Start with the easier, the thermostat. If it's sticking, it could cause the symptoms you described. To confirm my thought, I asked my AI mechanic about the symptoms. Here's what it said:
Yes—if the thermostat is sticking closed (or only cracking open a little), it can absolutely give you the one-two punch of:
- Overheating at idle or in stop-and-go traffic, and
- Little to no heat coming from the HVAC vents.
Why that happens
| Thermostat position | What the coolant does | Result at the gauge | Result at cabin heater |
|---|
| Normal (opens ~195 °F / 90 °C) | Hot coolant leaves the engine, circulates through radiator and heater core, returns cooled | Temperature stabilises in the 200–215 °F range | Hot coolant flows through heater core → warm air |
| Stuck closed or sticky | Coolant can’t leave engine block easily; flow through radiator and heater core is restricted | Heat soaks in the block, especially while idling (water pump spins slow) → temp climbs | Little or no coolant gets to heater core → “cold” vents |
Because the Jeep TJ’s heater core is plumbed in parallel with the radiator circuit (both branches sit behind the thermostat), any restriction at the thermostat reduces flow to
both the radiator
and the heater core. At road speed you sometimes get enough ram-air through the radiator to mask the problem, but at idle that extra airflow disappears—so the temp needle creeps up while the cabin stays cold.
Quick sanity checks
- Watch the upper radiator hose:
- Cold when you start → suddenly gets hot ≈ thermostat opened.
- Stays cool even as the gauge climbs ≈ thermostat stuck closed
- Feel the heater hoses (carefully): both should get hot once the Jeep is at operating temp. One cold hose usually points to low/no flow.
- Burp the system: trapped air can mimic a stuck thermostat, so top up coolant and bleed air first.
Other culprits that can look similar
| Symptom overlap | Possible cause | How to differentiate |
|---|
| Overheats at idle and no cabin heat | • Low coolant level / air pocket • Weak water pump • Severely clogged heater core | Coolant level check; squeeze test on lower hose; infrared temp gun across radiator/heater hoses |
| Overheats at idle but cabin heat is OK | fan clutch failure, clogged radiator fins | Heater output proves coolant is flowing; look at fan operation and radiator surface temps |
| Normal temp gauge but no cabin heat | Blend-door actuator or cable not moving, heater core clogged | Both heater hoses hot → blend door; one hose hot, one cold → core restriction |
Best fix path
- Replace the thermostat (use an OE-spec 195 °F unit and a fresh gasket). They’re under $20 and take <1 hour.
- Flush/burp the system and refill with 50/50 HOAT-compatible coolant.
- Check water-pump impeller if problem persists—high-mileage 4.0 L pumps can erode or loosen.
- Inspect fan clutch (for ’97-’99 mechanical fan) or electric fan relay (’00-’06).
- Back-flush heater core if you still lack cabin heat.