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Oldsmobile 350 Diesel

Oldsdiesel Let's step in the Wayback Machine for a second. Pretend it's the mid '70s. Disco is popular, Saigon just fell, Nixon was pardoned, a peanut farmer from Georgia was just elected President of the United States, and gas prices are about to spike to the highest they'll get, inflation adjusted, for the next 30 years. To add insult to injury, let's briefly pretend that you're in charge of GM's product development during this time. Your customers are abandoning your large, profit-friendly cars faster than the Italians abandoned their lines during the Battle of Caporetto. As for the smaller, more fuel efficient Vega ... well, let's just say the Italian Navy was more successful against the British at night than the Vega's metallurgically suspect engine ever was against rust and engine wear.  In short, you need the following

  • A more fuel-efficient lineup.
  • Better reliability.
  • A healthy enough profit margin where dealers will actually feel compelled to take care of their customers again (i.e. not the T-Platform).

Fortunately, the solution is staring you in the face: Diesels! Diesel engines inherently provide superior fuel efficiency, less engine wear, and as an optional engine, you even get to charge extra! As if all of that wasn't enough, diesels are not subject to the sort of pollution-mitigation requirements that your gasoline engines have been laboring through. The only question is whether there's a market in the U.S. for diesels. Thankfully, the results are rather encouraging on that front--domestic sales of Mercedes-Benz's 240D and 300D are solid, and even the Peugeot 504 isn't being completely laughed out of the showroom.

So, in your capacity of head of GM product development in the late-1970s, you take the plunge. You order your minions to create a diesel engine suitable for installation in larger Chevrolets, Buicks, Oldsmobiles, and even Cadillacs. Certain in the wisdom of your decision, you step back into the Wayback Machine and fast-forward to the present day, secure in the knowledge that you've saved GM and Detroit from further erosion against the import hordes.

So ... what happened? It really was a brilliant idea, after all--better fuel efficiency, world-class reliability, and higher profit margins, all at the cost of a little acceleration that people were increasingly willing to lose. How did GM go from this brilliant concept to an engine that would eventually spawn a class-action lawsuit and inspire the drafting of Lemon Laws throughout the country? Like the Italian invasion of Greece, the devil was in the details.

Cadillacdiesel Contrary to popular myth, the Oldsmobile 350 diesel was not a gasoline engine that GM hastily threw some glow plugs into. On the contrary, GM designed a block specially for the diesel 350, which was heftier and made of a different alloy than the block used in the gas-powered Rocket 350. Unfortunately, GM did cut some corners - the most egregious of which was the lack of a water separator. 

To understand the importance of a water separator in a diesel engine, it's helpful to understand how a diesel engine operates. Unlike gasoline, diesel fuel doesn't ignite explosively when exposed to an electric spark. It does, however, ignite quite nicely when exposed to warm, highly compressed air. Consequently, diesel engines are built to produce incredibly high compression rates (the Oldsmobile Diesel V-8, for example, used a 22.5:1 compression ratio).

Unlike petroleum, however, water is virtually uncompressible. Consequently, when water gets into the combustion chamber of a diesel engine during the compression phase, the effective compression ratio of the cylinder increases dramatically, leading to incredible pressures in the engine. This causes increased heat, since air warms as its compressed, which, along with the higher pressures, would strain vulnerable non-metallic parts such as head gaskets and rubber seals. Of course, none of this touches on what water-based oxidation did to '70s-era metal, or that metal's ability to handle extreme heat and pressure.

This brings us to the next corner that GM cut, which led to most of the misunderstandings regarding the Diesel 350's origins. Though the Diesel 350 was given an entirely new block, GM decided it could save costs by reusing some of the tooling and parts from the similarly sized gas-powered Rocket 350. Some of the reused parts were relatively harmless bits like the valve covers, oil pan, water pump and oil pump. Fatefully, though, this list also included the Rocket 350's head bolts and head bolt pattern.

