Mk1 1.6, luke warm heating

Hi all.

Taken my mx5 for a few drives recently in the cold and the heating takes ages to warm up, when it does, it’s luke warm. If I rev it hard I can feel it warm up more, but normal driving it’s just like warm. It gets up to temperature fine on the gauge. And I’ve felt the pipes going into the cabin and they are not.

I’ve recently replaced the radiator And flushed the system, is the cooling now too good? Or something wrong?

Thanks :slight_smile:

I’d say most likely your thermostat has stuck open, though if the temperature gauge looks normal that might suggest not.

But the coolant that’s flowing over the gauge sensor is the same coolant that’s on its way to the cabin heater, so if you reckon that’s not as hot as it should be then maybe the gauge is optimistic and it is a stuck thermostat after all.

Is there another way to see if the thermostat is stuck? I just did the test where I took the rad cap off, turned on the engine a left for a while. It looked like this…

After a while and with a bit of revs the fan kicked in and the water started flowing…

This to me says the thermostat is kicking in?? It then went back to normal after. But still no heat out the fans. Pipes going into the cabin were both red hot.

Heater matrix clogged?

Initial thought is airlock in heater matrix. Water can still flow through part of the matrix hence the heat in heater hoses.
Try burping it again, maybe raise the front end of the car (ramps).


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The simplest way to check if the thermostat is closing normally is to run the engine from cold and see if the top radiator hose and radiator top tank stay almost cold until the temperature gauge reaches the normal zone. A working thermostat should stay closed until the coolant reaches mid-80’s celsius. Only when it opens should the top hose and rad abruptly get hot.

If the pipes going to and from the cabin got very hot then its likely the problem is elsewhere. It’s possible the diverter flap that guides air through the heater rather than around it is not moving properly.

Ok great thank you, I’ll try looking at both of these tomorrow. Might be worth changing the thermostat anyway

Ok so the as the engine warmed up, the top pipe stayed cold for a bit but warmed up gradually.

The coolant pipes going into the cabin are both hot, the left one hotter than the other.

Still no heat in cabin, It a probably coming out at maybe 20 degrees.

Still worth changing the thermostat??

Ok so I purged the system with the front raised. Heating definitely warmer now (Not very hot but better) and no change in the temperature at motorway speeds. Temp gauge stayed rock solid in all situations too (I know my gauge is very accurate due to high temps in summer). Hopefully all is well.

Had a similar problem on old cars I’ve owned, especially the mini, have you checked the valve is opening when you move the lever, if the cable has come lose you may be feeling just the cable sliding in the locking hole on the arm of the valve. Sorry it’s not direct Mx5 experience but sounds plausible

There isn’t a valve in MX-5s, the hot coolant flows through the heater core all the time. The control moves a vane which diverts air through the heater or around it.

Thanks all, I just tried doing this and not much more happened. It did need adjusting though! I’ll give it a run tomorrow and see how much it heats up.

I’m pretty confident that all is ok at the engine end as I can see the thermostat opening and closing when the cap is off.

The heater pipes that go through the firewall are both hot (inlet very hot) meaning hot water is going in but the lots of heat is not coming out of the vents. Although roof off today o was still toasty. Therefore and to conclude, I will live with it (but change the thermostat just incase and to be safe as its a cheap and quick job)

I think there is still air in the heater matrix. It sounds like you managed to get some out but! Some cars can be a so and so to bleed. If you are going to do the thermostat then you may have to start all over.
Mean while carry some antifreeze mix and check the system when it’s cold and the opportunity arises. It may just bleed it’s self and need a top-up.


Thanks for that, I’ll remember for the future. It does explain why when working properly the Mx5 heater is very good.

Leave the engine running on tick over and if the level drops top it up this could happen a number of times.
My MK2 has a small vent bung situated on the top corner of the radiator on the drivers side,its awkward to get at but should if loosened release any remaining air in the system.

My 3.5 heater only seems to provide properly hot air with the distribution knob tuned all the way to the left.