1991 MAZDA COSMO

Typical onset: 60,000-90,000 mi

Symptoms: Loss of compression in one or more rotors, Hard starting when hot or cold, Blue smoke on startup or deceleration, Rough idle and significant power loss, Poor fuel economy even by rotary standards

Fix: Complete engine rebuild with new apex seals, side seals, rotor housings inspection/replacement, and bearings. 20B rebuilds are 25-35 hours of labor; 13B-RE around 20-28 hours. Parts availability from Mazda is extinct; aftermarket Racing Beat or Atkins Rotary required. Expect 3-4 weeks turnaround at a rotary specialist.

Estimated cost: $5,500-9,500

Typical onset: 70,000-110,000 mi

Symptoms: Sequential turbo system not transitioning smoothly, Wastegate actuators sticking or leaking, Intercooler piping cracking at welds, Oil leaks from turbo oil feed lines, Overboosting or underperforming boost levels

Fix: Turbo rebuild or replacement on the 20B's sequential setup is complex—12-18 hours labor depending on whether you're doing one or both turbos. Actuators, intercooler piping upgrades, and oil line replacement add time. OEM turbos are unobtanium; expect aftermarket or rebuilt units.

Estimated cost: $3,200-6,000

Symptoms: Coolant temperature climbing past halfway mark, Coolant loss with no visible external leaks, Heater blowing lukewarm air, Warped rotor housings after overheat event, Cracked or brittle original radiator end tanks

Fix: Rotaries run hot and the Cosmo's luxury weight makes it worse. Replace radiator (OEM discontinued, use Koyo aluminum 3-row), all hoses, water pump, and thermostat as a package—8-12 hours. Add an oil cooler upgrade if tracking or spirited driving. Flushing and bleeding rotary cooling systems is tedious due to air pockets.

Estimated cost: $1,800-3,200

Typical onset: 80,000-130,000 mi

Symptoms: Slipping between gears, especially 2nd to 3rd, Delayed engagement from Park or Reverse, Burnt transmission fluid smell, Transmission overheating warning light, Harsh or erratic shifting

Fix: The JATCO 4-speed behind these motors is not built for sustained high torque. Rebuild requires 14-20 hours; transmission removal, clutch pack replacement, and valve body service. Transmission oil cooler lines frequently corrode and must be replaced during service. Finding a shop that will touch this is harder than the work itself.

Estimated cost: $2,800-4,500

Typical onset: 80,000-120,000 mi

Symptoms: Rough idle or stumbling at part throttle, Fuel smell in cabin or engine bay, Hard starting after sitting overnight, Check engine light for fuel trim codes, Visible corrosion on fuel rails and hard lines

Fix: Fuel filter, injectors (primary and secondary on turbo models), fuel pressure regulator, and rubber fuel lines all degrade. Injector cleaning or replacement is 6-9 hours on the 20B due to intake manifold complexity. Fuel pump replacement adds another 3-4 hours (tank drop). Use OEM-spec injectors or quality aftermarket; cheap ones kill rotaries.

Estimated cost: $1,200-2,400

Symptoms: Rough idle that smooths out at higher RPM, Check engine light for air/fuel mixture codes, Hissing sounds under the hood, Poor throttle response and sluggishness, Failed emissions testing

Fix: Dozens of vacuum lines, solenoids, the air pump system, and emissions control valves turn brittle after 30+ years. Diagnosis is tedious—expect 4-8 hours to trace and replace all suspect lines and components. Delete kits exist but won't pass inspection in strict states. Use silicone vacuum line, not rubber.

Estimated cost: $600-1,400

Symptoms: Intermittent no-start with good fuel and spark, Gauge cluster acting erratically or failing, Power windows or climate control failures, Ignition system misfires without obvious cause, ECU throwing random codes or failing to store codes

Fix: The early '90s Mazda ECU capacitors leak and cause all sorts of phantom issues—ECU rebuild or replacement is 2-4 hours plus bench time (send out for recap service, 1-2 weeks). Wiring harnesses crack at engine bay flex points. Window regulators and HVAC servos also fail. Diagnosing electrical on JDM gray-market cars is a nightmare without factory manuals.

Estimated cost: $800-2,000