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Vintage Diesels, Part 1

Evaluating an older oil burner

All the recent attention given to alternative fuel vehicles has piqued interest in diesel-powered passenger vehicles. Their higher fuel efficiency has rekindled a desire in many drivers to join the "Church of the Lakka-Lakka-Lakka." Or perhaps you already own a vehicle powered by one of Rudolph Diesel's progeny. Either way, you probably would like to be able to make a quick evaluation of the engine, even with its legendary reputation for reliability.

Assessment

Let's start with the scenario of a potential purchase of an older diesel, and then go into the details of the assessment procedure (covered in more detail in part two).

Since diesels last a long time, you many not want to shell out the 25-large (or more) to own one of the newfangled, fully computerized, highly-fuel-efficient, direct-injected compacts, such as the new VW Jetta TDI or Mercedes E320 CDI. Even if you do, you might like to also have a backup compression-ignition-powered vehicle to augment it. Something at least remotely related genetically, but with a different mission statement—vintage funkiness would be a plus here, too.

Well, what do you have to choose from in that field, and how can you tell the good from the bad from the ugly?

Candidates

If you want to get completely technical, "vintage" refers to something no longer produced. If you're talking trucks, you don't have to go back much more than a decade to find a model that falls under that description. Dodge, Ford, and GM all produced trucks in that era powered by since-discontinued Diesel powerplants.

For a diesel pickup from the early Eighties, probably one of the best choices is the GM 6.2L. Although its availability and replacement parts aren't plentiful, the situation is probably a bit better than for the other brands manufactured during that period.

For passenger cars, you've got just a couple of truly viable candidates (Peugeot fans, please don't be offended): the VW Rabbit Diesel, and the Mercedes-Benz 240 and 300 series. The latter two had excellent build quality, are still reliable, relatively easy to perform service on (being mechanically straightforward), enjoy good replacement parts availability, and (due to the high mileage these cars usually would likely have by now) will easily show evidence of abuse if such occurred during its history.

While a complete inspection can (and should) be performed on one of these vehicles much in the same way as on any other, the question does come up as to how to determine the engine's condition. These engines can perform satisfactorily for 300,000 miles and more, under ideal conditions. Such conditions would include regular oil and filter changes, valve adjustments, fuel system service (including fuel injector and pump replacement, if needed), and cooling system service. The latter would involve preventive replacement of components, instead of waiting for failure, and subsequent engine overheating.

So, how do you determine the history of the engine and fuel system? Accurate service records, preferably showing that a reputable repair shop has performed the maintenance and repairs, help quite a bit. But whether the seller has these records or not, you can see quite a bit just by looking—and knowing what you're looking at.

Helpful Hints

Here are some helpful hints that can be performed in the seller's driveway, without scaring him or threatening any life, limb or property. The vehicle shown here is a rare 1981 Mercedes-Benz 300CD with 227,000 miles plus on the odometer).

First, make sure that the engine is cold; that is, it hasn't been run for a couple of hours, at least. The battery should be fully charged, and the starting system in good order. We will start with an "informal" compression test, as the formal one will require a visit to a repair specialist. This test is fairly accurate, but must be done with an ear that can distinguish and identify pitch easily. If you're tone-deaf, you might need to enlist the help of a musician friend. This test is performed by cranking the engine and listen for irregularities in the way the engine spins.

As each piston comes up on compression stroke, the resistance of the air being compressed within the cylinder momentarily slows the cranking speed, imposing a greater load on the starter. If all of the cylinders are sealing equally well, each momentary slowing would be more or less identical.

In the case of the 5-cylinder diesel engine we're testing here, every sixth pulse (from initiation of the test) would signal the beginning of the firing order again. For our purposes, we really don't need to be concerned with which exact cylinder we are listening to at a given moment, only whether all of these "compression loading pulses" are uniform or not.

In part two of this article, you'll learn how to perform the informal compression test, as well as a couple of additional tests, which will prove valuable in helping you get a more complete picture of the condition of the engine in the vehicle you're considering purchasing.