Main >> Engine >> Factory >> AMC 258 I6

The AMC 258 was introduced in 1971 in J-series pickups and Wagoneers and in 1972 in CJs. It continued to be used through 1990 in the Wrangler. It is basically a stroked 232 engine; some background about the AMC inline 6s (I6s) is given in the article about the 232. The great thing about this engine is it reliable, inexpensive, and has horse power and torque peaks at low RPM.

The 258 has a cast iron block and cylinder head, hydraulic lifters (with non-adjustable rockers), 7 main bearings, and cast iron crankshaft and rods. The 258 I6 engine was always carbureted from the factory. The 1-barrel engines came with the Carter YF carburetor, and the 2-barrel engines came with the Carter BBD. Many people complain about the Carter BBD carburetor and the maze of emissions vacuum hoses. There are a couple of articles below that can help you sort through those hoses and tune the Carter BBD. For a carbureted engine, the 258 with the Carter BBD performs very well off road.

There are two generations of 258 engines. The first generation was introduced in 1971. This earlier engine has cast iron intake and exhaust manifolds with the two manifolds bolted together by 4 bolts and a gasket between. In 1981, AMC revised the 258 for lighter weight. A new block casting and fewer crankshaft counterweights were used. A new smoother exhaust manifold and aluminum intake manifold were introduced. V-belts were replaced with a serpentine belt, and a new lightweight plastic valve cover was fitted. Although these two generations have the same bellhousing and motormount attachments, and are mostly the same internally, the differences must be kept in mind when choosing used replacement parts.

The 258 can be identified by the 4th character of the engine build date code ("A", "B", "C" or "M"), stamped into the engine block below and between the number 2 and 3 spark plugs. An engine stamped with "B" is an early 258 with a 7.6:1 compression ratio. "A" and "B" are engines with 1-barrel carburetors, while "C" and "M" are 2-barrel. "M" originated in Mexico. Other codes ("E", "F", "K" and "L") are 232 engines.

The 258 used in '87-'90 Wrangler YJs uses a computer controlled Carter BBD. It is very simlar to the 258 used in '82-'86 CJs, but performance suffers due to emmission controls.

 

The 4.0 L (3956 cc) straight-6 was an evolution of the 258 and 150 and appeared in 1987. It had the same 3.88 (98.4 mm) bore as the 150 with a longer 3.41 in (86.7 mm) stroke. The 4.0 has been discontinued at the end of the 2006 model year as the Jeep Wrangler will instead get Chrysler's 3.8 L OHV V6. This is at the same time other manufacturers are introducing new I-6 engines.

The first 4.0 engines in 1987 had RENIX (Renault/Bendix) engine control systems, which were quite advanced for their time, but are now handicapped because there are very few scan tools which can be "plugged in" to a RENIX system for diagnosis. The Renix also used a very advanced engine knocking sensor that allowed the computer to make the appropriate changes to prevent predetonation in each cylinder.

In 1991 Chrysler Corporation, then the owners of the Jeep brand, redesigned the engine control computer and the intake ports; the camshaft profile was also changed. The result was an engine that made 190 hp (142 kW) and 225 ft·lbf (305 N·m) of torque.

Small changes were made to the cylinder head for the 1995 model year. In 1996, the engine block was updated, it made use of more webbing cast into the block, and a stud girdle for added rigidity of the crankshaft main bearings.

The cylinder head was again changed in 1998 to a lower flowing, but more emissions friendly, design.

This engine was used in the following vehicles::

1987-2001 Jeep Cherokee
1993-2004 Jeep Grand Cherokee
1987-1990 Jeep Wagoneer
1987-1992 Jeep Comanche
1991-2006 Jeep Wrangler

 

AMC 258 I6
Bore x Stroke	3.75" x 3.90"
Displacement	258 (4.2L)
Compression Ratio	9.2:1
Horsepower (net)	112@3200
Torque (net)	210@2000
Main Bearings	7
Valve Configuration	OHV
Fuel	2bbl Carter BBD

 

 

The 258 used in '82-'86 CJs used a computer controled Carter BBD. The computer controls mixture based on an O2 sensor and other sensors.

 

AMC 258 I6
Bore x Stroke	3.75" x 3.90"
Displacement	258 (4.2L)
Compression Ratio	9.2:1
Horsepower (net)	115@3200
Torque (net)	210@1800
Main Bearings	7
Valve Configuration	OHV
Fuel	2bbl Carter BBD

 

 

 

The 258 used in '79-'81 CJs, some used a 1bbl carb and some used a 2bbl carb.

 

AMC 258 I6
Bore x Stroke	3.75" x 3.90"
Displacement	258 (4.2L)
Compression Ratio	8.3:1
Horsepower (net)	110@3500
Torque (net)	195@2000
Main Bearings	7
Valve Configuration	OHV
Fuel	1bbl and 2bbl

 

The 258 used in '72-'78 CJs, J-series pickups, and Wagoneers, some used a 1bbl carb and lower compression than later 258s.

 

AMC 258 I6
Bore x Stroke	3.75" x 3.90"
Displacement	258 (4.2L)
Compression Ratio	8.0:1
Horsepower (net)	110@3500
Torque (net)	195@2000
Main Bearings	7
Valve Configuration	OHV
Fuel	1bbl

 

 

The 258 was introduced in 1971 as the standard engine in J-series pickups and Wagoneers.

 

AMC 258 I6
Bore x Stroke	3.75" x 3.90"
Displacement	258 (4.2L)
Compression Ratio	8.5:1
Horsepower (gross)	150@3800
Torque (gross)	240@1800
Main Bearings	7
Valve Configuration	OHV
Fuel	1bbl Carter or Holley

 

 

 

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