NA still has lukewarm heater when driving

  1. My model of MX-5 is: '93 NA
  2. I’m based near: Blackpool
  3. I’m looking for technical help or recommendations on: The heater

The car now has a new water pump, all new hoses, new thermostat. When warmed up and idling and stationery with the heater of full fan and full heat the temp gauge stays all the time at 11.45. It also stays at 11.45 however hard you work the engine down country lanes.

When stationery it throws out even more heat than my NC, so that’s plenty, and would allow me to do my usual top down driving at low temperatures.

However, as soon as the car is on the move, the heater temp drops to only a few degrees above ambient.

Any ideas what’s going on here?

I was going to bump your other thread to see if you got your car fixed, sorry to hear it’s still not right.
Do you know if the cooling system has been checked or cleared of any debris or sludge

No, I don’t know if the coolant system has been cleared of debris or sludge. Would debris and sludge in the heater core allow it to work fine when parked but not while on the move?

How do you know the gauge is working?

Symptoms are otherwise suggestive of a thermostat stuck open.

Check heater hoses are hot or cold. Check levels, backflush heater core.

I reckon the fact that the gauge is at zero when cold then goes up to 11.45 when it’s warmed up and went off the top of the scale when it did the head gasket does suggest that the gauge is working.

Looking from front, left heater hose is at 70 degrees, right one at 54 WHEN PARKED. Not measured them while on the move.

There’s your answer. The heater core is blocked, and needs removing and clearing. Temperatures should be the same.

Easy repair:

Try replacing the heater resistor. Remove the glove box cover for easy access is best. See if you can borrow one from another MX5 NA owner. If it works simply get another from MX5 parts or one of the many good used part dealers.

Mk 1 cars are known to have problems with bleeding the air out of the heater matrix. ( it is higher than the rad cap so it traps air )
there are two ways of dealing with it, both involve raising the front of the car up, Either park on a steep hill nose upwards then pump the rad hose with the cap loose ( engine OFF ) if you get bubbles and the water level drops you are winning, the other way is to put the front of the car on ramps or axle stands and do the same thing.

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Thanks for the thought. Would a faulty heater resistor cause the heater to provide 40 degrees when parked but 12 degrees on the move? If so, how?

Thanks for the thought. Would air in the heater matrix cause the heater to provide 40 degrees when parked but 12 degrees on the move?

I know it’s a bit of a leap, but just cured the same issue on my son’s 2006 Corsa by changing the heater matrix

Thanks for that. Really helpful. Just so I understand so I can explain it to my non-specialist mechanic, how do the differing temps on the two hoses show that the heater core is blocked, and how can the heater give 40 degrees heat when parked, if it’s blocked?

How long of a job do you think that is for a mechanic of 20+ years’ experience?

For the sake of £15 or so, would it be better to have a used replacement heater box and matrix on hand? And is the NB one interchangeable and any better?

So the Corsa gave out hot heat when parked buy low heat when moving?

Not quite as black and white as that, but yes the same effect which was why I changed the thermostat first (which made no difference)

Read:

Its an easy job. But if a mechanic is following Mazda repair procedures, its a dashboard out job.

Thanks. I’ll be very happy for my mechanic not to follow Mazda procedures if there are short cuts.

How do the differing temps on the two hoses show that the heater core is blocked, and how can the heater give 40 degrees heat when parked, if it’s blocked?

How long of a job do you think that is for a mechanic of 20+ years’ experience?

For the sake of £15 or so, would it be better to have a used replacement heater box and matrix on hand? And is the NB one interchangeable and any better?

A mechanic won’t be happy following some instructions you got off the internet, and then for you to expect him to warranty his work. Most professional mechanics will use something like Autodata to price a job, irrespective of how they actually do it themselves. There was a recdent thread on this forum that included sumissions from professionals explaining why they will invoice, for instance, full price for parts to a customer, or why they won’t fit customer parts, or used parts. Best to speak to speak to your mechanic or do it yourself.

If I was to do it, I’d probably set aside a day of my time. I’ve been fiddling with MX5s for over 20 years as a hopeless amateur who reads, rather than usually asking.

If coolant is freely flowing in an out of the heater matrix, the hose temperatures should be more or less the same. That is obvious. If its hotter when you are not moving, I suppose its because the engine is hotter when the car is not moving, and you are seeing a heat soak effect.

A cartoon representation of the MX5 coolant system

Sounds like you have got a lemon of a car. I’d cut my losses right now, if you can’t DIY. I don’t think the work is technically difficult for an amateur, but its all labour that can mount up @ £90 an hour.

Fortunately there are many other things about the car that are very good indeed. One is the lack of rust, then the great bodywork and interior, and the fact that it goes like stink for a 1.6, although it appears to have no engine mods. It has, however, been a pain to sort out to the stage where it’s very useable. It’s about 90% right now and I’m not going to give up.

The temp “gauge”, which I understand is very inaccurate, goes from the bottom when fully cold to 11.45 when fully warm, and stays there when the engine is being thrashed, or when the engine is idling. It does not drop from 11.45 if it’s just cruising gently.

Well, you hope its rust free. It probably isn’t, if you follow a certain Canadian resto thread on an American forum… A shell that looked completely mint when stripped down, until the owner decided, for the hell of it, to take a peek inside a sill by cutting an exploratory hole, and found it rusted…

If you want a gauge that responds in the way you expect, you need to linearise the gauge. Its not that the gauge is “inaccurate” as you say, its that Mazda, for very good reasons, dampened the response.

There are loads of write=ups and videos to modify the gauge to give a linear response.

https://www.miataforumz.com/how-40/how-linear-water-temp-gauge-245/

I did something similar to this to a 1991 car I had following an overheating issue, but not bothered with it since, for the S-Limited, M2-1002 and S-Spec 2 I’ve had.

Its useful if you feel the information it gives is actionable by yourself. For instance, it allowed me to diagnose a blocked thermostat bypass line, strip it down, and then also to diagnose worn fanmotor bushes (and replace the motor myself). For someone who has no mechanical knowledge and is dependant on others to do the work, its probably only likely to lead to unnecesary worry, which gets that person nowhere.

Best thing I did was to do things myself. You should do the same.

It is not physically possible for me to do any work on my cars any more. That’s why I like to understand what needs doing, so I don’t get ripped off.