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Over many decades of service a W123 diesel engine is put through a lot. Over time the piston rings and cylinder walls will not seal against each other quite as well. It's inevitable. However, how much compression blow-by you get in to your engine's crank case depends a lot on how well the engine was maintained over it lifetime.

One quick and simple test for this is to check the engine for blow by while it's running by following the steps in this guide.

If you find that the engine has substantial blow by, don't necessarily fret over it! You'll want to follow up with a full compression test as that's the only scientific way to confirm the health of your engine. This is just a starting point.

  1. Open the hood.
    • Open the hood.

    • Remove the oil cap.


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  3. With the cap removed, start the engine.
    • With the cap removed, start the engine.

    • Now watch as the engine runs. Check for smoke coming out of the oil fill hole.

    • The video here shows little to no smoke; this indicates low blow-by.

    • Wisping or small puffs of smoke indicate normal blow-by for a higher mileage engine.

    • If it's puffing like a steam locamotive, you'll want to investigate further!

    • Then, as the video shows, set the oil cap back on without tightening it.

    • If it just wiggles a bit, you're in pretty good shape.

    • If it dances around, and hops up and down, there's some blow-by but probably nothing to get excited about.

    • If it is hard to keep it on, for risk of taking off a finger, there's an issue!!


If you find a lot of blow by, follow this up with 1) a valve adjustment and then 2) a compression test to verify the health of the engine.

15 other people completed this guide.

Nicolas Siemsen

Member since: 12/06/2013

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79 Guides authored


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Great test. Gary

gvonneida - Reply

interesting, my 81 300td has near 300k miles and likes to blow oil around the cap if i unscrew it, anyone have suggestions on that?

Conner Ketchum - Reply

Hi Conner. Good question. This "tends" to mean that the crankcase is seeing excess pressure which "tends" to indicate that pressure is finding its way out past the cylinders beyond the normal amount in these diesels. However, it does not necessarily indicate a problem per se.

So take it for what it is, a potential symptom, and go to the next step which is to perform a compression test on your engine. A compression test is the only way to truly know if an excessive amount of pressure is being lost during the compression stroke. There are two things you want to test for in this - one, that no individual cylinder is below the minimum compression specification listed in the factory service manual and two, that no two cylinders are too far apart in terms of compression again based on the specs listed in the FSM. I don't have these numbers handy but they should be easy enough to find with come searching.

Nicolas Siemsen -

Hi is there a difference between lateral movement and vertical movement. My cap gets a nice rock back and forth but nothing up and down. No smoke visible at all!

brenton.tylerc - Reply

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