Even with a water separator, this would have been problematic--the head bolt design was meant for use in an engine generating less than half the stock compression ratio of any diesel. Without the water separator, however, the results were apocalyptic. Water would enter the cylinders via the fuel lines, which would cause higher pressures and heat to further weaken (or snap) the weak head bolts. This, in turn, would lead to the head gasket failing, which would cause a flood of incompressible water-based coolant to enter the cylinders. Once that happened, it was a battle between the compressive abilities of liquids and the ductile strength of 1970s American steel. Needless to say, the battle was over before it even started.

Sadly, there's one more exit in this cavalcade of fail. There wasn't a single Buick, Oldsmobile, or Cadillac mechanic alive that had even the faintest clue what to do with a diesel engine. It showed. When faced with failing head gaskets, the mechanics of the time did what they always did--they removed the head bolts, milled the block, then reinstalled the same head bolts. Since the head bolts were stretched from heat and pressure far beyond what they were originally meant to ever handle, it wasn't uncommon for them to fail completely shortly after re-installation. This did wonders for GM's already spotty reputation for reliability and post-sales support.

After a few years, GM was able to produce a moderately competent passenger car diesel.  The Oldsmobile Diesel V6 addressed most of the problems that plagued the Diesel 350, including the spotty head bolt problem. Regrettably, the word was already out--the 350 tarnished GM's reputation in the passenger diesel market severely enough to convince GM to cease production of all domestic passenger diesels in 1985.

The first picture is from emmka on Flickr. The second picture is from the excellent collection of Cadillac ads being hosted by Flickr user "That Hartford Guy." In April 2006, Car & Driver did a "Battle of the Diesel Beaters" that featured a Diesel 350-equipped Oldsmobile 88.  Proving that, if something exists, there's a fan of it, is an impressive if mildly misguided one-stop repository of all things Oldsmobile Diesel.

--David Colborne


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Great, informative post, David! Another epic fail of The (Then) Big Three. These cars have a legacy of their own, and have prompted a few unbusted myths:

Myth #1: I heard that many owners of these cars, at their own expense, later converted them to gasoline power. The 2-3-year-old car bodies were still sound, and as trade-ins or private sales, their value was... diminished. The cost for conversion to gas was absorbed in the vehicle's value difference between Diesel and Gas power, not to mention less frequent major repairs.

Myth #2: I also heard that since the vehicle's VIN said "Diesel", that they were exempt from emissions testing, so any engine swap could be made. No catalytic converters or pollution controls whatsoever had to be fitted, and they were still legal. This may have pushed legalities... or did it?

Myth #3: That the engine blocks removed from these vehicles, after removal of any contaminates, made a very affordable boat anchor. No other purpose for their non-recycled use was ever found.

Should we call Adam and Jamie on this one?

You did a great job explaining why the Oldsmodiesels were such epic failures. As one whose other other hobby (besides cars and railroads) is military history and wargaming, I stand in awe of your ability to work references to the retreat from Caporetto, the Battle of Cape Matapan, and Mussolini's invasion of Greece into an article which is otherwise concerned with the geeky technicalities of 1970s diesel motor engineering.

I really enjoyed the mixture of automotive history, european military history, and applied physics.

@ That Car Guy: Well, until I was about the age of 7, my dad had an oldsmodiesel, but it was converted to a regular gas V8. I'm pretty sure someone just swapped out the diesel and threw in a 350. The diesel plaque was still inside the car though, I remember staring at it on road trips.

is there something between you and the Italians we should know about?

Ben: No, I have nothing against the Italians. I just think that Italian military failure is an oft-overlooked historical subject that deserves more scrutiny. I'm hoping to work in obscure Bulgarian battles next time around (as if there's any other kind).

Great article. I always knew the GM diesels sucked, but to now know WHY they sucked is a big bonus. I'd like to hear your take on the GM 8-6-4 cyl. deactivation engines of this same period. It took nearly 25 years for them to try that technology again, although now I'm sure it's far better planned.

I just found your blog and I love it. You give a more fair and balanced report on the Olds Diesel than most people. I think it was a good idea in concept, with poor execution. I have a Cadillac ad from the time that proclaims " Dawn to Dusk on a tankful, that's the Diesel Cadillac." The claimed range was 740 miles. If gas prices had continued to stay high, I would have considered an Olds Diesel-powered Caddy or Ninety-Eight and some new head bolts.

