and the valves. If you look very closely at how the interplay of pressure in the cylinder, the geometry of the piston, the ring grooves, the piston rings, the piston rod, the cylinder, and also the formation of the oil film affects the sealing, then you might understand me.

Hello Gustav,

The "Hoppeltest" (a test involving slight engine movement) with its artificially low compression reading cannot account for the fact that normal compression pressure, for example, increases the contact pressure of the piston rings, thereby improving compression.
If these processes are, for example, disturbed by deposits in the area of the rings and no other defect is present , the "Hoppel test" may actually perform better than a regular compression test.

However, even smaller damages, such as holes in the piston, significant scoring on the cylinder walls, broken piston rings, and leaky valves, will typically be noticeable during a "knock test" – and, in my opinion, will also be confirmed by a proper compression test.

So, the question is, what is the percentage of engine defects in which...
-> on the one hand, the function of the piston rings, etc., is "subtly" impaired, which only becomes apparent during a regular compression test.
-> On the other hand, both testing methods ultimately provide the same basic conclusion: that at least one cylinder is significantly weaker than the others. I suspect that this will be the case in the vast majority of instances.

Considering this aspect, I believe the reliability of the "Hoppel test" is good enough that it can be used as a low-cost initial check and to confirm suspicions before performing a proper compression test, although it is not 100% reliable.

But that's something a standard compression test doesn't offer either:
For example, significant deposits in one combustion chamber and a slightly leaky valve in the same cylinder can compensate for each other during the initial cranking speed, resulting in that cylinder producing a value that is practically the same as the other cylinders.
In the "hop test," however, the leaky valve would "take longer to act," thus revealing the defect through a shorter time interval between the hops.

Putting aside the question of proselytizing or not: what do you think about it (from a more objective perspective)?

Deleted as requested.

diesel.gustav wrote:

For me, a counter-question would arise now: How many diesel engines have you disassembled and examined for features relevant to compression? I am not asking this question, and therefore I do not expect an answer.

Hello Gustav.

Despite that, the answer, which you probably already suspect, is zero.

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My intention is not to tear down your expertise or your articles, but I think that forum participants expect concrete help with their questions and problems, and therefore, in my opinion, you should remove the "Hoppeltest." However: it is your decision. With that, I would like to conclude this topic for myself.

If a compression test were as straightforward for diesel engines as it is for gasoline engines, I would simply refer to that.
But because of the effort involved in expanding the ESD system (or at least the spark plugs, where fuel injector lines often get in the way), I want to give potential buyers a simpler guideline to help them initially assess whether a compression test would be worthwhile.

I will revise the passage again, add another "quick test," and then I would like to ask for your opinion (and also the opinions of other practitioners with relevant experience).
Okay, I'm ready. Please provide the German text you want me to translate.

Hello everyone,

Actually, changing the spark plugs wasn't a big deal (I've changed them before).

With a 1/4-inch extension and a long 10mm socket (due to the high glow plug connections), it took about 10 minutes to remove them.
By the way, I secured the spark plug connectors with a cable tie, and then you can pull the connector straight out from the ends of the cable tie without tilting it. It is particularly helpful for reaching those who are difficult to reach.

However, I'm missing a pressure gauge or something similar. What is the typical price range for such a pressure gauge?
And with that, we will only be able to assess the difference values. In any case, a specific VAG device is required as a prerequisite for measuring absolute pressure.

I'll probably only be able to work on my car next weekend, as I have a lot of other things to take care of in the meantime.

Regards,
Ernst.

Hello fellow diesel enthusiasts,

I found the error on the engine. The cause was actually quite trivial:

During the injector replacement, I tightened the clamping brackets that secure the injectors in the bore relatively according to the specifications (10 Nm + 25 degrees of rotation). However, over time, the solenoid valve injector in the 3rd cylinder apparently decided to free itself from the clamping. When this cylinder was compressed, exhaust gases leaked out of the valve stem, which in turn caused those unpleasant noises at low RPM.

I noticed this because, on the weekend, when I was listening to the engine at low speed, I thought I could hear the third cylinder misfiring again. Then, when I did a quick test on each cylinder individually, I could clearly smell exhaust fumes coming from the third fuel injector. The clamp has loosened to the point that I could even raise the nozzle by about 1mm...

Looking back, I have to admit that the engine was still relatively warm when I replaced the electric injectors at the time. Then the 10 Nm torque specification is probably no longer valid, right?

Thanks again for the tips from the forum!

Regards,
Ernst.