The Jeep 4.0 liter PowerTech Straight-Six Engine

Mopar has a 4.7-liter I-6 stroker long block, a powerful upgrade for the 4.0-liter I-6, designed to deliver around 265 hp and 290 lb.-ft of torque, and to fit in any Jeep ordered wtih the 4.0-liter I-6 engine from 1991 to 2006.

The Jeep 4.0 liter six-cylinder engine provided exciting acceleration in Jeep Wranglers for years, with excellent torque. Based on the old 199 cubic inch Typhoon Six introduced in 1964 (via the tall-deck 232), the AMC-engineered 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, due to the 0.10” larger bore, and the lack of mechanical fuel pump support; 4.0 heads can be bolted to the earlier 232 and 258 I-6 blocks (the ports on the right edge have to be sealed when this is done, to avoid seeping coolant).

amc engines

To create the 4.0, the older engines’ bore and stroke were also changed, but 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.

When the 4.0 first came out, AMC was using two six cylinder engines, a troublesome 2.8 liter V6 from General Motors (with a similarly troublesome Ford carburetor) and AMC's 258 CID (4.2 liter) straight-six, which had been derived from the 232 cubic inch six. 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.

Why was it dropped? Norm Layton wrote that in-line sixes “have trouble meeting emission standards at their low rpms. Engineers discovered raising the rpm levels more completely burned the fuel and lowered emissions. The in-lines could not operate in that range and compete with noise/vibration, top end, and emissions, without a lot of work and money.” Former Jeep engineer Bob Sheaves wrote that the 4.0’s replacement was the 4.7 liter V8 engine.

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.

When the 4.0 burst onto the scene, it had a full 180 horsepower in the Wrangler (a German publication listed it as 172 horsepower, and, in the Cherokee, it started out with 177 hp). One of its advantages was being designed for fuel injection: it had no mechanical fuel pump and was intended from the start to be injected, developed with help from Renault’s fuel injection techs (it used the Renault-Bendix, or Renix, system from 1987 to 1990).

In 1990, the GM 4.3L V6 only made 160 hp, the Ford 4.9L I6 (used in the F150 and Econoline) only made 145 hp, the Ford 4.0 V6 made 155 hp, the Chrysler 5.2L made 170 hp, the Jeep/AMC 2bbl 360 V8 made 144 hp, and the Nissan 3.0 V6 (used in the Pathfinder) made 153 hp.

Jeep upped the ante in 1991, when the Cherokee's version made 190 hp.

The YJ stuck with the old 258 six until the 1991 model year, when it gained the 4.0 — but only at 181 hp, possibly due to a more restrictive exhaust.

Peter wrote:

In 1987 I wondered why they didn't drop the 4.0 I6 into the YJ, which was still using the 112-117 hp 258 (4.2 liter) six; or the Eagle, Concord and Grand Wagoneer, which came with the four-barrel 360 V8, making just 144 hp. The 4.0 made more power than the Ford 302 V8, Chevy 305, Chrysler 318, and AMC 360, as well as any of the six cylinder engines the Japanese were putting in their trucks, and it had comparable or better fuel economy.

An AMC Concord with the 177 hp 4.0 I6 might have been a better police pursuit vehicle than the Dodge Diplomats (140-150HP), Chevy Caprices (160-170HP) and Ford Crown Victorias (160-165HP). Compared with other contemporary engines, the 4.0 was strong up until the end.

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.

Starting in 1991, the Jeep 4.0 engine used Chrysler engine computers, allowing for easier diagnostics; before that, Crysler had to honor contracts with AMC's ignition system vendor, Renix. There are two yellow rubber covers on the right side of the engine compartment; dealers can 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 CPS, no knock sensor or EGR, and complies with OBD requirements.

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

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; 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).

The reasons the 4.0L went away 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.

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,

4.0s are generally long-lived. 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:

I owned a 1992 Cherokee with a 4.0 I-6. It 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.

This engine also has cooling system corrosion issues. The thermostat neck looked like termites had been eating at it when I changed the thermostat at its first change interval. Some pits were going all the way through the neck and only the hose clamp and hose were preventing a leak. My fix was good old JB Weld slathered liberally over the entire interior surface and the outer surface of the hose neck. this not only filled the pits, but electrically isolated the pot metal thermostat housing from the coolant (aka electrolyte) which effectively shut down the galvanic corrosion cell. Admittedly this was a jury rigged fix, but since the dealer was closed and the part was a dealer only item and I needed the vehicle for Monday morning (it was Sunday Afternoon). I was surprised at how well the fix worked and how long it lasted (it was still fine when I got rid of the vehicle).

Specifications (2003)

  • Overhead valves (two per cylinder), flat-face followers, hydraulic lifters, cast iron block and head
  • 3960 cc (242 cubic inches)
  • 98.4 mm bore x 86.7 mm stroke
  • 8.8:1 Compression ratio
  • Redline: 5,300 rpm

Maximum power, 2003, Wrangler:

Wrangler: 2003 figures 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

Acceleration. In 2003, in the Wrangler, 0-60 in 9.9 seconds (manual; 10.6 seconds, four-speed automatic); top speed 108 mph - the Wrangler weighed over two tons, so those aren't bad times.

Liters per 100 km (EU), 2003 Manual Automatic
Combined cycle 13.2 15.0
Ex-urban cycle 9.0 10.7
Urban cycle 20.5 23.0
Combined CO2 315 g/km 362 g/km
USA (EPA) miles per gallon Manual Automatic
City 15 14
Highway 18 18

Built in Kenosha, Wisconsin (5555 30th Avenue) - the plant also 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 was built in 1917!

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