It was the 1970s and people were reeling from the shock of gas prices that had tripled in just a few short years. The effect was profound: a national 55-mph speed limit, government-mandated fuel economy standards, and consumers scrambling for vehicles with better fuel economy. Diesel engines, at the time not much of a presence in the American car and light truck market, were immediately eyed as a fuel economy answer. The collective American eyebrow raised at the idea of noisy, smelly, smoky diesels, not to mention the notable scarcity of diesel fuel stations at the time, but the diesel’s high fuel economy and low-cost fuel were attractive, especially in the truck world where a torquey gasser meant sub-10-mpg fuel economy.

Introduced in 1993, the original 6.9L diesel evolved rapidly. The first big update was a boost in the compression ratio from 20.7 to 21.5:1. The 1983 blocks, casting number 1805440C1, were problematic due to possible cracking issues. As a result, the rebuilding industry doesn’t rebuild these blocks. In 1984, an improved block design was introduced, casting number 1807996C1. It has some extra material and ribbing to reduce the potential for cracks. The 1983-1984 block also had some issues with cracks around the block heater area, but this was dealt with in 1985. The 1983 also had cracking issues with the heads (casting numbers 1805296C1 and 1809199C1). A stronger design was instituted from 1984 and remained the same all through the 6.9L run (casting numbers 1801809215C91 and 1805855C1). The late-1985-and-up engines are the best of the 6.9s.

In 1978, GM and Dodge tied for being the first to offer a diesel pickup. The Chevrolet entry was a C10 with the notorious Olds 5.7L V-8 making 120 naturally aspirated horsepower. Dodge fielded a 4.0L (243ci) Mitsubishi NA diesel inline six with 100 rip-snorting horsepower in half and 3/4-ton 4×2 and 4x4s. Dodge dropped the Mitsu after 1979 and remained diesel-less until 1989, but Chevrolet continued to offer the 5.7 in C10s through 1981, and then replaced it with the vastly better 6.2L in 1982. The stage was now set for a grand entrance by Ford.

Ford And International Harvester Team up

International Harvester started development work on a V-8 diesel for medium-duty trucks in 1978, with architecture based on IH’s 446-cid industrial gas V-8. “Aaakkk,” you say, “a conversion!” Not really—the diesel merely adopted similar block dimensions that made tooling-up easier.

It isn’t exactly clear when Ford Motor Company’s interest was piqued by the IH diesel, but in 1981 a $500-million agreement was signed for IH to develop the engine for Ford light and medium duty trucks and supply engines for five years. Because it was a hefty engine, weighing in at more than 900 lbs., it was soon clear this was going to be a ¾-ton-and-up powerplant.

The E-250 and E-350 vans were available with the IDI diesel, but only with an automatic transmission. The diesel E-350 dually chassis was popular for motor homes and well-liked for its fuel economy, which was significantly better than the big-block gasser. Jonathan Lalonde of Ontario, Canada, owns this 26-foot 1987 F-350 Corsair with a 6.9-liter IDI, C6 automatic and 3.54 gears. Last count, this rig had nearly 170,000 miles on it.

The Ford diesel light trucks debuted in late 1982 as 1983 models, with the new diesel engine offered as a $2,225 option. The lightest-duty truck in which the new 6.9L (420-cid) was available with a diesel was the F-250HD (8,600 lbs. GVW).

Availability went all the way up the light truck line including the E-Series vans and up to the medium-duty trucks. The first advertised rating was just 161 hp and 307 lb-ft, with a 19.7:1 compression ratio. These were likely preliminary specs because 170 hp and a 20.7:1 compression ratio soon became the advertised norm.


The 6.9 featured oil-cooled pistons, four bolt mains, a massive forged crank with 2.2-inch rod and 3.1-inch main journals, valve rotators, roller tappets, and a gear-driven cam and injection pump. It was naturally aspirated used the Ricardo V combustion chamber. Indirect injection came from a Stanadyne (Roosa-Master) DB2 rotary pump and pintle-type injectors that popped at 2,100 psi. Issues with cold starting appeared right away, so for the 1984 model year the compression ratio was increased to 21.5:1. Torque rose to 315 lb-ft as a result, and that’s where output would stay for the remainder of the engine’s run.

