Before you drive the Jeep hard, before you take it off road, before you do anything else mechanical, change all the fluids. Every one of them. This is not routine maintenance. It is diagnostics.

Fresh fluids protect your drivetrain. But what comes out of the old fluids tells you something important about the condition of everything connected to them. This step costs very little, and it gives you more useful information than almost anything else you can do in the first week of ownership.

Fuel

If the Jeep has been sitting for more than six months, the gasoline in the tank is no longer a fuel—it is a contaminant. Do not attempt to "run it through" the engine. Old gasoline breaks down into varnish and gum that can seize intake valves in their guides, bending pushrods and causing significant engine damage within minutes of startup.

Disconnect the fuel line before it reaches the carburetor and pump a small amount into a clear glass jar.

• Smell and Color: Fresh gas is clear or slightly yellow and has a sharp, familiar odor. Old gas turns dark amber or orange and develops a sour, acrid smell similar to paint thinner. If it smells like varnish, it must be drained.
• Sediment: Look at the bottom of the jar. Fine red or black grit indicates that the internal lining of the steel fuel tank is corroding or the original rubber fuel lines are disintegrating from the inside out.
• Water: Since water is heavier than gas, it will settle as distinct bubbles or a clear layer at the bottom of your jar. This is a sign of a failed fuel cap seal or a tank that has "sweated" from temperature swings.

If the fuel is contaminated, drain the tank completely and flush the lines with compressed air before reconnecting them to the fuel pump or carburetor. It is much easier to drain a tank now than it is to rebuild a gummed-up carburetor and replace bent pushrods later.

Engine oil

Drain the oil and look at what comes out before it runs into the drain pan. Fresh, clean oil that is simply dark from normal combustion is fine. Milky, gray, or foamy oil means water contamination, likely a head gasket issue or a cracked block. Metal particles in the oil mean internal engine wear. Thick, sludgy oil that barely drains means the engine has been neglected for a long time.

Change the oil and filter regardless of what you find. If you found contamination, that is a problem to address before you put any load on the engine.

Gear oil — front and rear differentials and transfer case

These are the fluids most previous owners skip. Pull the fill plugs on both differentials and the transfer case before you drain them. If the oil runs out when you pull the fill plug, the level is correct. If you have to reach a finger in to find the oil, it is low, and you need to know why before you just fill it back up.

Drain the old gear oil and look at it. Healthy gear oil comes out dark brown or black from normal oxidation. Fine metal particles are normal and show up as a light shimmer. Large metal chunks or flakes are not normal and indicate gear or bearing wear that needs to be investigated. Milky gear oil in a differential means water intrusion, probably from a bad axle seal or faulty vent tube.

Use the correct weight gear oil for each application. The Dana 30 and AMC 20 call for 80W-90 conventional gear oil or the equivalent. The Dana 300 transfer case also uses 80W-90 in most applications. Check your specific year. Do not use a limited-slip additive unless the differential is a Trac-Lok limited slip unit.

Transmission fluid

Manual transmissions use gear oil, typically 80W-90. Automatics use ATF. Drain and inspect just like the differentials. Dark, burnt-smelling ATF in an automatic transmission is a sign of heat stress. Metal particles mean internal wear. Low fluid level in an automatic means either a leak or it has been run dry, neither of which is a good sign.

Coolant

Drain and flush the cooling system. Old coolant loses its corrosion inhibitors over time and actually starts to corrode the aluminum and cast iron components it was designed to protect. Look for rust particles, oil contamination (a sign of head gasket issues), and the condition of the coolant itself. Bright green or orange coolant that simply looks old is fine. Rust-colored coolant or oil-contaminated coolant is a problem.

Refill with the correct mix for your climate. A 50/50 mix of ethylene glycol antifreeze and distilled water is the standard starting point.

Brake fluid

Brake fluid is hygroscopic; it absorbs water from the atmosphere over time. Old brake fluid with high water content has a lower boiling point, which means fade under hard use, and it accelerates corrosion inside the brake lines and calipers. Flush and replace with fresh DOT 3 or DOT 4. While the system is open, note the condition of what comes out. Extremely dark, contaminated fluid means the system has not been touched in a long time and every rubber component in it should be considered suspect.

Power steering fluid if equipped

Check the level and condition. Dark, burnt power steering fluid should be flushed. While you are there, check the hoses and connections for leaks and cracking.