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Honda Civic 8th gen, 1.8L petrol

Context 5k miles ago I observed no hot air coming into cabin, only stone cold air even when set to 'Hi'. Lived with it, car seemed to be running fine but suddenly one day engine cooked itself. Warning lights suddenly came on when I was driving on the motorway one day and then I had to buy a new engine. Dipstick melted, coil packs melted into the cylinders. At the same time I also replaced: radiator, thermostat, water pump, upper and lower rad hoses, aux belt, radiator cap.

Observations: New engine installed since they're affordable for this gen civic in the used market. The car feels like it's running like a dream, dash temp gauge exactly where it should be for this model (just under half way). Fuel efficiency is great. Everything appears to be working perfectly except still no hot air blowing into the cabin even when set to Hi. You understand my concern. The cabin air fans work fine but the air being blown is still always stone cold.

I've pointed an infrared thermometer gun at different places (all readings taken at the same time with a hot engine after a 1 hour journey):

  • various points on the engine block: 95-105°C
  • upper rad hose near engine: 80°C
  • upper rad hose near radiator (inches away): 65°C
  • lower rad hose leaving radiator: 40°C
  • heater core inlet & outlet hoses at firewall: both around 40°C, the lower one a few degrees hotter

I now regularly check my coolant and it's always visible when I remove the radiator cap. Can see it shimmering close to the top of radiator under there.

Also worth mentioning the engine temperature gauge on driver's dash shows it sitting at the right place within 3 mins of starting the car from cold. Maybe this is a little too quick?

With all this said, I'm frankly surprised the car feels "great" yet I appear to have severely impaired coolant flow. Are these mk8 Civics just so well screwed together they can cope with what must be a fraction of the normal coolant flow rate for thousands of miles before terminally overheating?

Would air in the system explain this? Worth back flushing the heater core before or after bleeding? I'm still so confused how my car is actually taken me thousands of miles with this (lack of) coolant circulation.

Your thoughts welcome:)

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    Did you try flushing the heater core? 40C/104F isn't hot at all when hot coolant should be flowing thru@95C. A blocked heater core is one possibility. Another possibility is incorrect heater hose routing. Hot coolant, usually from the water pump, feeds the heater core unless it's (incorrectly) being fed from the lower radiator hose(?) where the same 40C coolant is going into the heater core? Review the cooling system flow and how it feeds the heater core.
    – F Dryer
    Commented Oct 21 at 1:41
  • I will review the routing. I still don't get how my engine hasn't cooked itself by now if the coolant routing was wrong leading to such low temps away from the engine!
    – jsstuball
    Commented Oct 21 at 9:28
  • Heater cores do not help cool off coolant. The water pump, cooling passages in the engine, main radiator hoses and radiator move coolant absorbing engine heat to the radiator, The cooling fan should cycle on when coolant rises above (around) 220F), shut off below (around 210F). Ac running needs cooling since the ac condenser coil get hot in front of the radiator. The cooling fan should run when ac is running but not if ac is broken.
    – F Dryer
    Commented Oct 21 at 16:57
  • Ah you're suggesting that if the inlet and outlet hoses of the heater core were installed the wrong way around, I may experience sub-optimal cooling but not enough to cook the engine within a couple of hours driving?
    – jsstuball
    Commented Oct 22 at 21:49
  • Review heater hose routing; hot coolant should flow from water pump or engine to heater core, return flow to engine return flow. Try Civic forums for owners sharing info, online service manuals, etc, for cooling system routing of hoses. If cooling system sealer was used previously, this may inadvertently clog/block coolant flow in the engine block and/or heater core. Forcing water thru the heater core inlet/outlet ensures it isn't clogged or blocking hot coolant flow.
    – F Dryer
    Commented Oct 23 at 2:36

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