AI Mechanic

Symptom · Any water-cooled engine

Overheating with no visible leak

Heat has to move from the block to the air. It fails at flow, at airflow, or because combustion gas is getting into the coolant. Test in that order and you rarely strip a head you did not need to.

01Possible causes, none of them ranked yet

  • Air lock after a coolant change

    A trapped pocket stops flow through the head, so the gauge climbs while the bottom hose stays cool.

  • Stuck thermostat

    It never opens, so hot coolant never reaches the radiator.

  • Fan not running

    At low speed the radiator needs the fan; without it the temperature climbs in traffic and drops on the move.

  • Failed water pump impeller

    A corroded or slipping impeller turns without moving coolant.

  • Blocked radiator core

    Silt inside or debris outside cuts the heat the radiator can shed.

  • Head gasket passing combustion gas

    Pressure in the coolant pushes it out and stops flow, with no external leak to see.

Nothing on that list is the answer yet. A measurement decides which ones survive.

02The tests, in the order a senior tech runs them

Test 1

Warm the engine to temperature and compare the top and bottom radiator hoses by hand and with an infrared thermometer.

tool
Infrared thermometer
expect
Both hot, with roughly 10 to 20 °C between top and bottom once the thermostat opens.

A hot top hose and a cold bottom hose is a thermostat or a blockage, not a head gasket.

Test 2

Run the engine at idle from cold until the fan should cut in, and watch it.

tool
By eye, no tool
expect
The fan runs before the gauge passes the middle, usually around 95 to 105 °C.

No fan explains overheating in traffic and normal temperature on the motorway.

Test 3

Pressure test the cooling system cold and leave it for 15 min.

tool
Cooling system pressure tester
expect
Holds the cap rating, usually 1.1 to 1.5 bar, with no drop.

A drop with nothing on the floor points at the head gasket or a heater matrix.

Test 4

Run a combustion gas test on the coolant with the engine warm and the cap off.

tool
Block tester with test fluid
expect
The fluid stays blue. Yellow means combustion gas in the coolant.

This is the test that names a head gasket. Nothing before it should.

Never sell a head gasket on a temperature gauge alone. The block test is twenty minutes and it decides the job.

Your car is not this car.

Open a Case with the vehicle and the symptom in front of you. It asks the two or three questions that narrow yours, then names one test with the value to expect, and re-ranks the list on what you measure.

Open your first Case

Free and complete. No account, no card. A Bay Pass after that is $12.00 for 90 days.

Other symptoms