AMC 242 (4.0L) Inline-6 Engine

The Jeep 4.0L engine (and it’s 2.5L little brother) is one of the most iconic off-roader engines ever made. Originally designed and built by American Motor Company (AMC), the 4.0L engines have maintained a track record of being reliable and “has a reputation for getting people where they need to go” (Motor Trend). These engines were two of the four engines kept by Chrysler during the acquisition of Jeep in 1987. They have long been known to reach well into 300,000 miles with minimal repairs given adequate maintenance.

Jeep’s Legendary “Straight Six” 4.0L Engine

The AMC Inline-6 engine dates back as far as 1952, but the more iconic design was developed and released in 1964. Since then, that 3.8L (232 cu in) inline six has been modified throughout the years until 2006, when development of the Jeep Inline-6 engine came to an end in the Wrangler TJ.

When the modern 4.0L was released, it made around 177 horsepower which was later updated in 1991 to make 190 horsepower. The engine design also allowed for a high amount of torque to be produced at relatively low RPMs, making it ideal for off-roading and rock crawling applications.

The 4.0L engine was used in the Jeep Cherokee, Jeep Grand Cherokee, Jeep Wagoneer, Jeep Comanche, and Jeep Wrangler in various years.

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

Engine Power Specifications

1987-90177hp @ 4500rpm224lb-ft @ 2500rpm
1991-95190hp @ 4750rpm225lb-ft @ 4000rpm
1996-01190hp @ 4600rpm225lb-ft @ 3000rpm
2001-06190hp @ 4600rpm235lb-ft @ 3200rpm

Oil Types and Capacities

ViscosityAbove 0°F: 10W-30 (preferred)
Below 32°F: 5W-30
Capacity6 qts
Oil FilterMopar 5281090AB
WIX 51085
Drain Plug Torque30ft-lbs

Tune-up Parts

Spark Plug1987-1990: Champion 344
1991-1998: Champion 412
1999-2001: Champion 438
Plug Wires1987-1990: NGK 53229
1991-1999: NGK 53168
2000-2001: N/A
Coils2000-2001: NGK 48662

Common Problems

Lifter Tick

Lifter tick is a common issue on these engines. While essentially harmless, many people do not enjoy the sound of the top of the engine slightly ticking when running. Besides replacing rockers and lifters, the only solution I’ve seen to work is to use a zinc additive in your oil or switch to an oil type that includes higher levels of zinc. One of the most recommended oils to use in the 4.0L engine (and 2.5L & 4.2L for that matter) is the Shell Rotella Diesel Engine Oil. The oil works great in the 4.0L engine and includes lots of Zinc. As a result, this engine oil usually reduces lifter tick.

2000-2001 0331 Casting Cylinder Head

From 2000-2001 there were casting flaws in the 0331 cylinder heads that lead to a decrease in strength. A lot of owners have expressed issues with their 0331 head, with the majority of the failures being a crack in the coolant jacket between cylinder 3 and 4, visible when the valve cover is removed. This would leak coolant into the oil.

Despite this being a common issue, many high-mileage factory 0331 cylinder heads are still on the road with no issues today. The most likely way to cause an issue with this head is to let it overheat. If you do have this cylinder head, be sure to keep your coolant in good shape and turn your Jeep off if you ever start to experience any overheating.

Crankshaft Position Sensor Failure

If you’re Jeep suddenly decides to stop running after it has warmed up, there is a chance your crankshaft position sensor (CPS) has failed. The CPS in Jeeps that came with the 4.0L engine seem to have an intermittent thermal failure issue when approaching higher mileage. Most of the time when your engine shuts down because of this, you can pull over and wait for 15-20 minutes and you may be able to restart the vehicle. If your CPS does go out, it is in a tight location to replace it. This is the type of failure that will leave you stranded in the desert if you do not have a backup plan. As a result, I suggest all 4.0L owners keep a spare factory CPS on hand at all times.

Cracked Piston Skirts

The piston skirt failure seems to occur with the 4.0L engine between 1996-2000. This condition occurs when a piece of the lower part of the cylinder piston (called a piston skirt) cracks or breaks off. This may introduce a slight ticking or clacking noise coming from your engine, as the piston slightly hits the walls of the cylinder. This isn’t as common as the other issues mentioned above, but is something to look out for. Despite this sounding like a catastrophic failure, these engines have been known to run a while longer with a cracked piston skirt.


With millions of 4.0L engines made, and I’m sure many hundreds of thousands still on the road today – many Jeep and off-road enthusiasts have taken to modifying their engines for better performance on and off-road. In fact, there is a whole forum dedicated to modifying this engine.

A common major upgrade is to make a “stroker” engine. Simply put, these engines are usually equipped with the older 4.2L length connecting rods and various other modifications in order to increase there displacement to somewhere around 4.6L – 4.7L. With the added displacement and other modifications, such as camshafts, upgraded lifters, and ported cylinder heads, many “stroker” kits are able to push 250-300 horsepower out of these engines.


Engine Fluids

EngineFluid TypeCapacity
AMC 2.5L I4Above 0°F: 10W-30
Below 60°F: 5W-30
4.0 qts
AMC 4.0L I6Above 0°F: 10W-30
Below 32°F: 5W-30
6.0 qts
GM 2.5L I4Above 0°F: 10W-30
Below 0°F: 5W-30
3.0 qts
Renault 2.1Ltbdtbd
VM Motori 2.5Ltbdtbd

Transmission Fluids

TransmissionFluid TypeCapacity
Chrysler A904 3-Speed
Aisin-Warner AW-4 4-Speed
Chrysler 30RH 3-Speed
Borg-Warner T-4 4-Speed ATF Dexron II1.75qts
Aisin-Warner AX4 4-Speed75W-90 GL3 or GL4
10W-30 Motor Oil
2WD: 3.7qts
4WD: 3.5qts
Borg-Warner T-5 5-SpeedATF Dexron II1.75qts
Aisin-Warner AX5 5-Speed75W-90 GL3 or GL4
10W-30 Motor Oil
2WD: 3.7qts
4WD: 3.5qts
Peugeot BA-10/5 5-Speed
Aisin-Warner AX15 5-Speed75W-90 GL3 or GL4
10W-30 Motor Oil
2WD: 3.32qts
4WD: 3.27qts
New Venture Gear NV3550 5-SpeedMopar MT Fluid
SAE 30W Oil

Axle Fluids

AxleFluid TypeCapacity
Dana 3075W-90
75W-140 (towing)
Dana 3575W-90
75W-140 (towing)
Dana 4475W-90
75W-140 (towing)
Chrysler 8.2575W-90
75W-140 (towing)

Transfer Case Fluids

Transfer CaseFluid TypeCapacity

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