General Motors began developing the Iron Duke in response to the oil crisis of 1973. New demand for small, fuel-efficient cars required GM to take a serious look at an economical four-cylinder engine. Engineers initially attempted to modernize existing V6 and V8 designs, and once considered developing a V4 to meet the market requirement.
General Motors began experimenting with the 2.3L Chevy Vega inline-four (AKA Chevrolet 2300) but found its cast aluminum block too delicate. Inspired by the 2.3 Vega, GM engineers developed a cast iron inline four-cylinder engine with a slightly larger displacement. The name ‘Iron Duke’ came from GM’s decision to switch from aluminum to cast iron.
General Motors found numerous applications for the Pontiac Iron Duke. The company installed the engine in vehicles for 16 years; between 1977 and 1993. From a design standpoint, the simple and durable Iron Duke was an all-purpose winner for General Motors.
GM Iron Duke Specifications
The Iron Duke, also known as the Pontiac 2.5, has a displacement of 2.5 liters or 151 cubic inches. The engine produced between 85 and 110 horsepower and 123 to 135 lb-ft of torque. Dry weight was a mere 375 pounds.
ENGINEGM IRON DUKEMANUFACTURERGENERAL MOTORSYEARS PRODUCED1977 to 1993DISPLACEMENT151CI (2.5L)CONFIGURATIONINLINE FOURFUEL TYPEGASOLINECOMPRESSION RATIO8.25:1 (1977 to 1983) 9.0:1 (1984 to 1993)BORE4 INCHES (101.6 MM)STROKE3 INCHES (76.2 MM)HORSEPOWER85 to 110 hp (63 to 82 kW)TORQUE123 to135 lb ftGAS MILEAGE22-30 MPG (EST)
The Iron Duke is a single cam overhead-valve inline four-cylinder engine. This ‘pushrod’ motor features a tried-and-true two-valve design and a single timing chain located in front of the block. The 2.5 has a 4” bore and a 3” stroke. Most early engines came with a two-barrel carburetor, but later models after 1982 received throttle-body fuel injection.
The Iron Duke was used extensively across GM’s line in both rear-wheel-drive and transverse front-wheel-drive configurations.
Initially, the Iron Duke replaced the 2.3 Vega in the Pontiac Astre, Pontiac Sunbird, and Pontiac Phoenix. It was also used in the Chevrolet Manza and the Oldsmobile Starfire. Small changes were made to the engine for some applications, but the motor remained fundamentally unchanged between brands.
AMC and Jeep
General Motors sold the Iron Duke to AMC, who used it across their vehicle lineup as well. The AMC Spirit, AMC Concord, and AMC Eagle featured the engine. Additionally, AMC opted to install the 2.5 I4 in base-model Jeep CJ-5 vehicles.
The Grumman LLV (Long Life Vehicle) is perhaps the most notable application of the GM Iron Duke 2.5L engine. Grumman produced over 100,000 of the iconic LLV mail truck between 1987 and 1994. Most LLVs came with the Iron Duke, but later models utilized the GM 2.2 I4 as a replacement. The USPS initially specified a 24-year service life, but the Iron Duke-equipped LLVs have already surpassed expectations.
Overall, the GM Iron Duke is a robust and reliable engine. However, cost-cutting and minor design flaws are known to affect its reliability. The most common failure point of the 2.5 is its timing gear, which is known to chip a tooth at around 80,000 miles. Replacing the timing gear is laborious, and requires the replacement gear to be heat-fitted onto the shaft.
A common point of failure on fuel-injected models is the MAP sensor. When the antiquated sensor fails, it causes numerous issues from rough idle to poor acceleration. EGR valves also fail frequently, though emissions components are cheap and readily available.
The aftermarket and knowledge base for the GM Iron Duke isn’t nearly as big as the Chevy 350 or Ford 302. Nonetheless, there are numerous resources available to Iron Duke owners. Replacement parts (not performance parts) are widely available due to the Grumman LLV, and GM still produces many parts. Crate engines are available from Jegs and other parts stores.
Forums are an excellent place to look for information and troubleshooting tips. Popular forums with ongoing expertise include the S10 Forum, Pennock’s Fiero Forum, and the Jeep-CJ Forum. Additional discussions can be found all over the web, including an occasional Q&A on Pirate4X4 or Reddit.
About THE AUTHOR
I rebuild & restore classic cars and trucks when I'm not researching and writing about all things automotive. My current project is a 1978 Ford.Read more about Joshua Weinstein