In 1988, the 7.3L IDI debuted touting a number of improvements. Power was advertised at 180 hp and torque bumped to 338 lb-ft. The 7.3L blocks can be distinguished by a 10809000C1 casting number. With the heads off, they can be identified by their round coolant ports at the corners of the block deck, as opposed to the triangular ports on the 6.9. The 7.3 also benefitted from larger head bolts (1/2-inch vs. 7/16-inch) and of course the larger bore. Thinner cylinder walls, changes in coolant flow and an increased tendency towards core shift made the 7.3 vulnerable to cavitation and pinholes from the water jacket into the cylinder. The oil filler was cast into the pump gear cover. This is a 1992 engine, which featured the serpentine belt system.

For 1988, the engine got a makeover. The bore was increased by 0.18-inch, bumping up displacement to 7.3L (444 cid). The heads, head gaskets, rocker gear and combustion chambers were reworked and the glow-plug system was completely revised. The injection system also got some tuning alterations. As a result, output was boosted to 180 hp at 3,300 rpm and torque to 338 lb-ft (though some spec sheets show 345 lb-ft). In mid-1992, power output was increased to 185 and torque went up to 360. The 1992 model year also brought a serpentine belt system.
The revisions to the 7.3 were largely successful but there were stumbles. The overbore and cooling system changes resulted in an increased tendency towards cavitation damage on the cylinder walls in the water jacket. This was manageable using the right anti-corrosive coolant additives (called SCAs, supplemental coolant additives) but it became a well-known problem.

The IDI reached its zenith in late in 1993, when the first turbocharged 7.3 was introduced. It mounted a Garrett wastegated turbo with an A/R of 0.82. Advertised power and torque were 190 hp and 385 lb-ft, but this engine was somewhat underrated. If you look at the nearby power chart, you will see it’s mysteriously cut off at 3,000 rpm vs. the NA engine going to 3,300. The power line looks to still be climbing at 3,000 and extrapolations show the turbo engine would have developed more than 200 hp at 3,300 rpm. Speculation within the IDI enthusiast community is that with the new 210-hp Power Stroke engine on the horizon, Ford marketing wanted to make sure the new engine would be more appealing than the old one. The IDI turbo engine was hyped more for its high-altitude performance than its raw power, but tweaking in the intervening years have shown the IDI turbo is fully capable of 250 hp with only minor tuning.

The pinnacle of the Ford IDI was the 7.3L IDIT turbo engine. It was advertised at 190 hp and 385 lb-ft, but is generally considered to have been underrated and undertuned by Ford so as not to upstage the upcoming Power Stroke. It was a considerably more robust engine, with beefier pistons and rods, a stronger block (casting number 10809000C3) and head gaskets, Inconel exhaust valves and a larger oil cooler.

Internally, the IDI turbo received improved head gaskets with a heavier fire ring. The pistons were given keystone rings and anodized crowns. The wrist pin diameter was increased from 1.110- to 1.308-inch, Inconel exhaust valves were added, and the oil cooler bundle was increased from 24FPI (fins per inch) to 30. The injection pump calibration was altered and new injectors were fitted. Minimum boost was 5 psi but most developed 8-10 psi in service.

The Turbo IDI and the Power Stroke shared the stage in 1994 and sales overlapped a little after the Power Stroke’s mid-year intro. In reality, the Power Stroke is very much the IDI engine’s progeny. The IDI’s heritage is clear, especially in the first-generation Power Strokes, and there are even a few interchangeable parts.

The diesel badging was under the driver’s side headlight, even more low-key than previous years.

The IDI Today

Nearly 1.5 million 6.9L and 7.3L IDI engines were produced by International Harvester (now known as Navistar International). They are truly one of the cornerstones of the growth of diesel power for pickups. The IDI is an old-school engine and can’t hold a candle to the modern electronic engines in the power department, and yet it has a large and loyal following. There are countless IDIs still on the road, some still working in commercial livery, and they seem to just go on and on. No question, the Ford IDI engine is nowhere near the end of the road.

Corey Zakeosian’s OBS 1994 SuperCab turbo diesel was purchased with the Truxarossa body kit from the Ford dealer. Ford and the dealer network had agreements with various kit and coachbuilders to convert trucks.

The Ford Package

1983-1986 – The Bullnose Era

The Ford diesel was in full swing when this 1984 F-250HD 4×4 XL made its debut. In this era, the XL was a step up from the base model and sat in the middle between the Base and XLT (later XLT Lariat) models. The “HD” in the designation denoted the 8,600 lbs. GVW, the lowest GVW light truck in which the 6.9L diesel was offered.

