FAQ
TL;DR: Fiat Seicento 900 no-heat at idle points to weak coolant flow/pressure. One owner says temperature “remains at 80 degrees” after a winter radiator cover. Fixes: bleed air, verify heater valve, replace expansion cap, check pump/thermostat. [Elektroda, kierbedz4, post #16896945]
Why it matters: Warm cabins and proper engine temperature protect the engine and keep you safe in winter driving.
Quick Facts
- Symptom pattern: heats while driving, but no heat at idle or in traffic. [Elektroda, ERESZEK, post #16896816]
- Heater matrix and valve were replaced with new parts, yet issue persists. [Elektroda, ERESZEK, post #16896885]
- New thermostat already installed; still experiencing weak cabin heat. [Elektroda, ERESZEK, post #16897012]
- System was bled multiple times; air still suspected. [Elektroda, ERESZEK, post #16896885]
- Cheaper first step: replace the expansion-tank cap before major parts. [Elektroda, zdzichra, post #16897370]
How do I fix no heat at idle on a Fiat Seicento 900?
Restore coolant flow and system pressure. Start by re-bleeding the system. Confirm the heater valve opens fully. Replace the expansion-tank cap to restore pressure. Inspect the water pump for low-RPM performance. Verify the thermostat closes when cold. Avoid blocking the radiator until you confirm temperatures. [Elektroda, zdzichra, post #16897212]
Could trapped air cause this, and how do I bleed it?
Yes. Air pockets reduce heater-core flow. Typical 3-step bleed: 1. Park nose-up, engine cold. 2. Open bleed points, fill slowly until coolant flows bubble-free. 3. Run engine with heat on HOT, top up as the level drops. Recheck after cooling. [Elektroda, radek778, post #16896864]
Does the heater valve need checking or replacement?
Yes. A stuck or closed heater valve blocks flow through the heater core and kills cabin heat. Move the control from cold to hot and feel both heater hoses; both should warm. Replace the valve if motion or flow is inconsistent. “Opens the flow?” is the key question. [Elektroda, headshot, post #16896883]
I already replaced the thermostat and heater—what next?
Focus on flow and pressure. Replace the expansion-tank cap, which maintains system pressure. Inspect the water pump for worn impeller or slipping, especially at idle. Air re-entry or a minor leak can also reduce heat at low RPM. [Elektroda, zdzichra, post #16897212]
Is blocking the radiator with cardboard a good idea?
Use caution. It can mask other faults and risk overheating if ambient or speed changes. Install a temperature gauge before experimenting. As one expert warned: “CAUTION with this.” Prioritize proper thermostat, pressure, and pump checks first. [Elektroda, g107r, post #16897048]
What temperature can owners see after partially covering the radiator?
A forum member reported coolant temperature holding around 80°C during winter driving with a front radiator cover installed. This helped maintain stable cabin heat at speed but is a seasonal workaround, not a root-cause fix. [Elektroda, kierbedz4, post #16896945]
Can a brand-new thermostat be defective out of the box?
Yes. New thermostats sometimes stick open, preventing proper warm-up and heat. If heat is weak and the radiator warms early, recheck or replace the thermostat. As advised: verify the new part rather than assuming it’s good. [Elektroda, zdzichra, post #16897036]
Why does heat disappear when I increase the cabin fan speed?
Higher blower speed strips heat faster from a marginally warm heater core. If coolant flow is weak, outlet air cools and “heating sits down” at higher fan settings. Restore flow and pressure first; then retest fan speeds. [Elektroda, ERESZEK, post #16896914]
How can I tell if the water pump is weak at low RPM?
At idle, poor pump performance gives lukewarm or no heat, yet improves while cruising. Look for leaks, noise, or temperature swings when revving gently. Persistent no-heat at idle with better heat at speed suggests reduced pump output. [Elektroda, zdzichra, post #16897212]
Should I replace the expansion-tank cap first?
Yes. The cap is inexpensive and crucial for system pressure. Low pressure slows coolant circulation and collapses heater performance. Swap the cap before chasing bigger parts; it’s the fastest, cheapest diagnostic move. [Elektroda, zdzichra, post #16897370]
What does the expansion-tank cap actually do?
It seals the cooling system and regulates pressure. Correct pressure raises the coolant’s boiling point and sustains flow through the heater. A leaking cap reduces pressure and circulation, causing weak heat at idle. [Elektroda, zdzichra, post #16897212]
What is the heater valve on a Seicento 900?
The heater valve controls coolant flow through the heater core. When set to HOT, it should open fully and allow continuous flow. If it sticks closed or partially, the heater core stays cool and the cabin won’t warm. [Elektroda, headshot, post #16896883]
Is weak cabin heat harmful for the engine?
Yes. A thermostat stuck open or chronic underheating prevents the engine reaching operating temperature. That increases wear and fuel use. Fixing the cause protects the engine and restores cabin comfort. [Elektroda, zdzichra, post #16896983]
How do I quickly try the expansion-cap fix?
- With a cold engine, remove the old cap and inspect the seal.
- Install a new quality cap rated for your model.
- Warm the engine, set heat to HOT, and check for stronger, stable heat at idle.
“Cheaper version and faster.” [Elektroda, zdzichra, post #16897370]
Why does the heater work while driving but not at lights?
Vehicle speed can boost coolant flow and airflow across the radiator. At idle, weak pump output or low pressure reduces heater-core circulation, so vents blow cool. Fix flow, pressure, and valve operation to restore idle heat. [Elektroda, ERESZEK, post #16896816]
