My 1994 E420 with 150k miles, started and ran powerless and rough after standing for about 2 weeks. Ran fine before that. I had some rotten egg smell, but no smoke of any color. Also the water pump started leaking. I removed the water pump, and while waiting for the new part to arrive, I also removed all spark plugs, all looked nice and brown, except one that was black. Next, I did a compression test on the cold engine.

Cyl PSI
8 60
7 20
6 60
5 110
4 40
3 20
2 50
1 100

I read elsewhere in the forum that my compression should be around 200, but in any case there are huge differences and very low numbers by any standard. I checked the valve timing by inserting the pin into the right outer camshaft. the timing nicely lines up with the 45 degree mark on the harmonic balancer. (I reinstalled the old water pump and balancer for this).

Now I'm thinking it might be the head gasket, but it is unlikely that both sides blow at the same time. I don't know what I'm missing here, and I hope someone can help with some good advice.

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2004 E500 4Matic 138,000 miles

I re-ran the compression test and got different results for no apparent reason. Where it was 20 psi, now 120 psi. Still not 200 psi, but maybe because the engine is cold and has been sitting for many days, and the oil may not be currently around the piston rings. I don't know.

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2004 E500 4Matic 138,000 miles

Try changing the motor oil, old oil can give lower compression readings.

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.

Your compression gauge / method isn't reliable enough. Also, be sure to prop the throttle wide open, air is needed for a good compression test.

Do a leak down test next, this will tell you much about engine condition.

The single black plug is either a leaking injector , failing plug wire.

A head gasket is very low on the list.

I don't think low compression is your problem. I have not heard of an M119 engine with low compression yet; and the head gaskets are generally not an issue with this engine unless there was a massive overheat.

Have you checked the caps and rotors for corrosion? I'd start there. Also, make sure you have the correct Bosch F8DC4 plugs (absolutely no substitutes!).

If you want to measure something, I would check the resistance of the plug boots to make sure they are all about 2 K Ohm +/- .2 K Ohm. Start from the wire on the distributor caps all the way to where the boot meets the tip of the spark plugs. It should be about 2 K Ohm.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Andras:

There is a better than even chance that the hydraulic pedestals (compensators) have leaked down and are very slow to refill with oil. As a consequence, the valve timing and opening have been substantially reduced both in degrees and in lift. Less air drawn in during the intake phase results in less pressure during compression.