Top of the line was XLT, to which “Lariat” was added. Shown here is Frank Butt’s 1986 F-250HD 4×2 SuperCab, bought new by him in 1986. Besides the Regular Cab longbed and the SuperCab, Ford offered a Crew Cab version.

The 1983-1986 Bullnose era interiors ranged from a rubber mats, vinyl seat, bare-bones hose-out interiors to this plush velour cabin with carlike options.

Ford debuted a new line of trucks in 1980 and the diesel entered the game right in the middle of that 1980-86 generation. Today, this body style is often called the “Bullnose” for its broad snout. It came in a Standard (base) trim, mid-line XL and full-boat XLT. Body styles included regular cab longbed, SuperCab longbed and, in some years, a crewcab longbed. “Lariat” was added to the XLT designation in 1985.

In this era, you had the choice of two transmissions, the Warner T-19 four-speed or the legendary C-6 automatic three-speed. If you ordered four-wheel drive, it came with a BorgWarner 1345 transfer case. Axles varied by year. To mid-1985, the rear axle was a Dana 61 or 70 axle. In mid-year 1985, the 10.25-inch ring gear Ford Sterling debuted in semi or full-float version (full-float only on the diesels). F-350 DRWs most often had Dana 70 HD axles but there are DRW Sterling axles out there too. Up front, the twin I-beam non-driving axles carried the load for 4x2s. The F-250HD 4×4 had a twin traction beam (TTB) Dana 44 as standard with a Dana 50 TTB optional, while the F-350 came standard with the stronger Dana 50 TTB. In 1985, the Dana 60 solid front axle was introduced and most F-350s had it. Only two axle ratios were offered for the diesels, 3.55 and 4.10:1. This was to remain the practice all the way through the IDI era.

1987-1991 – The Flatnose Era

In 1987, Ford made some styling changes but did not alter basic body shape. To IDI diesel nuts, these trucks are known as the “Bricknose.” With their flattened nose and flush headlights, these trucks had a more aerodynamic look. Fenderwell shapes changed as well the interior. A small “Diesel” badge appeared under the driver’s headlight. Trim levels changed a little, with the base level now called “Custom,” and each level moved slightly upmarket.

The IDI-powered F-350 cab chassis was one of the most popular chassis cabs for conversions.

In 1990 you could get a diesel-powered SuperCab F-350 XLT Lariat dually with all the goodies, although this one is in a plain white wrapper.

The 6.9L engine soldiered on for 1987, but was replaced by the 7.3 for model year 1988. A big innovation that came later in 1987 was the ZF five-speed manual transmission option. At first, buyers could choose between the standard T-19 four-speed, the ZF five-speed, and the C-6 automatic. By 1988 the T-19 had been dropped and the five-speed became the only manual option. Like the T-19, it had a taller first gear than the gasoline version.

For 1989, the E4OD overdrive automatic was offered in the diesel line. That was a big innovation, offering the mpg improvements of overdrive to those desiring automatics, but it had taken Ford a while to build one stout enough for use in the higher GVW light trucks. Trucks were built with a mix of three-speed C-6 and E4OD automatics through 1994, though in later years the C-6 was available only in the commercial cab and chassis lines. Suspension and axles stayed the same as the previous trucks but in this era the Dana 44 TTB was dropped and the F-250HD inherited the Dana 50 as the standard front end for 4x4s. With a few exceptions, F-350s still used the Dana 60.

1992-1994 – The OBS Era

The front wrap changed again for 1992 but once again the basic body shape remained the same. For some reason, Ford diesel fans call this generation “OBS” for old body style. We suppose that’s because the Power Stroke engine was installed into this body well into the 1990s. The interior got a big makeover, but the powertrain options changed little.

The upgraded 1992 Ford truck line was where the IDI would make its final bow and the Power Stroke would take over. The only thing to mark a naturally aspirated diesel in this era was a badge on the tailgate. In the case of this 1993 turbo diesel truck, there’s an additional “Turbo Diesel” badge on the fender. This is Jon Miller’s stunningly restored 1994 F-350.

The big news came in 1993 when Ford introduced a turbocharged version of the IDI diesel. The naturally-aspirated IDI continued as the baseline diesel option but output had increased to 185 hp and 360 lb-ft, the highest power and torque levels the NA IDI would achieve in factory trim. The Power Stroke was a midyear entrant for 1994 and the IDI trucks were soon in the “Oh, by the way” category. DW