Need Definitive OM617 Compression Test Procedure - Page 3 - PeachParts Mercedes-Benz Forum

vstech · #31 11-23-2010, 10:44 AM

Quote:

Originally Posted by bustedbenz

I understood most of that diagnostic procedure posted above with the air fittings and such, but I'm confused on one point. What does the cranking speed have to do with compression?

Compression is the result of squeezing a fixed volume of air (what the cylinder can hold) into a fixed space (The remaining space when the piston has reached the very top of its stroke), true? The air volume is going to be the same, the cylinder capacity is going to be the same, and the top point of the piston's travel is going to be the same no matter how quickly or slowly it reaches that point. So why is having steady cranking speed so important for a compression test? Theoretically it would make the same compression even if you turned the crank with a wrench instead of the starter -- UNLESS you've got wear letting some of that compression pressure leak down as it's produced.

What am I missing?

yup. all engines have leakage in the rings and the valves. a wrench would NEVER build full pressure.
also, the fixed volume of space compressed into the remaining space is the basics for a compression ratio evaluation... stricly a measurement thing.
a compression test is measuring the pressure of the compressed gas to determine wear and leakage.
550 is AMAZING pressure,
300 is minimum acceptable and all pots need to be within 10% of one another or the motor will not operate properly
steady rotational speed is needed to maintain the pressure to the gauge.

JHZR2 · #32 11-26-2010, 11:56 AM

As far as the HF kit goes, I bought one and here are the pictures:

The kit:

The numbered image of the kit:

The key:

The two closest 1.25 thread (does it matter 10 vs 12x 1.25?), next to a new beru plug:

gastropodus · #33 11-26-2010, 12:50 PM

If you're doing the compression check using the injector holes I recommend disconnecting the glow plug wiring at the fuse; this will cut down on the current draw from your battery.

Kurt

JHZR2 · #34 11-26-2010, 01:12 PM

I'm planning to do mine through the GP holes...

bbboomer · #35 11-26-2010, 07:04 PM

We can go on and on over the value of a compression test and if all the pressure tests are necessary and valid. Personally I agree with the validity but not necessarily the need.
What I feel is most important is what the compression test is really best able to tell you.
Simply put it is this... a comparison of one cylinder to another.
Giving this as a measure it is important to measure each one the same.
Weather that be buy time or number of times the piston rotates keep it the same and you will have a good idea of overall comparison.
If overall it is good you are fine, if one or two are low you probably have a problem.
For all but the most anlytical that should give all the info you need.

Sev · #36 11-26-2010, 09:52 PM

everyone here should do a compression test on their om617 using the same harbor freight tester and using the same method and post up their results. curious to know how my engine compares. i'd be willing to loan my hf compression tester out, too

JHZR2 · #37 11-27-2010, 08:09 PM

Well mine is the hf tester shown above (93644), and I got the following cold:

Cyl 1 380
Cyl 2 385
Cyl 3 375
Cyl 4 355
Cyl 5 375

Number 4 I did have to do twice as I couldn't get a good seal of the parts the first time... The second time pressure held, but I'm not 100% sure it was perfect.

So now for the discussion, what does cold tell me versus doing it hot (those numbers are cold), and what am I going to miss out on if I do not do it hot (it would be a pain to remove the injector lines and all for a while...

These were done through the GP holes, btw.