My model of MX-5 is: NA 1.6 Eunos ('92/J), c. 100k miles
I’m based near: London / Brent Cross
I’m looking for technical help or recommendations on: Coolant temperature overheating
Hi all,
Joined the forum and had a good read through dozens of prior over-heating posts, as I’m trying to find inspiration after persistent overheating despite seemingly having thrown everything at it. Owned the car nearly 15 years / 10k miles, most of which time I had no overheating issues including an annual spirited track day where it would go all day. Its a ‘bone stock’ engine including timing that to my knowledge has never had a rebuild.
However last couple of years, I’ve notice on hot days on the motorway (at 70mph), especially on the way home from a day trip (i.e. car had 2 hrs + of running and/or a spell in traffic), and on track days after ~10 minutes of hard driving (after driving 1-2 hrs to track), the temp gauge starts creeping up from middle. Its never gone into red as I’ve been able to manage it by slowing down and/or stopping for breaks.
During that time I have replaced in my efforts to rid the issue:
All coolant hoses with new silicone
Thermostat with OEM
Thermostat housing with OEM
Waterpump with competition spec
Radiator with OEM
Coolant with 50/50 distilled water
I don’t think its a sensor or gauge issue - most of the time in normal driving it is fine
I don’t think its an electric fan issue - overheating is at high speed, and not in traffic
I don’t think its head gasket - passed the sniffer test and compression test
I don’t think its restricted overflow pipe / heater core or air bubbles as I have cabin heat, normal idle, and normal warm-up behaviour in hot and cold conditions and have done numerous flushes, I also don’t get any overflow in the expansion tank
I wonder if there could be small internal coolant passages in the head or block that have been gunked up from prior k-seal use? Sadly it has drunk a couple of bottles in the past and it took a lot of flushing to get to clear liquid. If such passages exist and are fully blocked, it may not be cleared via the coolant flush? How on earth could I clean it out if that’s the case?
Could there be a head gasket issue that would cause overheating without allowing coolant into the oil or exhaust gases into the coolant?
Any other ideas?! Am very reluctant to tear into head and/or throw money at professional diagnosis without a good hypothesis!
Thank you both. No l’ve never changed the cap, although I don’t seem to be losing any coolant so would be surprised if that’s leaking, although maybe it’s enough to lose pressure but not noticeable volume if temps are managed. Easy and cheap thing to try though!
Question is as I think 999to5 is right, does the temp gauge normally hold steady if both temp and pressure are rising proportionately? If not, hard to see how this could explain the temp rise.
What pump have you fitted? (Not MX-5 specific, but I believe that some, so-called “high efficiency” pumps, will cavitate at higher engine speeds and not pump effectively.)
(Also, have you verified that the electric fan comes on as expected?)
Changing the cap is more to allow a release of pressure than to fix any leaks. As far as I understand it, raising the pressure WILL increase the temperature!
Does your cooling fan actually turn on?
When you replaced the thermostat, did you align it correctly? They have a little notch or hole or some such that needs to be aligned by pointing directly backwards. I had a similar issue with coolant overflowing when coming to a stop and it turned out to be bad alignment of the thermostat.
It’s highly unlikely that your water jacket will be blocked, though I suppose not impossible. I would suggest compressed air might help dislodge any blocks, but surely you can test if the jacket’s blocked by disconnecting the return pipe when engine is running and thermostat open and seeing if you get a nice stream of scalding hot liquid? I think k-seal would struggle to block the water jacket, although it might be possible if there was a bit of vapour-lock in the wrong place. I feel like a blocked water jacket would produce a much more noticeable and severe problem, though.
50/50 coolant mix sounds a bit rich. Glycol is not actually a very good coolant; all the stuff in antifreeze is there to lubricate parts and help prevent corrosion, it actually reduces water’s ability to cool things. 1/3 antifreeze is the strongest mix that I know of, never heard of 50/50 being used. Maybe try a run with just water or a 1/4 mix and see if the same things happen?
Do you circulate and bleed properly when you change coolant? It’s easy to overlook simple things sometimes, such as putting the heat on full blast when draining/refilling the system or removing the expansion tank cap. Silly little things but they can make a big difference.
Doesn’t sound like head gasket at all, I don’t think you could have an issue with the HG without some other symptoms showing.
Appreciate the further thoughts and comforting that head gasket issues seem rather unlikely. Certainly possible it’s as accumulation of things - was never aware of stat needing specific alignment, so possible that’s causing a marginal flow restriction that matters at high load. Pretty confident otherwise it’s opening/closing normally
fan operates normally when stationary. I presume no one would expect it to come on at high speed, or should it (in which case how would I verify that)?
radiator cap: as mentioned I think, I’ve never noticed expansion tank filling. I think you’re saying under high load, pressure (and temp) would increase until releasing via expansion tank, which should then suck coolant back when reverting to normal. So could well be something in that.
You shouldn’t need the fan at high speed for obvious reasons, although it should still come on if the system reaches 90 degrees C as the car has no system that tells the fan not to turn on dependent on speed.
Yes, most coolant systems have a “pressure release” built in, often (almost always) to the radiator cap. That’s what the spring and rubber ring on radiator caps do. Note that rad caps have a pressure rating, and due to your non-stock water pump, you may have to buy a specific radiator cap. The manufacturer of the water pump should be able to advise you. They might even advise that you should be using a different radiator!
Is there any activity in the expansion tank when the car is running? You may not notice it fill but you should see some turbulence in there.
It’s interesting that the overheating is occurring when both water and airflow should be high.
Is it possible that the fan is blowing the wrong way??
Is the numberplate impeding airflow?
Could there be an airlock in the coolant?
I would say chances of airlock are so low as to almost be impossible, especially for a car that gets revved up nice and high. The only way you could get an airlock in this system is to not put enough coolant in. I also think you would be able to hear an airlock.