Heater not working whilst stationary (NB8B)

[mmxx55]

Hi folks,

About to take the car to MX5Factory or someone knowledgeable with MX5s in the SE Melbourne but just wanted to get people's thoughts on what this could be.

The heater seems to only work when the car is moving. When stationary, the fan blows cool air. Not long after this issue occurred, the top tank in the radiator cracked (it was the 15yo OEM unit though) so I'm not sure if the issues are related. I've replaced the radiator but the heater issue still remains, so I'm hoping that the heater issue didn't cause the tank to crack, but can't be sure.

Any ideas from the knowledgeable people on this forum??

Cheers
Kevin

[smy0003]

Could be as simple as a floppy flap (sorry it's early on a Monday and I can't think of a way to re-phrase that) in the blower unit. I'd get under the dash and check that all the HVAC actuator cables are doing their jobs.

Is the stationary air cool cool? Or still a bit warmish?

[KevGoat]

Sounds like possibly a bit of a blockage somewhere and the coolant only flowing through the heater enough at higher revs.

[manga_blue]

As smy0003 says, the most common cause is the linkage to the hot/cold air flap in the heater controls getting disconnected or out of adjustment. On an NA the linkage is deep down beside the clutch pedal on the tunnel. I don't know whether or not it's in the same spot on an NB but mostlikely it is the same. You need to take of the covers under the steering wheel and climb upside down underneath there to see if it's all working while you slide the hot/cold control back and forth. I hope you're a small skinny contortionist.

If you're getting cold air when stationery and warm air when moving it usually means that warm air is getting from the engine bay into the HVAC collector box between the windscreen and the firewall. There's a sealing rubber across the top of the firewall that's meant to stop that but they do get flattened over time and stop working. A Mazda replacement seal is the most effective and easiest fix, but not the cheapest ($40 now?).

[93_Clubman]

Depending where in SE Melb is convenient, Mammi Motors at Somerville are knowledgeable about MX5s.

I hope you're a small skinny contortionist.

Btw, if you're not, removing the seat helps so you can lay on your back with feet on rear shelf.

[davekmoore]

Depending where in SE Melb is convenient, Mammi Motors at Somerville are knowledgeable about MX5s.

How knowledgeable?

Recent events, for instance, would cause me to ask anyone who is about to do work on my car, how long and how perfectly would they expect to execute the diagnosis and replacement of a failed crank angle sensor.

If they could do that within an hour and without swearing they would be front runners for me for ongoing maintenance, especially given the geography.

[93_Clubman]

MrB & MrsB as well as Bensale take/ used to take all their cars there:
viewtopic.php?t=47111

[hks_kansei]

I'd first off flush and bleed the cooling system.

If there's air or blockages in the heater core you'll not get any heat.
With a partial blockage the higher coolant flow when moving may be enough to flow past the block.

If you rev the engine a bit when stopped do you get any warm air?

[manga_blue]

This article talks about adjusting heater air flow. Note that it's for LHD, so they get to it via their glovebox. On our RHD cars it's in the same place, you just need to get in from under the dash.

http://www.miata.net/garage/heater_cable.html

[mmxx55]

Hi all,

Thanks for the input!

Radiator has been replaced and coolant system flushed and bled. The system is free of air (problem was already apparent before the radiator top tank let go).

I've been scratching my head over the cable thing (been avoiding getting under as I'm recovering from a broken arm) but it looks like it might be related to that. Will climb under and make sure the cable adjustment is all good and report back.