78 300D low compression - Mercedes-Benz Forum

 
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post #1 of 5 (permalink) Old 01-23-2012, 02:04 PM Thread Starter
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78 300D low compression

I bought a 78 300D with 274K miles about 3 months ago. Now it will not start. After replacing the glow plugs, injectors, adjusting the valves and checking the fuel system, I have narrowed it down to low compression. My compression is below 200 in all cylinders. A shop checked compression a few months ago and said is was very good.

What could cause such a loss in compression in all cylinders? Is it more likely something with the head, or piston rings?

Last time I drove it, it ran fine. The next morning it would not start.

Thanks
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post #2 of 5 (permalink) Old 01-24-2012, 04:03 AM
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Did you get the compression readings from the shop? "Very good" readings could be simply the fact they are not familiar with diesels. A shop dealing with gasoline powered engines would be impressed with 250 p.s.i. whereas in the diesel world it is actually bare minimum.
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post #3 of 5 (permalink) Old 01-24-2012, 02:51 PM
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Did you check the compression with the engine warm (around 80º C)?
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post #4 of 5 (permalink) Old 01-24-2012, 05:58 PM Thread Starter
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Thanks,

The shop said it was close to 400 on all cylinders. They specialize in old mercedes.

The engine was not warm when I did the compression check, but it should be getting more than 160-200 when cold.

I am pulling the head this week. I have a bad feeling it's the piston rings, because the engine had a lot of blow by.
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post #5 of 5 (permalink) Old 01-24-2012, 10:56 PM
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The FSM says to check the compression with a warm engine. There can be 10 bar (145 psi) difference between cold and warm. Compression does not suddenly drop from 400 to 200.
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