Do i need to do a compression test before i buy? - PeachParts Mercedes-Benz Forum

ah-kay · #1 07-10-2016, 12:21 PM

Quote:

Originally Posted by funola

That is a cool trick! Where did you hear about it and do you have a pic of a storage scope waveform you can show us? I picked up a storage scope recently and will give it a try sometime.

It is a trick to check for consistency ONLY. It does not tell you much. It does not tell you which cylinder unless you rotate the engine to compress at a pre-defined cyl, say #1. It does not tell you compression numbers, if low, how low or how high? The equipment is expensive. The low compression could be due to valve adjustment so there are still a lot of unknowns. Just fire up the engine and do the simple checks.

leathermang · #2 07-10-2016, 12:29 PM

Quote:

Originally Posted by ah-kay

It is a trick to check for consistency ONLY. It does not tell you much. It does not tell you which cylinder unless you rotate the engine to compress at a pre-defined cyl, say #1. It does not tell you compression numbers, if low, how low or how high? The equipment is expensive. The low compression could be due to valve adjustment so there are still a lot of unknowns. Just fire up the engine and do the simple checks.

X - 2

funola · #3 07-10-2016, 01:14 PM

Quote:

Originally Posted by ah-kay

It is a trick to check for consistency ONLY. It does not tell you much. It does not tell you which cylinder unless you rotate the engine to compress at a pre-defined cyl, say #1. It does not tell you compression numbers, if low, how low or how high? The equipment is expensive. The low compression could be due to valve adjustment so there are still a lot of unknowns. Just fire up the engine and do the simple checks.

If the engine starts within a quarter rev of the starter and not having to crank it for 30 sec, you can assume it has good compression.

If the test can accurately discern uneven compression in cylinders, that is a goldmine of information for a in the field non invasive test that is quick and easy. It would determine for me a buy or no buy.

leathermang · #4 07-10-2016, 01:43 PM

Quote:

Originally Posted by funola

If the engine starts within a quarter rev of the starter and not having to crank it for 30 sec, you can assume it has good compression. If the test can accurately discern uneven compression in cylinders, that is a goldmine of information for a in the field non invasive test that is quick and easy. It would determine for me a buy or no buy.

If you have 375 in four cylinders....and 100 in one cylinder it will start right up...
the test is non invasive but almost no one has a scope in the first place...and even fewer I think which have ' recording 'scopes.. (?)....

But just for the record.. how does one set up the scope dials to do this test ? What is the least bandwidth a scope needs to do this ?

97 SL320 · #5 07-10-2016, 06:08 PM

Quote:

Originally Posted by ah-kay

It is a trick to check for consistency ONLY. It does not tell you much. It does not tell you which cylinder unless you rotate the engine to compress at a pre-defined cyl, say #1. It does not tell you compression numbers, if low, how low or how high? The equipment is expensive. The low compression could be due to valve adjustment so there are still a lot of unknowns. Just fire up the engine and do the simple checks.

What part of the first and last sentence in my post 14 didn't you understand?

"There is a way to do a non invasive compression balance test. ". . ..
" This won't tell you how low however. "