Most Cadillac fans are aware that for about six and a half years, many Cadillac models were available with the unfortunate Oldsmobile 5.7-liter diesel V-8. Towards the end of that time, there was also ANOTHER diesel option: the Oldsmobile 4.3-liter diesel V-6, offered only on the downsized FWD C-body 1985 De Ville and Fleetwood. Sold in tiny numbers, it was more reliable than the diesel V-8, but it was one of the least-powerful engines Cadillac had offered since the 1920s, with what might have been the worst power-to-weight ratio of ANY postwar Cadillac.
Oldsmobile’s diesel engines were first conceived in 1973 as a low-investment fuel-efficient conversion of existing Olds spark ignition V-8s. This project took on new urgency following the federal Environmental Policy and Conservation Act of 1975 (EPCA), which established new Corporate Average Fuel Economy (CAFE) requirements for 1978 and beyond. GM senior management soon decided that the diesel should become a corporate engine, not an Olds exclusive. Oldsmobile motor engineer Tom Leonard later recalled that the diesel “caught on at the Corporation level, and all of a sudden they were putting the heat on us to do it. We got kind of rushed into it.”
The first production Olds diesel, the 350-cid (5,733 cc) LF9, arrived for 1978, initially offered only on full-size Oldsmobiles. It became optional on the Cadillac Seville late in the model year, and was extended to the Eldorado, De Ville, and Fleetwood Brougham for 1979. By 1980, the LF9 V-8 was available on most GM full-size cars. (There was also a short-lived LF7 diesel version of the smaller Olds 260 V-8, which wasn’t widely used.)
This proliferation proved to be an expensive disaster: Oldsmobile originally had very little experience with diesels (“Truck and Bus Group kept telling us, ‘There are so many pitfalls here that you don’t know about,'” Tom Leonard admitted later), so the LF9 and LF7 engines were underdeveloped, and the other divisions knew even less than Olds did about diesel repair and maintenance.
Between that and the poor quality of American diesel fuel, the diesel V-8s suffered a whole host of problems, including injector pump and head gasket failure, oil leaks, and broken crankshafts. Angry owners got organized, leading to class action lawsuits, a three-year FTC investigation, an FTC consent decree, and eventually a $22.5 million settlement agreement for most owners of 1978–1980 diesel cars. The whole fiasco soured many Americans on diesel engines generally and GM diesels in particular.
However, for a brief period in the late ’70s, diesel engines had seemed to be the hot ticket, with customers nervous about fuel shortages sometimes paying up to $1,000 over sticker price for oil-burning engines.
In November 1978, Oldsmobile began planning another new diesel engine, a 263-cid (4,287 cc) V-6 that would be smaller and lighter than the LF9, better suited for the downsized FWD cars then in the planning stages. This engine required much new tooling and a new plant to put it in, but GM anticipated that by 1985, as much as 25 percent of its lineup would be need to be diesel-powered for CAFE purposes. So, in spring 1979, corporate management approved construction of Oldsmobile’s big Delta Diesel Engine Plant, which had the capacity to turn out up to 1,600 diesel V-6s a day.
The new 4.3-liter diesel V-6, dubbed LT6, went into production for the 1982 model year, for both longitudinal RWD and transverse FWD applications. The 90-degree V-6 had a split pin crankshaft for even firing intervals, similar in principle to the even-fire Buick gasoline V-6. Longitudinal engines were all cast iron, but the transverse engine saved 62 lb with a different block casting, an aluminum intake manifold, and aluminum cylinder heads — the last were not often seen on diesels of the time.
Benefiting from Oldsmobile’s painful experience with the diesel V-8s, the V-6 was smoother and more durable than its V-8 cousins, with quicker starting and a water separator to keep moisture out of the injection pump. The 4.3-liter V-6 had only 85 net horsepower, 20 hp less than the 5.7-liter LF9 V-8, but in the lighter FWD A-bodies, its 165 lb-ft of torque made for acceptable acceleration as well as good fuel economy.
