Ford 6.9 Diesel Review

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The Ford 6.9L Diesel engine was considered one of the most reliable engines that Ford has ever built. How did the 6.9 L motor start the diesel revolution?

The International Harvester 6.9 L diesel was a four-stroke V8 engine used in Ford F-Series trucks and E-Series vans from 1983 - 1987 model years. It was the first diesel engine Ford offered, built to offer durability and performance over gasoline V8s. In 1988, the 7.3L diesel replaced it.

In the late 70s, Ford needed to reassure the American public (and the government) that it was willing to combat the rising cost of gasoline by finding alternatives. As customers began to park their V8s, two oil embargos, a national speed limit, and an unwillingness to spend more than they had to, Ford looked to diesel for answers. Ford’s quest led them to undertake a joint venture with International Harvester to design and produce naturally aspirated engines with indirect injection that would fit into the same space as their V8s. Their collaboration resulted in a damn good engine with better fuel economy that could outlast and outperform any V8 on the planet. The 6.9 IDI diesel engine would go on to power many of their light-duty F-Series trucks and vans over the few years, leading to the development of the 7.3 and laying the foundation for modern diesels like the popular power stroke engine we know today.

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Ford 6.9 Diesel Review

While the general thought was that idi engines like the 6.9 were dirty, noisy, and bad for the environment, they did tend to be reasonably efficient in how they burned fuel. Buoyed by the low fuel cost and the greater torque that diesel engines provided, GM and Ford began exploring how to adapt the technology for a modern type of diesel engine in their trucks. (Dodge lagged with its Cummins Diesel and waited until 1989 to offer it in a RAM truck. They relied on a Mitsubishi diesel that had no power and required maintenance whenever it had to tow. Blown head gaskets and cracked blocks were a common problem).

When the Ford 6.9L was introduced for the 1983 model year for F-250 and F-350 trucks, the company was making some changes. The F-100 was scrapped in favor of the F150, and Ford shifted the “Ranger” name to their lower-end compact truck, replacing the Courier. Due to low demand, several V8 engines (the M Series, 5.8L 351M, 6.6L  400 V8) were discontinued for the reliable 7.5 L V8 gas engine. Even the blue oval emblem appeared on the hood, replacing the iconic lettering that Ford had used for so long.

The 6.9L was an indirect injection, a naturally aspirated engine the designers knew would be ideal for low-end power for heavy towing or hauling. The motor produced 170 hp and 310 lb-ft when first introduced. (This ratio was significantly more than the output GM’s 6.2L diesel was offering. Even though it isn’t close to the 475 HP and 1050 pounds-feet that today’s SuperDuty produces, it was a great start).

The cast iron engine block was brought over from Harvester’s V8 engines. The bore was not as large as the Ford 6.6 V8 and was only ⅛ of an inch smaller than the IH MV404. This smaller bore allowed the engine to work more efficiently, get better fuel economy and achieve more power. (At the time, diesel fuel was pretty cheap, and American farmers were constantly looking for the right combination of power and fuel economy to keep costs down).

Indirect injection systems were designed to meet emissions standards Ford figured might be coming down the pipeline. The fuel inputs were lower cost to build and maintain than direct injectors, which made them ideal for maximizing fuel. Since the motor shared some of the components of the MV 404 and the MV446 V8s, International Harvester could save some money by not having to reinvent the wheel completely.

While the Ford 6.9L shared many components, it was not just a gas engine with diesel heads slapped on top. The motor had roller tappets, an injection pump, and five bolts around each head to keep the cylinders in place. Each combustion chamber was also strengthened. Two head gaskets with reinforced steel increased the rigidity and kept the seal between the engine block and the cylinder head.

Specs of the 6.9 L Engine

Item Specification
Horsepower 161 HP (pre-1984)
170 HP (post 1984)
Torque ratio 310 lb-ft
Configuration V8
Firing Order 1-2-7-3-4-5-6-8
Displacement 6.9 L - 420 ci
Compression Ratio 20.7:1 (pre-1983) 21.5:1 (post 1984)
Bore Size 4.0
Stroke 4.18
Weight 990 lbs
Transmission 4-speed Manual trans.
3-speed automatic trans.
Oil Capacity 10 quarts
Cooling System Pressurized Flow
Aspiration Naturally Aspirated
Head Bolts 5 around each cylinder
Block Heater Yes

Where The 6.9L Diesels Began

The basic design for the 6.9L IDI engines began several years before the first engine rolled off the Melrose Park, Illinois, factory line. The project was approved in 1977 (although the design was a closely guarded secret) with plans for prototypes within the next two years. The design of the new diesel engine borrowed extensively from the new MV404, (particularly the cast iron block) and was one of the last V8 gasoline engines that International Harvester produced.

The first mechanical prototype could have been successful, but it showed some fine promise. (Over 160 others followed as the engineers worked and worked to refine and improve it. Many of the prototypes were tested for over 80,000 miles each in field testing, with extensive research to support both fuel and power changes in design. GM and Dodge had rushed their diesel into production, with reliability issues, and IH was not about to make the same mistake.

At the time, IH was hemorrhaging money due to the austere labor practices of their chairman, Archie McCardell, who cut costs to increase profit margins. When labor unions refused to comply with demands, they went on strike in November 1979. The strike lasted six months, but not before it cost Harvester over $600 million in money it could not afford to lose. When the dust settled, the execs discovered that their company was worth much less than it had been.

The Two Different 6.9L Diesels - The A and the B

The initial offering of the 6.9L IDI was highly successful in helping bolster Ford’s already solid truck sales figures. Due to the better torque and fuel economy, buyers turned to the truck as a heavy lifter for the work they needed. The new diesel increased the towing capacity of the ¾ and full-ton pickups, beating the competition by over 2,000 lbs. While GM had good sales figures powered by their smaller diesel, their primary focus was on the half-ton market.

Ford was so pleased with the initial product that they insisted that International Harvester increase the number of units, which the maker was happy to do.

In 1984, the engineers at IH boosted the horsepower of their 6.9L IDI from 161 HP to 170, and upped the compression ratio from 20.7:1 to 21:5.1 which further improved the thermal efficiency and boosted mileage. The second series of 6.9 diesel engines were distinguished by the “B” line as a way of setting them apart from their earlier counterparts.

The End Of An Era For the 6.9L IDI

The engine was proving so popular that IH continued to make the diesel engine, eventually converting their Indiana plant to the production of the motor to keep up with the demand.

It would not be long (1986) before International Harvester realized they could not sustain their tractor and truck lines effectively.  IH sold out to Tenneco, who bought the Harvester line of the company, which they absorbed under the Case name. What was left of IH was their truck division, which was renamed Navistar International.

The 6.9L continued through 1987, when the 7.3L diesel replaced it. (The 7.3L was almost an identical engine to the previous version, only larger). The new diesel engine had a more substantial block, an upgraded cooling system, an oil cooler, a new injection pump, and reinforced cylinder heads, among other things. The power it produced got a lift to 185 HP and 338 lb-ft.

What Were The Issues With the 6.9 L Diesel?

The 6.9L IDI wasn’t perfect, but it was close to it. Classic truck enthusiasts often seek out the 6.9L and the 7.3L as examples of the best Ford engines ever built. There are some reports of overheating (usually due to a faulty thermostat in the cooling system). A clogged fuel filter or failing glow plugs can occasionally occur, but compared to how bad the 6.0L engine was that replaced it, the 7.3 is the gold standard.