My favorite cars are the big engined, low compression boats of the 70's. While they may build better performance cars today, modern automakers can't touch the luxury standards set in the 70's. If you like a laid back ride with zero road feel and no pretensions of sportiness, then this was the golden age. Unfortunately, every new car has to handle like a sports car. There's no room left for the big cruiser.

Jed, you may consider a peek at the Grand Marquis/Crown Victoria or Town Car. They are the closest remaining vehicles to the luxo-barges of yore. A half-ton pickup is cushy as well, and many are true 4-doors!

We drove one of the big Oldsmobile diesels from the Midwest to Disney World, circa 1980. Plush as could be, and we had to strain to get less than 25 mpg. Today this is no big deal - my current car will do better than that, and will do zero to sixty on the same day - but back then, it was amazing.

I had a Ninety-Eight Regency that I took from my mom when I was broke and needed a ride, and she was getting a new car, and it had one of those motors. Drove it well over 100K miles with no major issues. It was a 30 MPG Barcolounger back in the day when 30 MPG was astronomically high. I loved that car. Seriously.

Everyone I tell this to says I was the ONLY ONE who never had a problem with one of those engines, but it was stone ax reliable for me. Hey, I'd rather be lucky than good.

The diesel 350 engine from gas 350 engine story is a myth? Darn! I am so depressed to learn that GM actually engineered a diesel engine that was such a epic fail. To think they actually spent time trying to make a proper diesel engine and came up with THAT as a result? Who knew? Is the story about GM replacing that disaster of a diesel with a gas engine for free in those cars a myth too?

The results of GM's efforts of the time are pretty amazing if you think about how they effectively eliminated the diesel passenger car in the US. The results were almost 100% for decades (save the occasional Mercedes). It is only now, after almost 30 years, that we are starting to see some of those diesel passenger cars the rest of the world has been driving all this time.

I owned a hand-me-down Cutlass Supreme with the diesel engine for several years. It had a DACOR water separator installed when the car was new by the dealer. It would get 34 mph on the open freeway. It did eventually have it's problems: It needed the high pressure fuel pump rebuilt a number of times, it did need the head gaskets replaced (upgraded to aircraft quality head studs at the time), it oddly broke a connecting rod (at the piston end where it cracked open - still have the rod on my toolbox), and eventually the glowplug system was too expensive to keep working. The nice thing about the Olds diesel was that it was so easy to replace with another gas fueled olds engine including the 6.6 used in the Trans Ams in many parts of the country. Also the upgraded diesel block was (and still is) a great foundation for a high performance gasoline engine build as very few mods were required to swap everything from a gas engine over; even the distributers bolted in. Very reliable 600 HP possible with a diesel block.

I have done some reading on this topic, and some people say that the problems with this engine had been sorted out by 1985. Is this true? RE: The Cadillac V-8-6-4 I owned one of those, the cylinder deactivation acutally worked great!! The actuators got a little bit noisy, so I replaced them with regular rocker arms, and disconnected the 3rd gear pressure switch, so the computer would not try to de activate cyclinders. I might just purchase one of these old Diesels, if any are still available. I would have to think a lot of the problems were also caused by people putting in the wrong oil, not changing it often enough, and constantly driving at or near wide open throttle for decent acceleration. Despite all the problems, who would not like to have a 30 mpg plus full size AMERICAN luxury car?

Nice read, but you have one big glaring problem.
Water or diesel will not be compressed any significant amount. The injection pressures with in a 350 would be around 5000 to 8000psi and the fuel still has very little compression at those pressures; cylinder compression would be at around 330psi. The water in the fuel will cause corrosion problems in the injection pump and injectors.
Diesel performance is my hobby and we spray water into the intake to COOL the engine to keep it from melting at high temperatures. Also the amount of water injected to a cylinder in 1 fire of the cylinder would be a very small amount. I don't know the amount of fuel in a 350, but a 215hp Cummins in a Dodge will spray 0.17ml of fuel into a 59ci cylinder. I would doubt a 350 would spray as much as 0.08ml in one injection. Do you think 0.08ml of fuel burning into a big fire would make more heat than 0.05ml into a fire and 0.03ml of water into steam? I bet the fire will burn better than water.
The head gasket problems were just a bad design. They knew how to hold 10:1 compression, 22.5:1 was a whole different animal.

