From material by Peter Stern;
with thanks to Frank Swygert

The Jeep 4.0 liter six-cylinder engine provided good acceleration from low speeds in Jeep Wranglers and Cherokees for years, with high torque at low engine speeds. It was used in other vehicles (such as the Grand Cherokee) as well, a strong, durable performer loved by its owners.

Based on the old 199 cubic inch Typhoon Six (launched in 1964 via the tall-deck 232), the AMC 4.0 liter engine used a shallow-skirt cast-iron block with evenly spaced cylinder bores, loop-flow combustion chambers, in-line valves, and a seven-main-bearing crankshaft.

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AMC historian Frank Swygert reported that the 4.0 block is around 1/8” wider than its predecessors, with the 0.10” larger bore offset by taking away support for a mechanical fuel pump. The heads can be bolted to the earlier 232 and 258 blocks (the ports on the right edge have to be sealed when this is done).

Engineers preserved parts and dimensions to cut tooling and inventory costs, so most internal parts interchange, according to Swygert, among the late-1964 and newer sixes in the 232/258/4.0 family. He wrote that it’s “relatively common to put a 258 crank and rods in a 4.0L to make a 280 inch six. This can be done with all stock parts (258 crank/rods, 4.0L block/pistons), but most often a special piston is used along with the slightly longer 4.0L rods.”

In 1986, AMC was using a troublesome 2.8 liter V6 from General Motors and their own 258 (4.2 liter) straight-six, which had been derived from the 232. The 4.2 was used in the CJ7, Concord, Spirit, and Eagle, after debuting as the sole Eagle engine and a Wrangler option.

The related 2.5 liter four-cylinder, which appeared in 1983, was based on the same architecture; Frank Swygert wrote that it was essentially the 258 with the center two cylinders removed and a new head. The four-cylinder produced 125 horsepower in its final years, and, causing some confusion, both replaced a 2.5 liter GM engine and co-existed with a Chrysler 2.5.

When the 4.0 burst onto the scene in the 1987 Cherokee and Comanche, it had 173 horsepower (215 pound-feet of torque). By comparison, in 1990 (three years after the 4.0’s debut), the GM 4.3L V6 only made 160 hp, the Ford 4.9L I6 only made 145 hp, the Ford 4.0 V6 made 155 hp, and the Nissan 3.0 V6 made 153 hp.

Jeep upped the ante a few times; by 1988, power had crawled up to 177 hp (224 lb-ft), and in 1991, the Cherokee’s version was boosted to 190 hp. The engine had a lightweight tuned cast-aluminum intake manifold with long runners for higher torque; the exhaust manifold was tubular steel.

The YJ “Universal Jeep” stuck with the old 258 six (4.2 liter) until the 1991 model year, when it finally gained the 4.0 — but only at 181 hp, possibly due to a more restrictive exhaust. Even so, the 4.0 remained competitive with small V8s and big truck sixes.

The 4.0 used a Renault-Bendix, or Renix, fuel injection system from its 1987 launch until it was replaced by a far more reliable Chrysler system in 1991. This used standard Chrysler engine computers, allowing easier diagnostics: two yellow rubber covers on the right side of the engine compartment let dealers get codes and do certain tests. (Rob Mayercik wrote that the Renix system did not store codes.) The Chrysler system had a different type of cam position sensor, with no knock sensor or EGR.

At the factory level, a new Ingersol engine block line was installed in 1996; a new head machining line was installed later.

Willem Weertman, in his definitive book, wrote that many changes were made between 1996 and 1998 to cut noise, vibration, and harshness from the 4.0 engines, including a new cam lobe profile to cut valve seating velocity, isolated valve cover fasteners, and adding a main bearing brace.

As for why the engine was dropped, according to Bob Sheaves, it...

was due mainly to age of the manufacturing tooling, which was worn out. The long stroke made it harder to clean up the emissions of NOx, and NIH (“Not Invented Here”), in my opinion, also reared its head.

To completely retool would have cost as much (almost) as the 3.7L V6 did, and the Dodges were going to use the 3.7 as a base engine. Logically, the engine group did what they were told to satisfy the dealers....“Make a modern engine, and junk the old ones.” This statement is my opinion, based on conversations at the time.

The 4.0 was also replaced by the 4.7 liter V8, codeveloped with the 3.7, and, in the Wrangler, by the “minivan” 3.8 V6, temporarily, and, later, by the 3.6 liter V6.

AMC - Jeep 4.0 liter engine repairs and such

Bob Sheaves wrote:

The 4.0L was a strong performing engine. There were a couple of issues with the basic design, including leaks. In addition, the oil pump was notorious for wearing out at around 145,000 to 155,000 miles, and the TPS (throttle position sensor) on the 1989/1990 models would fail if someone spit on the sidewalk. Make sure you have about 30 lbs oil pressure at idle (+/- 5 lbs).

Karl Stolz wrote that the pressurized coolant surge tank tended to get pinhole leaks, and that there are alternatives which can work better at cooling. Rob Mayercik wrote that Moroso sold an aluminum coolant tank that took a standard radiator cap; he added,

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For loping and rough idle, I’d start with a good throttle body cleaning (Mopar combustion chamber cleaner spray works), then progress to a full tune-up. You could also see about having a good injector cleaning done. The original Renix injectors have been known to leak from the seams; if there’s any indication of a leak, replace the injectors. I’ve heard good things about Five-O Motorsports’ Jeep injectors. [Bob Sheaves agreed but used 5.8-liter Ford V8 fuel injectors with a standalone Haltech sequential injection system.]

Check the EGR valve if you have a Renix system; later (1991+) 4.0s with the Chrysler electronics use exhaust scavenging instead. I’ve heard of clogged EGR valves causing a number of problems.

Stanley Burton wrote:

My 1992 Cherokee 4.0 I-6 had constant oil seepage from the rear of the engine. My dealer replaced the plastic valve cover with a die cast aluminum one with no success. I finally traced the oil to the oil filter mount. The O-rings that seal the oil filter mount are Nitrile (aka Buna-N, NBR) which have limited life. Replacing the O-rings with the same size but with 70 Durometer Viton rubber solved the leakage. My fix lasted the life of the vehicle.

Generally, though, the 4.0 engines were very long-lived, taking positions of honor in the Allpar 200,000 Mile Club.

AMC/Jeep 4.0 Six Cylinder Engine Specifications

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In the 1988 Jeep Cherokee, gas mileage was listed as 18 mpg city, 23 highway with the manual; and 16 city, 21 highway with the four-speed Borg-Warner automatic, regardless of RWD vs 4x4. This was around 4 mpg less than the 2.5-liter four-cylinder.

Displacement was 3962 cc or 242 cubic inches, with a 3.88 x 3.41 inch (98.4 x 86.7 mm) bore and stroke. Compression was 9.2:1. The engine had two overhead valves per cylinder, flat-face followers, hydraulic lifters, and a cast iron block and head.

By 2003, the compression ratio had been dropped to 8.8:1; the redline was 5,300 rpm. Power had gone up in the USA, and had fallen in Europe:

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The 4.0 was made in Kenosha, Wisconsin (at 5555 30th Avenue); “FSJCherokee” wrote, “My uncle worked for the Indianapolis foundry, where they also made the 4.0. I went in during their open house, and they had a cut away 4.0 just as you entered the area where they were assembled. They also had a virgin 440 block in the main reception area. A cool but dirty place.”

In 2003, the plant made the 2.7 and 3.5 liter engines at the same time. In 2003, it could make 405,673 4.0 engines per year (as well as 315,638 2.7s and 220,000 3.5s). The plant had been originally built in 1917.