| Engine oil & filter | SAE 10W-30 (Quadratec); ExtremeTerrain lists 10W-30 or 5W-30 · ≈4 qt (2.5) / 6 qt (4.0, 4.2) with filter (Quadratec) | Haynes lists every 7,500 mi or 12 months (3,000 mi in severe use) | Drain-plug torque: ExtremeTerrain's TJ-era table gives 25 ft-lb for the 2.5 and 4.0 confirm in manual for your yj. ExtremeTerrain lists separate filters for 87–93 and 91–06 engines, so check which one yours takes.Oil filter, 87–93 YJ · ExtremeTerrain ↗Crown oil filter, 91–06 2.5L/4.0L YJ & TJ · ExtremeTerrain ↗ |
| Air filter & PCV | Engine air filter and PCV filter/valve | Every 30,000 mi or 24 months (Haynes); sooner on dusty trails. Haynes also lists the fuel filter for 1987–95 models. | On the 4.2, Ludel lists the PCV valve and air cleaner among the first checks before blaming the carburetor.YJ engine & fuel system parts · Quadratec ↗ |
| Spark plugs, cap & rotor | Plug torque 27 ft-lb for all YJ engines (ExtremeTerrain's YJ engine torque table, matching its 1997–2006 TJ table) | Every 30,000 mi (Haynes) | Both the 4.2 and 4.0 in the YJ use a distributor. Haynes lists inspecting the plug wires, cap and rotor on 2003-and-earlier models, so do them with the plugs.YJ ignition parts catalog · RockAuto ↗ |
| 4.2 carburetor, fuel filter & vacuum lines (1987–90) | Carter BBD feedback carburetor · mechanical fuel pump | Now, then inspect at every service | Ludel says a clogged filter, a weak mechanical pump or a vacuum leak can mimic carburetor faults. Replace brittle vacuum hose before rebuilding anything.2-barrel carburetor kit, 87–90 4.2L YJ · ExtremeTerrain ↗ |
| Coolant | 50/50 antifreeze and water (ExtremeTerrain) · ≈9 qt (2.5), 10.5 qt (4.0 and 4.2) (Quadratec chart) · antifreeze type confirm in manual | Every 30,000 mi or 24 months (Haynes) | Inspect the radiator, hoses and fan clutch while you're there. ExtremeTerrain lists a fan clutch that fits all YJs.Crown 52080183 replacement radiator, 87–06 YJ/TJ · Quadratec ↗Cooling fan clutch, 87–95 YJ · ExtremeTerrain ↗ |
| Manual transmission (AX-5 / AX-15 / BA-10) | 75W-90 GL-3 or GL-4 for the AX-5 and AX-15 (ExtremeTerrain) · ≈3.3 qt AX-5/AX-15, 5.2 pints BA-10/5 (Quadratec chart) confirm in manual | Change now, then every 30,000 mi | Retailer charts disagree slightly on AX-15 capacity (3.25–3.32 qt), so fill to the bottom of the fill hole. Haynes lists a 30,000-mile change.Manual transmission gear oil search · Quadratec ↗ |
| Automatic transmission (999 / 32RH / 30RH) | Mopar ATF of the type on the dipstick or in the manual confirm in manual · total capacity 8.3 qt TF999, 17.1 pints 32RH, 7 qt 30RH (Quadratec chart); a pan drop takes much less | Every 30,000 mi, or 12,000 mi in severe use (Haynes) | Change the pan filter at the same time. ExtremeTerrain's YJ torque table gives 35 in-lb for the 30RH/32RH filter screws and 14 ft-lb for the pan bolts.YJ driveline parts · Quadratec ↗ |
| Transfer case (NP207 / NP231) | ATF (ExtremeTerrain lists ATF+3 or ATF+4 for the NP231) · NP231 2.2 pints, NP207 4.4 pints (Quadratec chart) · fill until it runs out of the fill hole | Every 30,000 mi (Haynes) | ExtremeTerrain's YJ torque table gives 15–25 ft-lb for the NP231 drain and fill plugs. Inspect the output seals: Quadratec lists them among common leaks.Crown NP231 transfer case overhaul kit, 87–06 YJ & TJ · ExtremeTerrain ↗ |
| Front & rear differentials | GL-5 80W-90 or 75W-140 synthetic · +4 oz limited-slip additive for Trac-Lok (Quadratec chart) · Dana 30 2.5 pt, Dana 35 3.5 pt, Dana 44 4.0 pt (Quadratec chart) confirm in manual | Every 30,000 mi (Haynes), and after deep water crossings (ExtremeTerrain) | Cover bolts take 30 ft-lb in ExtremeTerrain's TJ-era table for the same Dana axles confirm in manual. Check the Dana 30 disconnect housing and pinion seals for leaks.Gear oil and friction modifier search · Quadratec ↗ |
| Vacuum disconnect system | Shift motor, check valve, reserve tank and hoses | Now, and whenever 4WD is slow to engage | Quadratec lists dry-rotted or cracked vacuum lines as the usual cause of a YJ that won't engage 4WD.4WD vacuum switch, 87–95 YJ · ExtremeTerrain ↗ |
| Brake & clutch hydraulic fluid | DOT 3 (ExtremeTerrain); Ludel uses DOT 4 in the clutch for heat resistance | About every two years | The clutch shares brake fluid type. On 1987–92 trucks a falling clutch reservoir points to the internal slave.YJ steering & brakes parts · Quadratec ↗ |
| Chassis grease, U-joints & spring bushings | Lubricate chassis components (Haynes) | Every 7,500 mi or 12 months (Haynes) | Grease the steering joints and driveshaft U-joints, and check the shackles and spring bushings for cracks.Old Man Emu OMESB57 leaf spring bushing kit, 87–95 YJ · Quadratec ↗ |
| Electrical grounds & battery | Firewall ground straps and battery terminals | Now | Quadratec suggests cleaning the grounds first when you see flickering lights, jumping gauges or intermittent starting.YJ electrical catalog · RockAuto ↗ |