1986 Toyota Supra Problems: 7 Known Issues

On the 1986-1992 Toyota Supra, the distributor-less 7M uses a dedicated cam position sensor to generate the G and NE signals the ECU needs for spark and injection, and after decades of heat-cycling the sensor's internal windings and its wiring become brittle, corroded, or open-circuit. A failed CPS produces a crank-no-start with no spark and no injector pulse, and the ECU logs Toyota flash code 12 or 13 (no NE/G RPM signal — these are pre-OBD-II Toyota two-digit codes). Good used and refurbished sensors are increasingly scarce, which has spawned aftermarket digital replacements.

On the 1986-1992 Toyota Supra, the original electrolytic capacitors in the MK3's engine ECU leak electrolyte with age, heat, and power cycles, and the spilled electrolyte etches the copper traces on the circuit board, eventually shorting or open-circuiting the ECU. Symptoms start as odd running behavior — rough idle, random cutting out, stalling — and can progress to hard-start or no-start conditions that mimic sensor failures. This is now an expected failure on any unopened 30-plus-year-old MK3 ECU, with forum members documenting burned and popped capacitors.

On the 1986-1992 Toyota Supra, the defining flaw of the A70 Supra: Toyota's factory head-bolt torque spec of 58 ft-lb (78 N-m) on the 7M-GE and 7M-GTE was roughly 10-20 ft-lb too low, allowing the composite head gasket to lose clamp and fail prematurely under normal driving. Combustion gases push into the cooling system, causing overflow-tank 'burping', overheating, milky oil, and brown coolant, and repeated overheating can warp the aluminum head. The problem is so widespread that unmodified MK3s that have never blown a head gasket are now rare, and owners reported repeat failures every 30k miles in period; NHTSA's complaint database holds only a handful of complaints for these years, and no recall was ever issued.

On the 1986-1992 Toyota Supra, beyond the head gasket itself, the 7M cooling system is unforgiving: it traps air pockets if not bled with the correct procedure, and 30-plus-year-old fan clutches, corroded radiators, and missing fan shrouds are common causes of creeping temperatures. Because the engine 'does not tolerate' overheating, any cooling fault rapidly accelerates head-gasket failure and head warpage, making this the most consequential secondary system on the car. Forum diagnostic threads consistently trace MK3 overheating to improperly bled coolant, weak fan clutches, blocked radiator cores, or absent shrouds/ducting.

On the 1986-1992 Toyota Supra, the gas struts supporting the MK3's large, heavy glass hatch lose pressure with age, leaving the hatch unable to stay open — a near-universal annoyance on surviving cars and a head-knock hazard while loading. Replacement is mechanically simple but access to the upper mounting nuts requires removing brittle 30-plus-year-old interior side panels, which crack easily if rushed, turning a 15-minute job into a multi-hour one.

On the 1986-1992 Toyota Supra, sport-roof (targa) MK3s commonly leak water into the cabin headliner, footwells, and seats as the targa weatherstrips harden and, more often, as the factory sealant between the removable panel and its seal-rail (held by four screws) fails. Forum consensus is that many 'bad seal' leaks are actually failed sealing between the seal-rail and the targa panel itself rather than the visible rubber. Chronic leaks soak carpet and promote interior rust and electrical gremlins.

On the 1986-1992 Toyota Supra, mK3 dashboards crack with near-universal regularity, typically across the top pad above the instrument cluster and defroster vents, driven by UV exposure and the vinyl-over-foam construction drying out with age (silicone-based dressings are widely blamed for accelerating it). It is a cosmetic but value-relevant defect — uncracked original dashes are scarce and command a premium, and replacement requires substantial dash disassembly.