Fault Tree: Coolant Loss / Pressure Test Workflow

Quick triage & tools

Never open a hot system. Let it cool fully first.
Look for clues: crusty residue, damp carpet, sweet smell, white smoke, oil contamination.
Best tool: a cooling system pressure tester. Without it, you can still do useful checks but slower.

Safety: if it overheats quickly, stop driving. Overheating can warp the head and turn a small leak into a big job.

Decision flow

Cold level + cap check Check level when cold, inspect the cap seal and the filler neck.
Find: cracked cap, damaged seal, or stained filler neck → replace cap first (cheap) and re-check.
External leak check (cold) Inspect radiator end tanks, hose junctions, thermostat housing, water pump area, expansion tank, and undertray.
Tip: dried coolant often leaves a white/pink crust. A mirror helps under the pump.

If you find a leak: fix it, refill/bleed correctly, then monitor.
Pressure test (best confirmation) Pressurise the system to the cap rating (often ~1.0–1.4 bar, check the cap). Hold for 10–15 minutes.
Pressure drops and you see coolant externally → you’ve proven an external leak.

Pressure drops but no external leak visible → suspect internal leak or hidden leak (heater core / under intake / turbo coolant lines).
Heater core / cabin checks Feel carpets (front footwells), look for misting on windows, sweet smell, greasy film.
If damp/smell inside → heater core leak likely. Also check heater hoses at the bulkhead.
Oil + coolant cross-contamination Check oil cap/ dipstick for milky residue, and coolant for oily sheen.
Note: short trips can cause light mayonnaise under the cap without a head gasket failure. Use multiple clues, not one.
Exhaust + running symptoms Look for persistent white steam when fully warm, rough running on startup, pressurised hoses quickly, or bubbles in expansion tank.
Strong signs of combustion gas → head gasket / cracked head / block or (on some engines) EGR cooler failure.
Rule out “coolant into intake” (some diesels) Certain engines can leak via EGR cooler. Look for coolant loss + white smoke + no obvious external leak.
If suspected: inspect EGR cooler area for dampness and consider a specialist pressure/smoke test.
After repair: refill and bleed correctly Air pockets can mimic overheating and push coolant out.
Re-check: cold level after 2–3 heat cycles, then weekly for a month.

Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

Freeze-frame captured / conditions noted
Battery voltage checked (resting + cranking)
Basic visual checks (hoses, connectors, grounds, fuses)
One test at a time (don’t change multiple variables)
Confirm fix by reproducing the original condition

Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.