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Old 02-10-2014, 01:53 AM
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Join Date: Aug 2013
Location: South Bend, IN
Posts: 80
Compression test questions....

Hey all,

Before I get my turbo rebuilt on my 81 300SD (185,000 miles), I am going to do a compression test. There are a few things that I am not clear on that I hope you guys could help me out on, as always.

I do have some blow by, but I am thinking this is caused by the turbo which has a lot of shaft play and is leaking A LOT of oil.

If my compression test results are good, would I need to do a leak down test or should I only do a leak down test if my compression tests are bad?

What exactly is the benefit of a "wet" compression test, and could I use MMO instead of engine oil for a wet compression test and/or leak down test? I figured since I'll have the injectors out I could pour some MMO to clean the cylinders/pistons and free any stuck rings if there are any. Speaking of having the injectors out, I figured I'd replace the nozzles and get them pop tested, but that's a different topic .

I appreciate your input.

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Old 02-10-2014, 02:54 AM
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Join Date: Sep 2007
Location: Long Beach,CA
Posts: 36,735
When you remove the Turbo and the Manifolds look inside the Intake and Exhaust ports and see if either Exhaust or the Intake ports have Oil in them. That will give you an idea where the Oil is coming from and going to.

I have more Blow-by than I would like as do quite a few others. The consensus is that if the Blow-by is not having a poor effect performance or anything else just drive the Car. (If you have enough Blow-by or a restriction in the Vent system it will shut your Engine down or has been know to blow the front Oil Seal out or push the Dip Stick out.)

If your Compression Test comes out well with in a normal range and they different pressures of the Cylinders does not vary too much I see no need to do a leak Back Test.

If Your compression test is close to being too low or the Cylinder pressures are too unbalanced then you might want to do a leak back test and hope the low compression issue is being caused by the Valves.

Few people do it this way but the Compression test is supposed to be done with the Engine at operating temp. That is what the specs in the book are geared to.

I am pretty sure if you dump Marvel Mystery Oil into the Cylinder and Crank the Engine with the Compression Gauge on you would end up igniting the Marvel Mystery Oil and the Cylinder Pressure is going to ruin your Compression Tester.
There is a tiny chance that could happen with Engine Oil.

If you just want to see where the leaks are you can simply use your compression Tester adapter hooked to Filtered Water free compressed Air to pressurize the Cylinder and listen to see if you hear Air leaking past the Valves or into the Crankcase.

If you go to the Harbor Freight site and click on the Leak Back Tester and look close you will see that you can access the PDF Instructions. Note that the Leak Back Tester is setup for Gasoline Engines and you would need to find a way to hook it to your Compression Tester adapter.

84 300D, 82 Volvo 244Gl Diesel
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