Diagnose an overheating Aceca
An Aceca that runs hot almost always has a findable cause: a furred or blocked radiator core, a weak water pump, a stuck thermostat, retarded ignition or a weak mixture, a slipping fan belt, or simply air in the system after coolant work. Work through them in order — cheapest and most likely first — before assuming the worst.
Tools
- Separate thermometer or infrared temperature gun
- Feeler gauge and spanners for the fan belt
- Torch and inspection mirror
- Radiator cap tester (useful)
Parts / supplies
- Replacement thermostat if faulty
- New radiator/heater hoses and clips if perished
- Correct coolant / inhibitor for refilling
Procedure
First confirm it is really overheating and not just reading high. Check the gauge and sender against a separate thermometer at the top hose or thermostat housing; a faulty gauge or sender is a common false alarm.
Check the simple things cold: coolant level and condition, the radiator cap's seal and pressure rating, the fan belt tension, and that the radiator core and fins are clean and not blocked with debris or bent flat.
Look for air locks, especially after any recent coolant work — bleed the system with the heater on hot until the level settles and no more air rises.
Feel the flow at temperature: with the engine warm the top hose should be hot and the radiator should warm across its face as the thermostat opens. A cold radiator with a hot engine points to a stuck-closed thermostat or a failed water pump.
Test the thermostat: if in doubt, remove it and check it opens at the temperature stamped on it in a pan of heated water. Replace it rather than run without one — a missing thermostat can actually make cooling worse by letting coolant pass through too fast to shed heat.
Check ignition timing and mixture. Retarded timing and a weak, lean mixture both make these engines run hot; confirm both against the workshop manual before condemning the cooling system. See the ignition-timing and triple-carburettor-balance guides.
Assess the radiator itself. An old core silts up internally and loses efficiency even when it looks sound; a flush may recover it, but a tired core often needs recoring by a specialist. See the cooling-system flush guide.
Use when it happens as a clue: overheating mainly at idle or in traffic points to airflow and pump — a marginal fan, a slipping belt or a weak water-pump impeller; overheating at speed points to the core, the timing and mixture, or a blockage.
If the engine loses coolant with no external leak, or the overheating is sudden and severe, stop. A head gasket or internal fault is specialist territory, and running on risks the alloy head.
Warnings
- Never open a hot, pressurised cooling system — wait until it is fully cold.
- Do not keep driving a classic that is genuinely overheating. These alloy-headed engines are expensive to repair, and a sustained overheat can warp or crack a head.
- If the car loses coolant with no visible leak, or overheats suddenly, suspect a head gasket or internal fault and consult a specialist rather than continuing.
Related faults
About the author
An independent classic-car enthusiast in North Yorkshire with decades of hands-on experience on British classics — including the AC Aceca — writing the single-model reference he always wished existed. Read the full bio.