Jeep 4.0L Inline-6: Anatomy of an Indestructible Engine

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There isn’t a better engine for Jeep lovers than the 4.0L straight-six Jeep used in the late eighties, but was it as good as they claim?

The Jeep 4.0L inline 6 engine (developed by American Motors) was used in Jeep vehicles from 1987 - 2006. The engine produced 173 hp and 220 lb-ft of torque with a 9.5:1 compression and was considered very reliable and virtually indestructible. It helped boost sales of Jeep in the late 80s and 90s.

One of the best things to come from the acquisition of Jeep by Chrysler in the late eighties was the emergence of the 4.0L Inline-six PowerTech engine. The motor had been in development even before Mopar brought Jeep Eagle under its wing, and after restructuring the brand, the robust powerplant was allowed to continue. The PowerTech engine appealed to loyal Jeep fans over the next twenty years, providing an important legacy in Jeep development. Even though Chrysler tinkered with the 4.0L motor a bit over the years, this engine was responsible for helping make Jeep a viable option for many American families. Even today, many classic Jeep vehicles, like Wagoneers and Grand Cherokees, rule the back roads of the American wilderness with their rock crawling prowess.  Let’s look a bit closer at this remarkable, long-lasting powerplant.

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How Did The 4.0L V6 Engine Come To Be?

It is no secret that when Chrysler purchased American Motors in the late eighties, they only wanted the company’s “Jeep” division. At the time, Chrysler recognized that the emerging SUV market would continue to grow and needed viable options. The company knew they had no SUV to contend with Ford and GM models, so they sought to acquire Jeep in response. When financial pressures started squeezing Renault’s stake in AMC, Chrysler saw its opportunity. With the stroke of a pen and a 1.5 billion dollar check, Chrysler acquired a worldwide brand, expanded the number of dealerships they owned, and set themselves up to compete in what was growing to be a very competitive market.

Chrysler wanted to demonstrate to a nervous public that Jeep would continue to be the favored brand people loved. To their credit, they phased out the smaller AMC vehicles that hadn’t sold well, restructured the new company to include Jeep and Eagle, and gave the green light for Jeep to continue to produce the second refreshed models they had intended to introduce with their new straight-six engine. Wanting to tap into the lucrative SUV market as quickly as possible and knowing that the Jeep models were some of the most profitable vehicles, Chrysler left Jeep pretty much alone in the early stages.

Jeep would utilize the engine until 2006, offering it as an optional engine for the Cherokee, Comanche, Wagoneer, Grand Cherokee, and Wrangler. The engine was replaced by the 3.8L OHV V6, which the company used as a minivan engine. The new engine produced 205 horsepower and 240 lb-ft of torque.

The Features Of The 4.0L V6 Engine

Many people believe that the 4.0L engine evolved from the 258 ci (4.2L) inline-six that AMC had used since 1971, but in truth, the 4.0L shares more in common with the 2.5L four-cylinder that AMC developed in 1984. While both engines (4.2 & 4.0L) share the same valvetrain, they are, in fact, very different engines. Jeep had to modify the engine bay to accommodate the new engine.

The Jeep 4.0 L straight-six used a cast iron block and a iron cylinder head design for stiffness and rigidity while, at the same time, strengthening the motor with more solid internals. The 4.0L had a better port setup, larger overhead valves, an aluminum intake manifold, and an improved combustion chamber to burn fuel/air more efficiently. It was designed as an over-stroke engine with a 3.875-inch bore diameter and a 3.414-inch stroke with a 9.5:1 compression ratio. The engine debuted in the 1987 Cherokee, Wagoneer, and Commanche models. Jeep lovers became excited when it was announced that the new straight-six produced more power than the previous 4.2L, which only managed 110 hp.  The original engine produced 173 hp and 220 lb-ft of torque.

Initially, the motor had a Renix (Renault/Bendix) engine management system, which incorporated a high-tech ECU monitoring the motor’s spark advance, octane levels, and load stresses. While many early owners complained about the system’s responsiveness, the engine was a capable performer early on. Jeep modified the engine the following year with more robust higher flowing fuel injectors and new intake ports, which raised the power output to 177 hp and 224 lb-ft of torque.

Early reviews of the 4.0L engine were positive, and Jeep wasted no time promoting that the 4.0L Powertech was the most powerful engine in its class. Jeep offered the 4.0L engine as standard on the Cherokee Limited 4x4 in 1988, but it was also an option on other models. Owners soon discovered that the engine could take a lot of abuse (which made it perfect for off-road trail work).

In addition, the motor was virtually maintenance-free, and as owners started piling miles on their Jeeps, the 4.0L straight six kept going and going. It is not unusual to see a 1988 - 2006 Jeep with over 300k miles and a viable 4.0L inline-six still powering the model today.

In 1991, Jeep tweaked the engine again, scrapping the Renix system and installing multi-port fuel injection with larger injectors and a cam update. The engineers updated the intake and exhaust manifolds and an increased throttle body. The power raised to 190 hp and 225 lb-ft. Jeep began badging the revised 4.0L as a “High-Output” variant to trumpet the improved performance and started offering it as standard on most Jeep models.

In 1995, changes were made to the cylinder heads, followed by an updated block a year later. Jeep slapped the PowerTech name on almost everything in the late nineties, even though Jeep had stopped offering it on the Waggoneer and Commanche. The Cherokee, Grand Cherokee, and Wrangler continued to use the engine. The modification didn’t affect the horsepower output, but in 2001, torque increased to 235 lb-ft.

Were There Issues With The 4.0L Engine?

While the 4.0L was virtually indestructible, there were some issues with the motor. We’ve listed some of the more common complaints listed in Jeep forums.

Oil Leaks

One of the more common places for the 4.0L engine to leak was from the rear main seal, valve cover, and around the oil pan. Owners often describe a burning smell (which indicates that the leak is coming from the valve cover and dripping onto a hot exhaust manifold). If the leak is on the ground, then the leak is likely coming from the rear main seal. Many aftermarket purveyors make steel gaskets for the valve cover, which are a definite improvement to the oiled paper units that initially came on the vehicle.

Cracked Exhaust Manifolds

The OEM exhaust manifold routes off of all six cylinders with welded V-shaped pipes into a single large exhaust. Over time, the piping can become brittle (due to the hot gases going through the pipes). Owners complained of exhaust smells or unusual engine noises like a ticking. If the engine ticks during the initial startup and goes away as the motor reaches operating temps, you may have a small manifold leak. (The cracks tend to be wider when the engine is colder).

Early Rod Issues

One of the most frequent complaints concerning the pre-1991 4.0L engines is that the block tended to weaken, and create catastrophic engine failure. Any rod issue is a serious matter, and the later engine versions are more stout with increased stiffness and better internals.

Poor Fuel Economy

The Jeep 4.0L was not known for its excellent fuel economy, but most Jeep lovers don’t care much. The inline-six produced 15 mpg in the city and 20 mpg on the highway. It is not unusual for the throttle body to get carbon buildup and affect fuel efficiency. While Jeep corrected this issue to some extent in 1991, these engines can still have this issue if not maintained and cleaned regularly.

The Specs Of The 4.0L Inline-Six Engine

Item - 5.0L H.O. V8 Specification
Actual Displacement 4.0L (224 ci)
Cylinder Count 6
Aspiration Multi-port fuel injection
Valves 2 per cylinder
Block Material Cast iron
Cylinder Heads Cast iron
Horsepower 173 hp (1987)
190 hp (1988)
220 hp (1991)
Bore 3.875 inches
Stroke 3.414 inches
Compression Ratio 9.5:1
Fuel Economy 15 city/20 hwy - 16 combined