Do i need to do a compression test before i buy? - Page 2 - PeachParts Mercedes-Benz Forum

MagicBus · #16 07-09-2016, 10:43 PM

Quote:

Originally Posted by ah-kay

Doing a compression test on a gas car is more straight forward then a diesel. No heat shield to replace ( unless it is done from the GP hole ), no injector line to remove etc. it is a 15 minutes job compared to a 1+ hr job. Some V6 front wheel drive may take longer. if I were the seller, I would decline. If I were the buyer then I will look for tell tale signs and take my chances.

Well aware. I didn't mean to suggest that the testing my Subaru and an OM616/OM617 were the same. I'd also decline.

In my case, I think the seller agreed to let me do it because I let him know I was serious about buying the car.

97 SL320 · #17 07-10-2016, 06:23 AM

Quote:

Originally Posted by leathermang

So you are measuring the load on the starter ?

Measuring load of starter on battery. Can pick large pos or neg wire near battery for easy access, no need to get near starter. The key is to record data in strip chart form so it can be viewed.

funola · #18 07-10-2016, 10:19 AM

Quote:

Originally Posted by 97 SL320

Measuring load of starter on battery. Can pick large pos or neg wire near battery for easy access, no need to get near starter. The key is to record data in strip chart form so it can be viewed.

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.

ambush276 · #19 07-10-2016, 11:49 AM

Quote:

Originally Posted by 97 SL320

To expand on the low end buyer. If someone comes to me says the right things and has the proper attitude, I'll make sure the down on their luck person gets the best value.

It's the "I gots no money because I have poor planning skills" person I avoid. Typical signs are lotto scratchers in the car , smokes / chews , has anything more than a basic cell phone, the "I should not have to work for anything because the world owes me" attitude.

There is a way to do a non invasive compression balance test. Place a clamp on amp meter attached to a recording oscilloscope on the battery cable and crank the engine. You should see a current spike for each compression stroke. On a 5 cyl if you see a low spike every 5th place, there is a cylinder with lower compression than the other ones. This won't tell you how low however.

it would be awesome if you have a pic of a chart showing that lower dip. That is a really neat trick.

ah-kay · #20 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 · #21 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 · #22 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 · #23 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 · #24 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. "

97 SL320 · #25 07-10-2016, 06:21 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.

If I remember correctly some 20 years ago, there was a gas engine test system that used this along with a cylinder balance test ( dropping out the spark on each cylinder . ) They used the coil wire and #1 plug wire to sync everything up.

Doing a cylinder balance by selectively shorting the ignition and watching for an even RPM drop is old hat. The starter draw balance isn't that common but is effectively a reverse balance test . I don't have any screen shots.

As for another post asking for bandwidth, any scope can see this since we are looking at very slow data. A 4 cyl has 2 compression strokes per rev, using a 200 RPM cranking speed we are looking at 6.67 Hz.

Remember , this is just a quick check.

funola · #26 07-10-2016, 07:02 PM

Found a youtube vid on such a test demo. They pulled a spark plug simulating a dead cyl with zero compression and it shows up on the scope waveform. With a 20 % low compression cyl, I wonder if it will be as easy to discern.

https://youtu.be/kWvII9Vsdpw

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