Overheat Only in Traffic With A/C On
A 2016 half-ton pickup with a 5.3L V8 and 128,000 miles overheats only in stop-and-go traffic. On a 95°F day with the A/C on, the gauge reaches 235°F; at 60 mph it drops to 210°F. Coolant is full, the mix is 50/50, and there are no external leaks. The thermostat was replaced three months ago. The upper radiator hose is hot, while the lower hose is warm but noticeably cooler. The scan tool shows ECT 233°F at idle and a commanded fan clutch of 65%. High-side A/C pressure is 275 psi at idle. With the A/C off, idle temperature stabilizes near 225°F. No DTCs are stored. The condenser has about 30% of its fins blocked with bugs and leaves; the shroud is intact and the belt tension is correct.
1. Does the evidence point more to airflow or coolant flow? Explain using the idle-versus-highway and A/C-on-versus-off comparisons, and say what the lower-hose temperature does and does not prove.
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The idle-versus-highway split is the strongest clue. A vehicle that overheats at idle but cools at 60 mph is usually airflow-limited, because road speed supplies air the fan cannot. The A/C-on versus A/C-off comparison adds heat load and, more importantly, raises condenser pressure and restricts airflow through the stack; the 275 psi high side at idle supports that. The lower hose being warm rather than cold suggests coolant is circulating, but it does not prove adequate flow: a partially restricted radiator or weak pump can still leave the lower hose warm. Coolant flow faults usually show overheating at speed or a cold radiator/lower hose. The blocked condenser fins are a concrete airflow restriction, so I would treat airflow as primary and verify fan clutch performance before condemning the thermostat.
2. With a scan tool that can command the fan clutch and an infrared thermometer, outline a test sequence to separate a weak fan clutch from a restricted condenser. State the results that would change your decision.
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First, verify actual fan speed or air velocity at 0%, 50%, and 100% command. With the engine hot and A/C on, command 100%: a healthy clutch should produce a clear roar and a measurable air-velocity increase. Then use an infrared thermometer across the condenser and radiator. A large top-to-bottom or inlet-to-outlet drop across the condenser points to restriction; a small drop with low air velocity points to the fan. Clean the condenser and re-test. If ECT drops more than about 15°F at 100% command and stays controlled at idle with A/C on, the clutch is likely weak; if cleaning alone fixes it, the fan is not the cause. If neither changes the temperature, move to coolant-flow tests such as radiator inlet/outlet comparison and heater output.
3. The customer wants another thermostat. Is that justified? What finding would make you agree?
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Another thermostat is not justified by these data. The thermostat is three months old, the upper hose is hot, and the lower hose is warm, so the engine is opening and circulating. A stuck-closed thermostat typically causes rapid overheating at speed, a cold radiator, and a cold lower hose; a stuck-open thermostat causes low operating temperature, not idle-only overheating. I would agree to replace it only if the lower hose and radiator stay cold while ECT is above 220°F, or if a scan-tool graph shows ECT rising without a corresponding drop when the thermostat should open. The current pattern points to condenser airflow and fan-clutch performance.