Cadillac didn’t immediately adopt the diesel V-6, although they continued to offer the Olds diesel V-8 on RWD full-size cars and the FWD Eldorado and Seville. The LF9 V-8 was actually standard on the wretched bustleback Seville in 1980–1981 and a no-cost option thereafter, but Road & Track found the diesel V-8 hopelessly overmatched by the Seville’s 4,200+ lbs of glitz and dreadful neo-Classical styling. With 40.5 lb of curb weight for each overworked horsepower, the oil-burner Seville needed over 21 seconds to reach 60 mph and wasn’t happy getting there — truly one of the worst Cadillacs of all time.
With the latest fuel crisis over, American buyers’ willingness to put up with diesel engine foibles quickly waned. Part of the problem was that the price of diesel fuel had risen until it was equal to or greater than the price of gasoline, but word had also gotten around about the terrible repair record of the earlier Olds diesel V-8s. Oldsmobile insisted the 1981 and later engines were much improved, but resale values of GM diesel cars had tanked, and fewer and fewer buyers were willing to take the chance.
Here’s the trajectory of Cadillac diesel installations through 1983:
However, Cadillac nonetheless decided to offer the 4.3-liter V-6 in the downsized FWD C-body, which arrived in the spring of 1984, somewhat behind schedule. Interestingly, while Buick and Oldsmobile charged a premium for the diesel V-6, Cadillac again made it a no-cost option, presumably more concerned with its anticipated effect on Cadillac CAFE calculations than with trying to separate buyers from a few hundred extra dollars.
The 4.3-liter V-6 wasn’t quite Cadillac’s least-powerful postwar engine — that particular booby prize goes to the 1982 Cadillac Cimarron, whose 1.8-liter four also had 85 net horsepower, with only 100 lb-ft of torque — but if you discount the Cimarron as not a REAL Cadillac, the LT6 diesel was the least-powerful engine Cadillac had offered since 1927.
If Cadillac had installed the LT6 diesel in the Eldorado, Seville, or RWD Fleetwood Brougham, all of which still weighed 2 tons or more, it would have made for the slowest Cadillacs in living memory. However, the FWD C-bodies were about 600 lb lighter than the older cars, which seemed more workable. The 1985 De Ville brochure offered this rather desperate-sounding appeal:
New for Cadillac, this dependable Diesel will give you the best of both worlds with all the positive characteristics inherent in a Diesel plus surprisingly responsive and smooth performance. The even-firing Diesel V6 furnishes the power to help you move “off the mark” smartly. But you really won’t realize just how responsive this Diesel engine is until you drive it yourself. People who have driven the new Diesel-powered Cadillac have been surprised and delighted. It’s that impressive.
I didn’t find any instrumented road tests of the new diesel De Ville, and the only driving impressions I found were a couple of one-line remarks in contemporary newspaper stories saying the diesel seemed surprisingly not bad.
How did the 4.3-liter diesel De Ville stand in terms of power-to-weight ratio? The base curb weight of a 1985 Cadillac Sedan de Ville was 3,396 lb; Car and Driver‘s well-equipped test car had an actual curb weight of 3,477 lb. Both of those figures were with the aluminum-block HT4100 V-8. While the LT6 diesel V-6 was lighter than the LF9 diesel V-8 (not offered in the FWD De Ville), there was still a weight penalty with the diesel, which included additional sound insulation and other equipment changes (like a standard engine block heater). I unfortunately don’t have the 1985 Cadillac MVMA specifications, which would give an exact figure, but from what I could determine, the weight penalty for a diesel De Ville was probably at least 100 lb, which would put its curb weight between 3,500 and 3,600 lb.
If we assume 3,577 lb (adding 100 lb to the C/D figure), that works out to a grim 42.1 pounds per horsepower, even worse than the diesel Seville Road & Track tested in December 1980, and MUCH worse than the similarly powerful but substantially lighter 1982 Cimarron. Even a late ’40s Fleetwood 75 limousine with the old L-head V-8, with 124 net horsepower to haul a curb weight of about 5,100 lb, had a fractionally better power-to-weight ratio!
