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 disassembly and cleaning of the engine.
Summing It Up
An engine that passes the inspection and tests could be considered worthy of modification. A minor rebuild during modification, such as new gaskets and seals, should be considered, as well as replacement of certain components that are typically a hassle to replace when the engine is fully assembled, such as the thermostat, hoses, belts, and water pump.
Rebuilding
If, after inspection, a rebuild is in order, it is best to pull the engine and further inspect crankshaft and connecting rod clearances to determine if machine work is needed. Consult an engine rebuild guide for your particular engine to determine inspection specs.
If the engine checks out, a rebuild can be far less expensive than a new engine and can be performed by a do-it-yourselfer to save some expense. Certain things such as engine cleaning should be performed by a professional to ensure all contaminants are removed from even the hard-to-reach places in an environmentally sound and efficient manner.
In addition, consider a professional valve job while the engine is out and apart, again another overlooked job so easily done at this time. If rebuilding an engine is not in your bag of tricks, a good engine shop should be able to perform the rebuild, including checking proper clearances and machining if needed.
Bent pushrods from a past broken timing chain combined with age, dirt, and grime made this engine run poorly, and after years of neglect a rebuild was needed. Bored .020 over, cleaned, and new bearings made this engine like new again. During assembly, a new performance cam, intake, and a fresh coat of factory color paint made this engine ready to go back into place.
Replacement or Swap
If the existing engine is beyond repair, a decision to replace the factory engine with a same type or swap to another type needs to be made. Typically, replacing the engine with the same type is the least expensive option because the expense of a swap and the costly items such as adapters, radiators, mounts, exhaust, etc., that go with it will not be incurred.
If the choice is replacing the engine with a similar type, consider factors such as engine size; perhaps an AMC 304 would be best replaced with a 360 that bolts right in.
Finally, deciding to install a used replacement engine in good shape is considerably less expensive than a crate engine.
Performance Parts and Modification
Increasing the performance of your Jeep’s engine can be achieved with a variety of additions and modifications. Because the goal is to build a Jeep that performs well on- and off-road, the modifications are not to build a 1,000-hp race engine but rather a good-torque and good-power engine. The beauty of off-roading a Jeep is that horsepower is not always as important as it may be in other automotive circles.
A rebuilt AMC 401 in this Scrambler provides enough horsepower to easily propel this Jeep in the passing lane. The nature of the 401 and thoughtful performance additions keep the engine producing plenty of high horsepower for some fun while maintaining the low-torque “tame” for those slow, controlled trail maneuvers.
Much of the day on the trail is spent moving at slow speeds and excess horsepower usually results in poor drivability, overheating, and broken parts. This fact allows a Jeep owner to invest a reasonable amount of the Jeep funds into performance parts that actually make a difference without going overboard. The saved funds can be put into things that matter more on the trail, such as traction systems, gearing, suspension systems, and everything else.
Here, I generalize on performance parts and focus on common modifications that enhance performance.
Factory Engine Modifications
Because most