i was perusing car lots the other day after work and came across a decent looking 83 300d...i didnt even want to talk to the guy about it, i just wanted to give it a look over...but, you know he just had to give me the whole damn history of it and tell me how much i needed it and "how long these diesels last", blah blah blahhh....so, he goes on to tell me that theyve done a lot of tests on the engine, including a blow-by test....i asked him if theyd done a compression test on it...he says, "yeah, thats what a blow-by test is, a compression test." i told him that it was called a leak down compression test...then he says,"well, on other cars thats what its called, but these diesels are different and a lot more complicated." then i asked him to follow me to the other side of the building, where my car was parked....we just got into view of it and you could hear the engine still running and i said," you see that? i just pulled up in that car." the guy didnt even say a word, he walked off....

now...my ego got the best of me with this guy...i really had no clue what i was talking about, but im pretty sure he knew that i knew he was full of crap....tell me, so im a little better armed should this come up again....whats the difference between a blow-by test and a leak down compression test?

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1985 300SD 192,000miles...Astral silver/ Grey MbTex

1983 300D 194,000miles...White/Tan MbTex...my newest addiction

Leak down compression test...is two parts..
#1 the actual compression test...
#2 gets a figure on percentage of leakage and you can listen as to weather its valve or cylinder leakage.

Blowby test mearures the volume of blow-by gasses that exit from the crankcase by mearuring volume exiting at the valvecover vent.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

two seperate tests for engine health.

#1. Compression test = dynamic cylinder compression reading.
http://64.78.42.182/sweethaven/MechTech/Automotive01/default.asp?unNum=2&lesNum=7&modNum=6
http://automotive.mytelus.com/automotive/tips/view.spy?artid=593

#2. Cylinder leak down test = Cylinder Leakage Tester gauges the amount of leakage in an engine cylinder, and can help pinpoint the source. When performing a leak down test, it is best used to compare a suspect cylinder to a known good cylinder on same engine.
http://www.jpmagazine.com/techarticles/73178/index4.html


http://www.findarticles.com/p/articles/mi_m0BQA/is_12_80/ai_81104065

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ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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