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When Diesel Was Dreadful: The Oldsmobile Diesels

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author photo by Doug DeMuro December 2016

Back in the late 1970s, when we were at OPEC's mercy, domestic automakers were struggling for ways to make their oversize offerings more fuel-efficient. Then, as now, diesel seemed like a good idea -- and then, as now, the exercise ended in tears.

At the time, it seemed like a good idea: Not only were the European automakers successfully selling diesels in the U.S., but the diesel engine was exempt from many of the emissions standards the domestics were struggling to meet. These were the days when General Motors divisions still developed their own engines, and Oldsmobile was charged with the task of delving into diesel.

Olds started with their 350-cubic-inch (5.7-liter) engine (no relation to the Chevrolet 350 we discussed last month). Contrary to what some believe, the Olds diesel was not just a gas engine "converted" to a diesel; the block was of similar dimensions because using the same bore and stroke meant Oldsmobile could build the engines using the same tooling as the gassers. The block was a beefed-up design intended to deal with the engine's 22.5:1 compression ratio, which was nearly 3 times the CR of contemporary gas engines.

Unfortunately, one thing Oldsmobile didn't change was the head bolts -- type, pattern and number. And that would prove to be the engine's undoing ... or at least part of it.

When the diesel-powered GMs made their debut for the 1978 model year, the public was blown away: Here were American full-size sedans that could achieve 30 miles per gallon. The memories of waiting hours in line for fuel were still fresh, and no one seemed to mind that the power figures were pitiful (120 hp, 220 lb-ft), or that the cars were mind-numbingly slow (a typical 0-to-60 time was 16.5 seconds), or that they sounded like they were grinding their own internals to bits.

Of course, when the engines actually started to grind their own internals to bits, people began to care. A lot.

There were two key problems with the Olds diesels. First, the head bolts simply weren't numerous or strong enough for the diesel's high compression ratio, so they started blowing head gaskets.

One of the consequences of a blown head gasket is that coolant can enter the cylinder, and unlike air (which, along with fuel, is the only thing that belongs in the cylinder), coolant does not compress. If a given cylinder took on enough water, a piston on its upward compression stroke would literally run into the immovable object. The piston would stop, but the crankshaft wouldn't; the connecting rod would bend and much mechanical malaise would ensue. In most cases, this would render the engine irreparable -- but in the event it didn't, repairing the engine using the same type of head bolt would simply give it a further chance to destroy itself. (Incidentally, Oldsmobile also developed a 262-cubic-inch (4.3-liter) V6 version, basically a 350 with two cylinders lopped off. It had a better head-bolt design and wasn't nearly as prone to failure.)

Second big problem: GM's cost-cutters decided not to fit a water separator. Unlike gasoline, diesel fuel is subject to water condensation -- hence the need for a water separator. Without one, water in the fuel becomes water in the engine, where it can rust either the cylinders or the complicated mechanical fuel injection pump. The former could destroy the engine, while the latter would denigrate the engine's running characteristics and possibly deep-six the pump -- which, in a mechanically-injected diesel, is an incredibly intricate and complicated device that is very expensive to replace.

Some owners had the idea to pop a little Dry Gas into the tank. This works wonders for gas engines, but the alcohol eats away at the rubber seals inside the complicated and expensive diesel fuel pump. Oops.

And if all of this isn't bad enough, GM paired early Olds diesels with their THM-200 automatic transmission, a lightweight 3-speed initially intended for the Chevrolet Chevette. The THM200 wasn't particularly robust to begin with, and mating it with the Olds diesel resulted in a race to see which of these two large and expensive components could fail first.

It should come as no surprise that the diesel woes soon resulted in an apocalypse of warranty claims. GM made improvements, but it wasn't until 1985 that they finally gave up on the diesel idea. The fuel crisis had passed, and no one wanted a car powered by one of these lumps. Worse, the diesel's reputation of "smoky, smelly and slow" had been secured, and there were stories of owners having their diesels replaced with gasoline engines.

The effect on the market was even more pronounced: While diesel flourished in Europe in the 1990s, its reputation as car fuel was ruined in America -- and GM didn't attempt to sell another diesel-powered car in the United States until the 2014 Chevrolet Cruze.

And just when diesel was starting to look like a viable alternative again, along came Volkswagen. At least General Motors no longer has to shoulder all the blame...

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This image is a stock photo and is not an exact representation of any vehicle offered for sale. Advertised vehicles of this model may have styling, trim levels, colors and optional equipment that differ from the stock photo.
When Diesel Was Dreadful: The Oldsmobile Diesels - Autotrader