The 242 cu in (4 L) engine was developed by AMC in just 26 months using many off the shelf components while featuring, among others, additional strength, improved combustion chamber, port setup, and cam profile.[12] The total weight of the new engine was 483 pounds (219.1 kg), only one pound more than the 258 six despite its heavier components and parts.[12] For example, the cast aluminum valve cover featured 15 bolts, the industry's most, to achieve a positive seal.[12]
The new engine, which was introduced in 1986 for the 1987 model year, was a further evolution of AMC's 258 six.[13] It had a 3.875 in (98.4 mm) bore and a 3.414 in (86.7 mm) stroke giving it a displacement of 241.6 cu in (3,959 cc).[14] Connecting rod length was 6.125" - similar to the discontinued 199 which was phased out in 1970.
The 4.0 is one of AMC's best-known engines.[15] It was one of four AMC engines kept in production when Chrysler bought AMC in 1987. Chrysler engineers continued to refine the engine to reduce noise, vibration, and harshness. The last in the line of the AMC inline sixes, the 4.0 is regarded as one of the best Chrysler 4x4 off-road engines.[16] A Motor Trend long-term test of a 1997 Cherokee XJ noted "this long-lived OHV powerplant has a reputation for getting people where they need to go" as well as "much love expressed by owners for the torquey 4.0-liter/190-horsepower inline six."[17] The engine is known for longevity, and can sometimes go more than 300,000 miles (482,803 km) without rebuilding.[18][19] There are many aftermarket parts available.[20]
When introduced, the block-mounted oil filter check valve was eliminated on the 4.0 (along with the 2.5 and 4.2s manufactured after September 1986) when AMC engineers standardized their oil filters. The pre-1987 engines had an oil filter adapter with 3/4 (cylinder block end) and 13/16 threads (which used a GM oil filter common to Buick, Oldsmobile, Pontiac, and Cadillac V8 engines) while the 1987-06 engines were originally fitted with a 20 mm metric thread filter (later revised in 1991 for the common 3/4 threaded Mopar and Ford V8 oil filter; the oil filter adapter was redesigned where the oil filter is positioned 90 degrees offset adjacent to the starter motor - a Ford V8 oil filter can be used in place of the short filter increasing oiling capacity).
The first 4.0 engines in 1987 had a RENIX (Renault/Bendix) engine management system considered quite advanced for their time.[10] A knock sensor allowed the ECU to control spark advance in response to fuel octane and engine load. Unfortunately, there are few scan tools capable of interfacing with the system to pull diagnostics codes. RENIX systems also have no permanent memory for diagnostics codes thus making the diagnosis of intermittent problems more difficult.[21]
The 1987 RENIX 4.0 made 173 hp (129 kW; 175 PS) and 220 lb·ft (300 N·m). In 1988, the 4.0 received higher flowing fuel injectors, raising output to 177 hp (132 kW; 179 PS) and 224 lb·ft (304 N·m)—more power than some configurations of the Ford 302, Chevrolet 305, and Chrysler 318 8-cylinder engines, and more than any of the Japanese 6-cylinder truck engines, but with comparable or superior fuel economy.[21]
In 1991, a Chrysler MPI fuel injection system replaced the RENIX system, and the intake ports were raised approximately .125 in (3.2 mm) for a better entry radius. Chrysler also enlarged the throttle body and redesigned the intake and exhaust manifolds for more efficiency, and the fuel injectors were replaced with higher flow units. Camshaft timing was also changed. The net result was an engine that made 190 hp (142 kW; 193 PS) and 225 lb·ft (305 N·m). Badging on most Jeeps equipped with this engine read "4.0 Litre HIGH OUTPUT." The new cam profile combined with altered computer programming eliminated the need for an EGR valve and knock sensor, but made the engine more sensitive to alterations, especially where emissions are concerned.[citation needed] The OEM fuel injectors used with the Mopar MPI system (manufactured by Siemens) have been known to leak fuel especially with OBDII where plugged catalytic converters are common which usually throws a P0420 code - one modification is the use of factory Bosch fuel injectors from a 1995/96 Dodge Neon which will provide quicker acceleration and gain extra horsepower.
Small changes were made to the cylinder head for the 1995 model year. In 1996, the engine block was redesigned for greater strength. The new block had more webbing and a stud girdle for added rigidity of the crankshaft main bearings. Engines installed in 1999 Grand Cherokees carried the "PowerTech" name that had been used intermittently in prior years and on other Chrysler truck and SUV engines. The name was subsequently passed on to 4.0s in the other Jeep models that used the engine, the Cherokee and Wrangler. The cylinder head was again changed for the 2000 model year to a more emissions-friendly design.[citation needed] This head was designated as "0331" in the casting number. Early 0331 heads are prone to cracking, causing coolant to contaminate the oil, which can lead to catastrophic engine failure. The head cracks in the center between #3 and #4 cylinders. The crack is usually discernible with the valve cover removed as a "milky" tan line. This condition is usually discovered before catastrophic engine failure, but can lead there if not corrected in a timely manner. The casting was fixed in mid to late 2001, but the same casting number was retained. The "fixed" heads have "TUPY" cast in the center where the cracks used to occur.[22] Also new for the 2000 model year, was the distributorless, coil on plug ignition system.[23] Option code: ERH.[WIKI]
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