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 excellent torque. In the Cherokee it did well, in the Grand Cherokee it was a bit sluggish due to the weight of that vehicle.
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 problematic Ford carburetor) and AMC's own 258 CID straight-six, with 4.2 liters of displacement, that the 4.0 itself was based on. The 4.2 was used in the CJ7, Concord, Spirit, and Eagle. While the 4.2 and 2.8 were both relatively strong for their day - keeping in mind that the Chrysler 318 V8 was producing 132 horsepower! down from 150 - the 4.0 beat just about all competing engines, including those of Japanese automakers. (The 4.2 produced about 112 hp in 1990, the 2.8 about 120 hp, and the 2.5 about 117 hp - torque was naturally higher in the 4.2).
(AMC's own 2.5 liter four-cylinder, producing 125 horsepower in its final years, replaced a 2.5 liter GM engine; to complicate matters, Chrylser made a 2.5 liter four as well, producing a meager 100 hp without a turbocharger).
When the 4.0 did 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. (The Wrangler also sold with the 124-125 2.5 four, and, in Germany, a diesel). The Cherokee's version made 190 hp starting in 1991.
In 1990, when the 4.0 still only had 177HP, the GM 4.3L V6 only made 160HP, the Ford 4.9L I6 (used in the F150 and Econoline) only made 145HP, the Ford 4.0 V6 made 155HP, the Chrysler 5.2L made 170HP, the Jeep/AMC 2bbl 360 V8 made 144HP, and the Nissan 3.0 V6 (used in the Pathfinder) made 153HP.
The YJ didn't get the 4.0 until the 1990s (it was still using the old 258 six). When the YJ did get the 4.0, it only made 181HP... probably something to do with 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 112-117hp 258 I6, as well as the Eagle, Concord and Grand Wagoneer, which came with the four-barrel 360 V8, making just 144 hp. The 4.0 I6 made more power than the Ford 302 V8, Chevy 305, Chrysler 318, and AMC 360, as well as any of the 6 cylinder engines the Japanese were putting in their trucks... and it had comparable or better fuel economy.
An AMC Concord with the 177HP 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). The Dodge Dakota, Ram, and vans would have been better with this engine rather than the 3.9L V6 or the pre-Magnum 318 V8.
Compared with other contemporary engines, the 4.0 was strong up until the end - when it was finally dropped from the Jeep Wrangler.
As Jim Stone pointed out, the 4.0 “is descended from the ‘Typhoon’ 232 cid straight-six that came out in 1964. It may have inherited some elements from earlier Nash/AMC poewrplants, but it was essentially a whole new design in 1964.”
Starting in 1991, the Jeep 4.0 engine used Chrylser 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.)
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 [except in the Wrangler, so far] 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 suprised at how well the fix worked and how long it lasted (it was still fine when I got rid of the vehicle).
Specifications (2003)
Description: in-line six, overhead valve (two per cylinder), flat-face followers, hydraulic lifters, cast iron block and head
Displacement:
3960 cc (242 cubic inches)
Bore and stroke 98.4 mm x 86.7 mm
Compression ratio 8.8:1
Redline: 5,300 rpm
Maximum power, 2003, Wrangler:
- USA - 190 hp (142kW)@4,600 rpm; 235 lb-ft (319 Nm) @ 3,200 rpm
- Europe - 130 kW (177 hp) @ 4600 rpm;
296 N·m @ 3500 rpm
Fuel injection (final form): sequential multiple-port, electronic
In the Wrangler, 0-60 in 9.9 seconds (manual; 10.6 seconds, four-speed automatic); top speed 108 mph - the Wrangle weighed over two tons, so those aren't bad times.
| European fuel economy (litres/100 km) |
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) fuel economy (mpg) | ||
| 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!