Without instrumented performance tests, I can’t say for sure how “smartly” the diesel De Ville actually accelerated, but my guess is that its main performance advantage over the LF9 was that it was quieter and smoother, so trying to get it up to speed was less miserable. Throughout this period, Cadillac repeatedly insisted that its buyers were less interested in 0 to 60 or quarter-mile times than in step-off performance (0 to 15 mph) — the kind of thing that could be easily demonstrated in a leisurely test drive — and the diesel V-6 probably did all right there.
Very few De Ville owners were keen to find out for themselves. Looking at the production figures, it’s very clear that the decision to offer the diesel at all had been made well in advance of its actual introduction. By the 1984 model year, Cadillac buyers’ interest in diesel engines had fallen off so much I had to make a separate chart with a shorter Y-axis to illustrate it properly:
The diesel V-6 did offer encouraging 22/32 EPA ratings (adjusted combined rating of 23 mpg), compared to 17/24 for the HT4100 V-8 (18 mpg adjusted combined rating), but only 788 De Ville/Fleetwood buyers ordered it. Cadillac customers were no more interested in the LF9 diesel V-8, which was installed in only 207 1985 RWD Fleetwood Broughams, 76 1985 Eldorados, and a mere 40 1985 Sevilles. Here’s the complete tally of Cadillac diesel engine installations by model year for 1978 through 1985:
| Model Year | De Ville/Fleetwood RWD | De Ville/Fleetwood FWD | Eldorado | Seville | Total |
|---|---|---|---|---|---|
| 1978 | N/A | N/A | N/A | 2,800 | 2,800 |
| 1979 | 7,640 | N/A | 8,176 | 10,731 | 26,547 |
| 1980 | 18,765 | N/A | 12,424 | 14,461 | 45,650 |
| 1981 | 23,175 | N/A | 11,731 | 7,250 | 42,156 |
| 1982 | 13,003 | N/A | 4,786 | 2,380 | 20,169 |
| 1983 | 639 | N/A | 1,205 | 3,337 | 5,181 |
| 1984 | 1,317 | N/A | 420 | 384 | 2,121 |
| 1985 | 207 | 788 | 76 | 40 | 1,111 |
| GRAND TOTAL: | 64,746 | 788 | 38,818 | 41,383 | 145,735 |
(The 1978 total is approximate; the diesel was first offered on the Seville late in the model year, and so it ended up being omitted from contemporary tallies of Cadillac engine installations, or erroneously lumped in with the gas 350, which was also an Olds V-8 of the same displacement.)
Like the ill-starred Cadillac V8-6-4 “modulated displacement” engine, the 4.3-liter V-6 was another Cadillac engine option that was doomed before it even went on sale. Diesel engines were facing tougher particulate emissions standards that would require new technology to manage — in 1985, Mercedes-Benz introduced trap oxidizer systems for diesel cars sold in California and some other Western states — but with the costly FTC settlement agreement and rapidly shrinking sales, GM was not eager to make any further investments.
On December 5, 1984, GM publicly announced that all of its passenger car diesel engines except the four-cylinder Isuzu diesel (offered in the Chevrolet Chevette) would be dropped after the 1985 model year. The Oldsmobile Delta plant ended production of the 4.3-liter diesel V-6 on April 23, 1985, and the plant was retooled to build the powerful but raucous Quad 4 engine instead.
Back in the fall of 1983, a couple of aggrieved owners of 1976 Cadillac Eldorado convertibles had sued GM, claiming that by reintroducing the Eldorado convertible, Cadillac had defrauded buyers of the 1976 model, which had been advertised as the “last” American convertible and a future collector’s item. The plaintiffs demanded that GM compensate owners for lost value (to the tune of $50 million!), arguing that the introduction of newer convertibles would make their 1976 cars less valuable because they would never be truly rare.
