Blow-By Tests

[BillGrissom]

A quick poor-man's compression test I posted to youtube:
https://www.youtube.com/watch?v=-wUdHjN5tq8

I've been prepping my 1984 300D to sell and compared with my 1985 300D. I saw similar tests on youtube. This one is more quantitative than judging "oil cap dance". Variance might come from how much force is required to pull the shutdown lever in the injection pump and if you have any leak paths to outside in the crankcase or injection pump seals.

Both engines seemed to pop off about the same pressure when I released my thumb from the vent port, so perhaps their IP's cut-off at the same force from the shutdown valve. To quantify, I could connect a vacuum pump to each shutdown valve and measure what vacuum shuts off the engine. Would also be good to run this blowby test right after measuring cylinder compressions to better judge how well it works as a stand-in, though likely mine haven't changed much since the last measurement. I'll let someone else take it to that level when doing so.

Note how well back-lighting shows particles in the blow-by gas. I learned that from long-ago research with liquid sprays (Mie Scattering). You barely see anything with back-scattered light. Might be a way to search for leaks in crankcase and IP seals (partially plug vent, hold flashlight behind suspect areas), since oil leaks can be hard to pin down.

[vwnate1]

Thanx Bill ;

Interesting, I'd be concerned if that much smoke was coming out the vent .

The car looks great .

[Rooster300SD]

Thanks for showing us the blow by test.
I tried it today and it took the engine 15 sec to die, so I'm happy with that.
Would be a good test to use when looking to buy a car.


Jeff

[BillGrissom]

Originally Posted by vwnate1

Thanx Bill ;

Interesting, I'd be concerned if that much smoke was coming out the vent .

The car looks great .

Concerning, but I haven't noticed any difference in mileage between my 1985 w/ like-new engine and 1984 with half-worn engine, nor in how easy they start on winter mornings, though <32 F isn't common here (have orange trees). As long as the air filter isn't getting gunked with oil, you probably wouldn't notice the blow-by, and just lets you predict how many more miles your engine might go before needing a rebuild (new cylinder sleeves then honing, and piston rings).