Avoidable Classic: GM Diesels

Have I Got a Smokin' Deal for You.

When pundits ponder why diesel cars failed in the U.S., the infamous engines built by Oldsmobile from 1978 to 1985 come to mind instantly.

Critical engineering flaws, consumers who ignored strict maintenance schedules, and the handicap of a casually water-logged diesel supply turned the cackling diesels into rolling grenades. Most have gone off by now.

Since the last Oldsmobile diesel rolled off the line in 1985, a class-action lawsuit from owners against GM has been settled, but the car remains the subject of cruel jokes that can still cause former owners to furrow their brows over their (often 100%) financial losses.

A fixture on "worst" lists everywhere, the Oldsmobile diesel suffered a public relations fate worse than the Edsel. But like the Edsel, GM diesels may be staging a (suitably slow and rattly) comeback.

Built to fulfill the CAFE (corporate average fuel economy) regulations of 1978, which required a fleet average of 18 mpg for cars, and to contend with rising fuel prices of the second OPEC crisis, the diesel option was intended to boost fuel economy without sacrificing vehicle size or luxury.

Initially offered exclusively in the downsized Oldsmobile 88 and 98, plus GMC and Chevy trucks for 1978-Oldsmobile sold 31,600 units in 1978-the 350-ci diesel LF9 engine clattered into all GM showrooms by 1980.

One myth that took hold was that the diesel was merely a gasoline Oldsmobile V8 converted to run on diesel fuel. Although bore spacing between the two engines remained the same and the engines were machined on the same line, the diesel engine's high internal pressures required numerous changes, including a heavy-duty block, different heads, unique internal parts, and a rotary-type Roosa Master diesel fuel injection system. We're not saying any of these parts were any good, they were just different.

In reality, the only parts in common between the gas and diesel 350 were the water pump and the valve covers. But a quick glance under the diesel's hood doesn't reveal many modifications beyond two batteries to power the large starter motor required to handle the diesel's 22.5:1 compression ratio.

Contaminated fuel and weak head bolts

Many of the problems that befell diesel owners could be traced to weaknesses that are largely avoided these days. The biggest problems were caused by a diesel fuel supply contaminated with water and the engine's insufficiently strong head bolts.

Before GM thought to install a fuel tank water separator and warning light, owners could unknowingly introduce water into their engines, which would cause head gasket failures. Survivors of this fault were caught by another design flaw that endured until the end in 1985.

Although the engine was beefed up in nearly every way, in a cost-cutting move, GM elected to keep the same number of head bolts as the gasoline engine. Under the stress of diesel combustion, the bolts stretched, leading to leaky head gaskets and steady work for tow trucks.

Surviving GM diesel enthusiasts often credit their cars' longevity to a long warm-up period, which they claim allows the bolts to heat up slowly, thus reducing stress. Stronger bolts were eventually offered by GM, but the aftermarket bolts available from ARP (www.arp-bolts.com) are recommended.

Other issues arose from the high sulfur content of the diesel fuel, which after combustion produced sulfur oxides. When combined with the water produced by combustion, this lead to sulfuric acid. This inevitably contaminated engine oil and corroded internal parts. Forget to change the oil at the specified 3,000- or 5,000-mile interval (as many diesel owners did), and the engine essentially eats itself from the inside.

Continuous improvements befell the 5.7-liter diesel engine throughout its life, and the later cars (especially after 1981) command higher prices than earlier, more powerful, examples. Second-year upgrades in 1979 brought faster-acting glow plugs that reduced cold-start times from 60 seconds to a more convenient six seconds.

A short-lived 4.3-liter V8 was offered only in '79 and is considered terrible even by permissive Olds diesel standards. The addition of an exhaust gas recirculation valve and a change from pencil-type to smaller and more accurate poppet-type injectors in 1980 reduced emissions but dropped engine output to 105 horsepower at 3,200 rpm and 205 ft-lb of torque at 1,600 rpm (down from 125 hp at 3,600 rpm and 225 ft-lb at 1,600 rpm).

In 1981, the beefier DX block replaced the earlier D block, roller valve lifters increased oil change intervals from 3,000 to 5,000 miles, and an optional fuel heater was offered to keep the diesel from waxing in cold weather; paradoxically, the fuel heater didn't seem to be designed with cold weather use in mind. A water separator was fitted in the fuel tank along with a sensor that illuminated a dashboard light when more than three gallons of water were in the tank. Starting in 1982, GM offered a transversely mounted 4.3-liter V6 diesel with 85 horsepower in a number of new front-drive intermediates. However, most collectors agree that the V8 diesel is the only one likely to appreciate-and I use that word advisedly.

Gathering speed gracefully

As one would expect of a 3,500- to 4,000-plus pound car with 105 horsepower, the cars don't so much accelerate as gather speed gracefully, like a Citroën DS19 with a plug wire removed. Zero to 60 times rest in the high teens, with quarter-mile speeds around 65 mph. If you can deal with that, diesels can keep up with modern traffic and cruise comfortably at 70 mph. Car and Driver's test of the diesel-powered 1980 Cadillac Seville observed: "At 70 mph the diesel would just as soon go home and put up its feet rather than produce another 10 mph." Editor David E. Davis sniped, "If the Seville is the answer, I obviously misunderstood the question." But consumers were still jumpy about gas shortages and bought roughly 400,000 of the cackling engines in 1981-their best year.

The trade-off for power was economy. Owners reported mileage in the low- to mid-20s, higher for fans of the 55 mph speed limit. Coupled with the 27-gallon tank on the large cars, 600 miles between fill-ups was possible.

Prices follow brand hierarchy

Typical diesel-powered GM prices range from $500 to $3,000, depending on mileage and condition-about double what they were a few years ago. Prices follow GM's brand hierarchy: Cadillacs are the most costly, followed by Buicks, Oldsmobiles, Pontiacs, and Chevrolets.

No matter the brand, exceptional low-mileage vehicles can command $5,000 or more. To avoid involuntary commitment papers, buyers should beware of head gasket and overheating issues; a car with such signs had better come with a replacement engine.

Many cars received updated DX engines in the 1980s; a sticker on the valve cover will indicate if the engine is a GM Goodwrench replacement. In general, post-'81 cars are more reliable, easier to maintain, and more likely to have survived. Expect to see black smoke at start-up and under full-throttle acceleration, but none at warm idle.

GM diesels appeal to contrarians, and the ideal Oldsmobile diesel buyer has an instant two-fer-the most defunct model of a defunct brand. Noted GM diesel expert and 350Diesel forum founder Chris Richert sums up the ownership experience perfectly. "When you own an Olds diesel, you are alone in an entirely different dimension."

Seeing one on the road today, amidst the current rebirth of diesel interest makes one want to paraphrase the Rick Cole collector car slogan thusly: "You can never pay too much for an Oldsmobile diesel, you can only buy too soon." And anytime before 2058 may be much too soon.