Vintage Diesel Car Care, Part 2

Part one of these articles about evaluating diesel engines ended by discussing the principles and the importance of performing an informal compression test. Let's now move on to how that's actually done, and then let's look at some additional tests and inspections which can show a complete picture of the state of the engine in the vehicle you are considering for purchase (or perhaps currently own).

Compression Test
The engine will need to spin for at least three to five seconds in order to properly execute this test. Chances are, unless something is done to avert it, the engine will actually start during this time. To prevent this from happening, you could disconnect the fuel supply to the injector pump, but it could get messy if you're not careful, plus it'll take a while to purge the air that the pump will ingest before the engine can be restarted.


A much simpler way to do this (here's where the cold engine helps) is to crank the engine before the glow plugs heat up. You'll get your three to five seconds, and won't have a mess all over the driveway. Although I wouldn't recommend this method frequently of starting an older diesel engine, a couple of times won't hurt. All you need to do is turn the ignition key immediately to the start position (as in, not waiting for the glow plug pilot lamp to cycle on and off like you would normally do when starting the engine) and observe the sound it makes while cranking.


Now, the sound that should be heard on this particular engine is sort of like a ruhhh-ruhhh-ruhhh-ruhhh-ruhhh, indicating even compression. Unfortunately, the Mercedes-Benz we checked exhibited more of a ruhhh-ruhhh-ruhhh-ruhhh-reehhw, indicating low compression in one of the cylinders (and reduced cranking resistance, which altered the pitch of the starter as it was spinning the engine).

Warm-Up Lap
In order to get the complete picture of the engine's condition, you'll need to go ahead and start the engine and take the car for a drive to warm it up. This gives you an opportunity to notice any noises or other drivability problems relating to the engine as well as the rest of the drivetrain and chassis.