Heater problems

[a30guy]

Good morning,

I'm hoping I can get some guidance on a slight problem I have with my 1990 NA.

I bought a cheap NA a couple of weeks ago as a project car with a view to bringing it back to spec and a few tasteful modifications.
It goes really well and I am confident that it is basically sound and the main issues are cosmetic.

Anyway, on to the problem. Symptoms are:

The temperature gauge sits slightly to the left of vertical. Looks ok.
The heater has hardly any warmth to it - lukewarm at best.
Both heater hoses at the firewall are hot.
Top radiator hose is hot.
Bottom radiator hose is almost cold.
Oil and water are clear, so I think the head gasket is ok.

The radiator is reasonably new - alloy, and the hoses and thermostat have been recently replaced, according to the previous owner.

Should I look at the thermostat, the heater core...? Any suggestions are welcome.

Thanks

David
Hobart

[hks_kansei]

Does the car have any issues warming up?
Does the temp gauge drop on highway trips?

Either of those could point t a thermostat stuck open.
A thermostat stuck closed usually just causes overheating.

I'd just replace it anyway, 10min job and a new thermostat only costs $20 or so.




As for the heater, since both hoses leading to the core are hot it seems like coolant is flowing alright.

I'd be confirming that the flaps in the air mixer box are working (ie: when you set it to heater, the flap is actually opening to allow the hot air into the vents)
If that looks ok, i'd be thinking the heater core is blocked. If the radiator and all that have been replaced recently it's possible the old one was clogged and some of the crap in it's passage through the system ended up in the heater core.
Buy some radiator flush and use it, might dislodge any sh*t caked in there. Otherwise you may need a new core.

Alternatively, if the hoses were all replaced, maybe they put the heater ones on backwards (not that it should matter, but worth confirming)

here's a diagram on how the system flows:
http://users.telenet.be/miata/images/co ... outing.jpg

[smy0003]

Most likely the hvac temperature control cable has come off it's post. Pull the glove box out and have a look for The cable that moves when you slide the temperature control back and forth.

Sent from my LG-H815 using Tapatalk

[KevGoat]

Along the same lines as smy0003 mentioned, check that all the cables are connected properly to their posts. They have a tendency to dislodge slightly and not open or close the vents properly.

May as well remove the plastic air vent cover under the bonnet on the passenger side and see how much rubbish has collected in there blocking air flow.

[a30guy]

Thank you all for your replies, and for the circulation diagram.
To answer a couple of questions - the car warms up normally. A couple of kilometres from home and the gauge is almost vertical. I haven't taken it on a long drive yet to see if it cools down.

It seems it's normal for the lower radiator hose to be cool, so I won't worry about that. I'll investigate the heater cables at the weekend, and clear out the intake vent, and see how it goes.

Thank you again for you suggestions.

Cheers

David

[a30guy]

Here's a quick follow up.

I changed the thermostat on Saturday and took it for a run. The heater definitely seemed better.
However, when I got home there was a big puddle of coolant under the mat on the driver's side! :-(
Haven't had time to look yet, but it looks like I'll have to pull the heater core out next weekend.

I guess it could just be a leaking connection, but I'm not hopeful...

Cheers
David

[93_Clubman]

If you've not already, would be worth checking coolant not pooling on pside floor as ECU is under the carpet in the footwell.