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Fiat Seicento 900: Poor Heating Issue, Thermostat & Heater Replacement, Traffic Jam Concerns

ERESZEK 11322 16
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Why does my Fiat Seicento 900 only heat properly while driving and lose heat at idle, even after replacing the thermostat and heater core?

Poor heat at idle but normal heat while driving usually points to a coolant circulation problem, not the heater core itself: re-bleed the system, make sure the heater valve opens properly, and check that hot coolant actually reaches the heater matrix [#16896864][#16896883][#16896894] If the system is already vented, the next suspects are low cooling-system pressure from a leaking expansion-tank cap or reduced water-pump performance [#16897212] The fact that the heating improves with engine speed also fits weak circulation at low RPM [#16896894][#16897212]
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  • #1 16896816
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    Hello, I have a problem in my recently purchased seicento 900. It doesn't heat very well when I start it, it doesn't heat anything but when I drive at high speed, it heats on average as soon as I stand on the traffic light or in the traffic jam there is no heating. I have already replaced the thermostat and heater and still
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  • #2 16896864
    radek778
    Level 10  
    Posts: 60
    Are you sure you have it well vented ??
  • #3 16896883
    headshot
    Level 20  
    Posts: 875
    Help: 32
    Rate: 128
    And the heater valve? OK? opens the flow?
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  • #4 16896885
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    Yes, I vented several times as recommended, but still nothing that suggests that it may be a water pump that is weak and can not pump at low speed I don't know what it can be

    Added after 59 [seconds]:

    Heater I bought a new one with a new valve
  • #5 16896894
    g107r
    Level 41  
    Posts: 5252
    Help: 809
    Rate: 985
    Well, because you write as if the pump pumped poorly.
    Is enough hot liquid flowing into the heater? There is no plug in its power supply?
  • #6 16896914
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    when the heating works like a jade and I have the airflow set on the first line, it heats up when switching to a larger heating blanket, the heating sits down

    Added after 4 [minutes]:

    when the heating works like a jade and I have the airflow set on the first line, it heats up when switching to a higher heating power, the heating sits down
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  • #7 16896945
    kierbedz4
    Level 36  
    Posts: 2708
    Help: 307
    Rate: 1006
    Cover the radiator. Safety cover is not foreseen in Safety Deposit Box. CC900 has this cover provided and is assumed for the winter period after removing the front license plate, installing the aperture and re-mounting the plate, and for me this cover installed in the bumper is covered with half of the air inlet on the radiator. I made my CC900 by installing the pointer temperature indicator and in winter, after covering the radiator, the temperature of the coolant in the radiator during driving remains at 80 degrees
  • #8 16896952
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    So put the cardboard on the radiator in the front?
  • #9 16896983
    zdzichra
    Level 32  
    Posts: 1531
    Help: 167
    Rate: 305
    Check if you have a functional thermostat because if it is damaged (open permanently or only permanently ajar) it causes not only poor heating but also the engine is not heated which is not healthy for it. The radiator cover (when the thermostat is damaged) is a half-measure and guerilla
  • #10 16897012
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    I set up a new thermostat
  • #11 16897036
    zdzichra
    Level 32  
    Posts: 1531
    Help: 167
    Rate: 305
    I understand, but check it out, you may have hit the sell. How it's working is just a cover.
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  • #12 16897042
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    ok tomorrow I will try to cover the radiator I will write if it helped
  • #13 16897048
    g107r
    Level 41  
    Posts: 5252
    Help: 809
    Rate: 985
    Eh, really with this veil?
    Someone recently asked "how do you know what the real temperature of the engine is?"
    @ERESZEK CAUTION with this
    ERESZEK wrote:
    Heats poorly at a standstill, when I start it, nothing heats but when I drive at high speed, it heats on average as soon as I stand on the traffic light or there is no heating in the traffic jam.

    And did I not understand something?
    At a standstill, it doesn't heat, but when driving? This cardboard box for a Christmas tree?
  • #14 16897078
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    Well, I do not know what the cardboard is for, since when I go it heats up that if I give more power to the heating, the heating disappears, if I switch to a smaller airflow force, the heating comes back crazy with no heat. The heating does not work at a standstill.
  • #15 16897212
    zdzichra
    Level 32  
    Posts: 1531
    Help: 167
    Rate: 305
    There should be adequate pressure in the cooling system for the fluid to circulate quickly enough. You may have a leaking plug or reduced coolant pump performance.
  • #16 16897274
    ERESZEK
    Level 6  
    Posts: 86
    Rate: 18
    There should be adequate pressure in the cooling system for the fluid to circulate quickly enough. You may have a leaking plug or reduced coolant pump performance.

    that is, replace the cap from the expansion tank and how can it not water pump?
  • #17 16897370
    zdzichra
    Level 32  
    Posts: 1531
    Help: 167
    Rate: 305
    Cork is a cheaper version and faster. Someday babol just has to be pity to replace it one after another because half the car can be replaced.
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Topic summary

✨ The discussion revolves around a heating issue in a Fiat Seicento 900, where the heating system fails to operate effectively at low speeds or when idling, despite the replacement of the thermostat and heater. Participants suggest checking the venting of the system, the functionality of the heater valve, and the performance of the water pump. Concerns are raised about the coolant circulation and pressure in the system, with recommendations to cover the radiator for improved heating during winter. The author confirms the installation of a new thermostat and heater but continues to experience heating problems, particularly when increasing airflow. The conversation highlights the importance of ensuring adequate pressure in the cooling system and the potential need for further diagnostics on the water pump and expansion tank cap.
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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

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?

  1. With a cold engine, remove the old cap and inspect the seal.
  2. Install a new quality cap rated for your model.
  3. 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]
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