of reliable service to its owner. The low-end torque of this engine and the low gears of a CJ is a near-perfect combination. However, there are some gotchas with this engine, and perhaps the biggest one deals with the horrific carburetor system found on 1981 and later CJs.
The AMC In-line 6-cylinder powered the CJ and so many other Jeep vehicles reliably and effectively for more than 30 years. This Jeep’s 258 is showing its age and use from all the years of mud, grime, and dirt present on it. These engines were true workhorses that were capable of 250,000 miles from brand new to totally abused.
AMC V-8
The AMC 304 V-8 is a fun engine for a CJ and even in totally stock form can provide plenty of power and torque to throw a CJ around on the street and trail. Although the 1970s smog-laden V-8s weren’t known for their performance, with a little help they can come alive. AMC V-8 engines were popular in the AMC muscle cars they came in, such as the Javelin and AMX. Thanks to those cars, a space was created in the performance aftermarket for the AMC V-8, and many upgrades and modifications are available. V-8–powered CJs running fenderwell headers and glasspack mufflers are an icon of the late 1970s.
The AMC Tall Deck V-8 engines were often accused of being copies of other manufacturers’ engines when in reality the engine was its own creation. The thing that separates the Jeep from other types of automotive enthusiast crowds is that a dirty, muddy engine is considered more attractive than shiny parts. Evidence that the Jeep is used the way it is intended only makes onlookers more interested.
Modify, Rebuild or Swap?
Before modifying your factory Jeep engine, consider its condition. Is it worth modifying, is it too far gone and not worth rebuilding, or even further swapping? Some inspection and a few tests should be performed on your Jeep’s engine before making this decision. A good-condition engine that is ready to modify should stand up to the added performance from the modifications. A rebuild candidate may be a fair-running engine but due to maintenance neglect or excess wear, this engine would benefit from a rebuild, along with modifications. A freshly rebuilt engine can be as good as new and can revitalize a Jeep. In the case that the engine may cost too much to rebuild or is beyond rebuildable condition, a replacement or swap should be considered.
Inspection and Testing
An engine in running condition is a good start, but how well it runs and how well it was maintained are just a couple of items that need further examination. Starting on the outside, inspect the cleanliness of the engine. Excess grease and gunk on the outside of the engine can mean there were unfixed leaks from old gaskets or worse, cracks. This grime hides problems and can cause the engine to run hot or possibly even catch fire. This book doesn’t focus on the specifics of rebuilding engines; consult an appropriate guide specific to your engine for more detail.
Finding grime like this inside your engine may mean a rebuild is in order. Grime in the rocker arm area usually means it’s everywhere. Changing oil and filter according the manufacturer’s specifications prevents this, even on an old engine. Complete disassembly and professional cleaning is the only real way of removing the contamination.
Compression
Compression testing can indicate the condition of the valves, valveseats, and piston rings. This simple test is performed using a compression tester that threads into the spark plug hole, cranking the engine (without it starting), and noting the pressure measured. Pressure numbers vary across engine types and years. In reality, the actual pressure number isn’t as important as the consistency (10-percent variance) of pressure among cylinders. If each cylinder measures a similar pressure, the engine’s valves, valveseats, and piston rings could be considered in sound condition.
Test an engine’s compression with the ignition system disconnected and all spark plugs removed. Thread in the compression tester and crank the engine using a remote starter switch or with the aid of another person on the key. Be sure the transmission is in park or neutral. Take note of each cylinder’s pressure and compare after the test is complete. Similar pressures in all cylinders indicate good balance and even wear, desirable if the compression is within specification, which is around 120 to 140 psi for both the I-6 and V-8.
AMC I-6 and V-8 engines in good condition generate pressures greater than 100 psi with 120 to 140 psi being within spec. A consistent low reading across all cylinders could be normal but may indicate a flat, worn-out engine. If a cylinder measures low, adding a teaspoon of oil to the low cylinder and repeating the test will indicate if bad rings or valvetrain cause the low pressure. No change in pressure from the oil test can indicate a holed piston or blown head gasket. Low pressures might warrant a full inspection and rebuild.
Engine Oil
Draining and inspecting the oil can give an idea of the conditions inside. Most people know that the engine oil provides lubrication to the close tolerances of the engine’s bearings but it can also suspend contaminants within. New oil is a nice, clear amber color and quickly turns to a blackish color from contaminants. Inspecting the oil for its texture and smell can indicate a problem; gritty oil can be a sign that the oil is very old, along with an old filter.
If other fluids come out with the oil, such as coolant, this indicates the likelihood that there is a gasket leak or crack in the block. Frothy or milky oil can contain coolant or water. If the Jeep was driven in deep water, it’s possible that water has entered the engine through the vents.
Coolant
Draining the coolant from the engine and inspecting it for rust or corrosion can indicate the condition of the cooling system within the engine and radiator. The presence of rust can indicate the water-to-coolant ratio was too weak and the water was causing the cast iron to rust. In addition to rust, a weak coolant ratio can result in corrosion to copper components. Periodic testing of the coolant ratio is simply done using an inexpensive coolant tester.
Interior
Interior inspection of the engine is definitely more involved and requires some disassembly of the engine. Starting with the valvecover(s), remove the cover and inspect the rocker/valve area and components. If this area of the engine is covered with grime and old, thick, sticky oil (known as sludge), this is a good indication of very old oil, overheating, and/or poor-quality oil. If the condition of this area is very poor, it’s likely the rest of the inside of the engine looks the same.
Inspection of the interior of an engine is a good indication of the extent of a rebuild. The removal of the oil pan or intake manifold on a V-8 (requires draining antifreeze) gives a view of the condition of the interior of the engine. The smell or presence of non-oil fluids such as water, antifreeze, or gasoline can indicate trouble.
A few products can be purchased to help remove sludge; many (if not most) aren’t very effective and can result in engine damage by releasing stuck sludge into the engine, clogging bearings and passages. Proper sludge removal is generally best done by full
