The Jeep 4.0 liter PowerTech Straight-Six Engine

The Jeep 4.0 liter six-cylinder engine provided exciting acceleration in Jeep Wranglers for years, with high torque at low engine speeds.

amc engines

Based on the old 199 cubic inch Typhoon Six introduced in 1964 (via the tall-deck 232), the AMC 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.

According to AMC historian Frank Swygert, the 4.0 block is around 1/8” wider than its predecessors, with the 0.10” larger bore mostly countered by the lack of mechanical fuel pump support. 4.0 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, to avoid seeping coolant). mopar

big half engine

To create the 4.0, engineers tried to preserve parts and dimensions to reduce the need for new tooling and inventory; all 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.”

4.0 liter engienWhen the 4.0 first came out, AMC was using two six cylinder engines, a troublesome 2.8 liter V6 from General Motors and AMC’s 258 CID (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 having debuted as the sole Eagle engine and an optional Wrangler motor.

A related engine, the 2.5 liter four-cylinder, which appeared in 1983, was based on the same basic architecture; Frank Swygert wrote that it was essentially the 258 with the center two cylinders removed and a new head. The 2.5 was engineered by AMC to be “part of a two-engine set,” according to Willem Weertman; the other engine would be the 4.0 liter six. The four-cylinder produced 125 horsepower in its final years, and replaced a 2.5 liter GM engine; to confuse matters, Chrysler made a 2.5 liter four as well, producing a meager 100 hp with throttle-body fuel injection.

Mopar sold a 4.7-liter I-6 stroker long block, designed to deliver around 265 hp and 290 lb.-ft of torque, for any Jeep 4.0 from 1991 to 2006.

When the 4.0 burst onto the scene, it had 180 horsepower in the Wrangler (a German publication listed it as 172 horsepower, and, in the Cherokee, it started out with 177 hp). It was being designed for fuel injection from the start, with no mechanical fuel pump. The injection was developed with help from Renault, and used the Renault-Bendix, or Renix, system from 1987 to 1990.

half viewBy 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 in 1991, when 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.

Starting in 1991, the Jeep 4.0 engine used Chrysler engine computers, allowing easier diagnostics; before that, Chrysler had to honor contracts with AMC’s ignition system vendor, Renix. Two yellow rubber covers on the right side of the engine compartment let dealers use the DRBII and the Jeep adapter to get codes and do certain tests. (Codes might not be stored in the Renix system and would have to be regenerated while the adapter was connected, according to Rob Mayercik.) Frank Swygert wrote that the 1991 system works in a similar manner, but uses a different type of camp position sensor, with no knock sensor or EGR.

Jeep 4 liter engine

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,

For loping and rough idle, I’d start with a good throttle body cleaning (the Mopar spray combustion chamber cleaner works), then progress to a full tune-up. You could also see about doing the "seafoam" ritual, or having a good injector cleaning done. If it still has the factory injectors, I’d keep a close eye on them - Renix injectors have been known to leak from the seams. If there’s any indication you might have a leak, I’d strongly recommend replacing the injectors. I’ve heard good things about Five-O Motorsports’ Jeep injectors. [Bob Sheaves agreed but said he used 5.8-liter Ford V8 fuel injectors with a standalone Haltech sequential injection system.]

Also check the EGR valve - Renix 4.0s have this, unlike the later 4.0s with the Chrysler electronics that use a different cam configuration to promote exhaust scavenging. I’ve heard of clogged EGR valves causing a number of emissions 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.

1987 4.0

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

  Horsepower Torque
1987 173 @ 4,500 215 @ 3,000
1988 177 @ 4,500 224 @ 2,500
1992 190 @ 4,750 225 @ 4,000*
2003 190 235
* 190 / 220 on Wrangler

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:

2003 Wrangler 4.0 USA Europe
Horsepower 190 177
Pound-feet 235 218
Kilowatts 142 130
Newton-meters 319 296
Power peak 4,600 4,600
Torque peak 3,200 3,500

In 2003, the two-ton Wrangler 4.0 could do 0-60 in 9.9 seconds (10.6 seconds, with the four-speed automatic).

2003 economy Manual Automatic
EU combined l/100 km 13.2 15.0
City mpg, USA (EPA) 15 14
Highway mpg, USA 18 18

The 4.0 was made in Kenosha, Wisconsin (at 5555 30th Avenue); 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.

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