I've seen so many folks just fire up an engine and not give it time for proper oil pressure or operating temperature. Just fire up a diesel especially and GO! If there's anything George ( ) taught me, it's to let the engine have a moment before you work the beans out of it.

If anyone with mechanical skills or access to same finds one of these cars and wants to get it going, the flaws in the engine can all be addressed. Go to, then to Other Diesels, then search for "The Infamous Olds Diesel" for the story and the fixes.

Amen, That Car Guy! You are correct everyone should let the engine get close or to full operating temp before the engine is put under load and it is more critical for a Diesel.
This is better for head gaskets and head bolts.
And remember to always let the engine idle for about 3 minutes before shutdown after the engine has had a hard pull.

I considered purchasing a diesel back in those days. Reasons that I didn't had more to do with the availability of diesel at the local gas station. In my area, it was rare to see a diesel pump except at a truck stop. My wife didn't like the noise and smell.
The diesel models were also pretty expensive and not in great supply.

So don't be too hard on GM. At least they tried.

i searched for a rwd gm with the diesel for 5 years. i have one now and love the looks people give me in traffic when they hear the clacking from within the wheel wells.

not all the diesel were junk the 1984 and 85 dx block were the best they fixed all the flaws heater in the tank and on the fuel line and in the block warning light if there was water in the tank the only reason they stoped in 85 was because the epa cracked down they were leaking oil everywhere tons of black smoke if someone floored it in the front of you you couldnt see anything at all it smelled bad and for some reason the decided to run the diesel engine oil in to the air intake and the oil would go in there that realy poluted alot doing that but anyways people didnt know how to drive them they would race them most people were puting regular oil in them when you have to use diesel oil and the biggest problem was people would only add 5 quarts not 7 1/2 like they were supposto and wouldnt let them warm up for atleast 5 mins before takeing off i know because my 1984 cadillac ran 230,000 miles because i knew how to run those diesels were very diffrent than a regular diesel you realy have to warm them up for a while......

My father, a conservative driver if there ever was one, purchased a new 1978 Chevrolet Custom Deluxe pickup with the diesel engine. As a child, I remember turning back on vacation trips and even being stranded away from home because his brand new truck had a p.o.s. engine.

In 1981, my father had enough of the G.M. diesel and traded it in on a Chevrolet Luv (made by Toyo Kogyo, Japan) even though he previously refused to buy any Japanese ("I fought them during the war") products.

He sent all his repair receipts to G.M. with a letter of his experiences and lack of dealer support only to receive silence in return.

Since then, my family of 5 siblings can count 1 Honda and 9 Toyotas as our vehicles of choice.

I, for one, have still not forgiven G.M. for the '78 diesel.


i work at a junk yard and have always been a big fan of the gas 350 but a couple days ago we got in an olds 350 diesel. the guy who brought it in didnt know what was wrong with it. is it worth pulling the motor out of the car. it will only cost me $50 for the motor n i would just pull the motor after work, its a complete engine.

I remember my grandfather had an 83 Olds Delta 88 with a 350 diesel. My grandmother traded it off in 1994 with about 362,000 miles on it. It got about 30 mpg but as I remember it was kind of a turd. There were a few other diesel 350s in town that were also high mileage when disposed of. I think we must have had better quality diesel fuel around here, idk. I wanted the car as a novelty item, but it was traded off anyway, dang it. All these engines need to last are stronger head bolts and stronger bolts for the main bearing caps and if you want, a water/fuel seperator although the newer of the cars had a water in fuel indicator.

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Pictured above: This is a forlorn Chevy Vega photographed by reader Gary Sinar. (Share yours)

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