Probably going to the dealer auctions to look for a good Diesel Mercedes for a cheap amount of money. They are checked out by mechanics and the problems are listed beforehand, but quick question: is there a quick way to do a rough on spot compression test to tell if the motor is shot? What should a good engine do, what should a bad one do? Thanks. Also, any cheap tools I can buy beforehand to check?

You would need a diesel test gage, an adapter to attach to the emgine, a glow plug adapter might be quicker and easier than an injector adapter, you would not have to remove the injector fuel lines if you go the GP route, but it may be harder to access the GPs as opposed to accessing the injectors! A boy scout would be prepared for either! You will also need an assistant because someone has to turn the key to cause the engine to rotate at least eight revolutions to get an accurate reading, and at the same time someone must hold the IP shutdown lever down to prevent the engine from firing! One person can do both if you have a remote starter switch to allow you to hold the shut down and use the starter to turn the engine over.

If you get this far and you get compression readings within specs your probably in pretty good shape. The problen with the 5-Cylinder MB diesels is if the valve clearances are out of adjustment then without the proper valve clearances you could get a reading that will show as low compression on a cylinder or cylinders that is cause by valves out of adjustment! So without knowing that the valves are adjusted properly a low compression reading would be inconclusive as to the cause of the low compression! Could be valves out of adjustment or could mean a valve job or worse a cylinder job! You could check the valve clearance first and if they are within specs then do a compression test, but then you've got to remove the valve cover!

Engine health is better acertained by a combination of compression test and a cylinder leakdown test. If you have the time and the oppurtunity that would give you the best indication, but if your doing all this testing and it comes out good I'll bet others will notice the results and the bidding will maybe get higher!

Do a search and you canfind a couple of threads that will point to diesel compression test sets for $30, you can buy a remote starter switch for $10-15 or make one for $2, and you'll need the usual assortment of ratchets, sockets and the like to remove and reinstall the parts and test fittings. It's a good idea but it is more than a 5 minute process.

You could quickly do a valve cover breather hose block test where by blocking the beather hose and determining how long it takes to cause the engine to die, this test is described somewhere in the threads, the time (in seconds) is supposed to be a fairly good representation of overall engine health, but I have never used it myself, do a search it sounds quick at least.

I don't think the auction folks will take kindly to you removing injectors and/or glow plugs to do a compression test.

Try to start the engine with the glow plug harness disconnected from the relay. If the engine starts within a couple of cranks and there's only an initial puff of whitish gray smoke that smells like charcoal lighter fluid, and the idle is smooth and even, compression shouldn't be a problem.

I don't think this is the only thing you should check but your resources are limited at an auction lot.

Sixto
91 300SE
87 300SDL

and then leave the glowplugs disconnected so everyone else thinks its a horrid piece of junk that has major starting problems. This will probably help your odds and keep others from driving the price up.

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Matt
2003 Jetta Wagon TDI 145,000MI
1999.5 Jetta TDI 287,000MI
2001 Suzuki SV650S 26,000MI
2008 Yamaha Vino 125 11,000MI
http://www.crazymatt.org

I am not sure if the 5 cylinder differs much from the 6...but when my engine was on its way out it would still start readilly without the help of glow plugs(in warm weather)...actually didn't smoke that bad either at startup...it was just when you accelerated that the smell of burning oil and the clouds behind the car would start....ironically :-) at that time fuel mileage was incredible...of course the quart of oil it was burning every 200 miles more than made up for the fuel savings


Warren
1992 300SD 142K
Columbus Ohio