However, Cadillac’s regrettable diesel interlude makes clear that rarity is no guarantee of either collectability or value. Plenty of cars that have never been rare still have robust collector followings and command high prices. Cadillac diesels were built in much smaller numbers, but their values were low even as late-model used cars, and the number of people interested in one today — at any price — is very limited. There’s a YouTuber (Cary’s Garage) who has one. I found only two sales listings for 1985 De Ville diesels, and neither seller thought to offer even a single blurry underhood shot of their car’s extremely rare 4.3-liter V-6 engine.
Related Reading
Vintage Review: 1978 Cadillac Seville Diesel – Puffing And Chugging To Deadly Sin Status (by GN)
Automotive History: 1978 Oldsmobile 5.7L Diesel V8 – GM’s Deadly Sin #34 – Premature Injeculation (by Paul N)
Vintage R&T Reviews: 1978 Oldsmobile 88 and 98 Diesels – A Decent Start, But Some Questions (by Paul N)
Curbside Classic: 1978 Oldsmobile Delta 88 Diesel – In Defense of the Olds 350 Diesel V8 (by BigOldChryslers)
CC Weird Engines: Oldsmobile V5 Diesel – Grasping at Straws (by Aaron65)
Cadillac spent the 1980s putting nails in their coffin at an astonishing rate. The 4.3 V6 diesel was somewhere in the periphery of my memory, but I didn’t know it was used in Cadillacs. A Cutlass Ciera seems like a much better home for that engine.
The 4.3 may have been a decent engine, but it had 5.7 disadvantages before it was even introduced.
Am I the only person who loves the bustleback Seville? I also like the aeroback 442, so I guess there’s no accounting for taste.
Yes. You’re the only person who loves the bustleback Seville. You are correct in your guess there’s no accounting for taste. Or lack of it.
No, you are not the only person who loves the bustleback Seville. I think the design is amazing!
Well, it WAS amazingly destructive to Cadillac’s image and to any chance the Seville had of being taken seriously as an import alternative.
I have seen exactly one 4.3 V6 diesel, it was in a Pontiac 6000.
The top speed is 90 mph. I think even the Peugeot 504 diesel and the Mercedes 200 D were faster (and also fuel-efficient).
The GM diesel was one of the biggest failures of the american automotive industry.
To my understanding, the 4.3 V-6 diesel ended up being basically fine except that it wasn’t very powerful (and really not powerful enough for the heavier FWD C-body cars). However, the earlier diesel V-8 had burned SO much goodwill by then that it was really a lost cause, at in the absence of much higher gas prices, so the V-6 diesel ended up not selling well even in the FWD A-bodies.
The 4.3 diesel was also available in the C-body FWD ’85 Buick Electra, known as the Electra 430.
There were Electra 300 (3.0) and Electra 380 (3.8) models the first year of the downsize.
The 3.0 was discontinued early on due to being underpowered.
The Electra 380 later became the Electra Limited, which was the base model below the Park Avenue.
I was the fleet manager at Fanning Cadillac-Buick at the time, and ordered all of them, but only one diesel Electra as a sold unit for some poor soul…
The FWD C-Bodies both Cadillac and Buick were released in April ’84 as ’85 models to build acceptance of the downsize, having both years’ models in the showrooms at the same time.
We had a Lucite hood for the Cadillac that showcased the transverse mounted V8, and a 24″ mailer that went out, which folded open to show the amount the cars had been downsized.
The Park Avenue proved more popular at first.
So I took to ordering Electra 380s and dressed them up with aftermarket top and Vogue Tyres and wheels.
Which made great switch cars for those who could not afford the Cadillacs or Park Avenues.
We could remove most or all of the profit from the add-ons, and still get close to sticker price for the cars, which made customers feel they’d gotten great deals which they did.
Some early ’86 FWD LeSabres also had the 3.0 V6 before it was discontinued and the optional 3.8 became standard.
The Electra 430 ended up being even rarer than the De Ville 4.3: Only 411 were built for 1985.