How hot is too hot?

[OurCognitiveSurplus]

Hi everyone

I've been having an 'overheating' problem with my MX5. The only symptom of this problem is that the temp gauge can start to sneak slightly past the normal vertical position. Yesterday at Marulan after a 10 minute session, when I was coming in for cool-down, the gauge snuck just past vertical but returned to vertical after a minute or two of idling (both fans come on). A similar thing has happened after driving up the mountains to Canberra from the coast on a very hot day.

I’ve recently had a new radiator installed (similar to stock design with the black top tank - Dave from Carco Canberra who did the install said it’s a thicker radiator than the stock model. But it’s not as big as the 52mm~ aluminium radiators). The new radiator doesn’t seem to have helped.

Dave did note that the system was hard to bleed. He said that may mean the heater-core is blocked – but the heater blows hot, so I don’t think that’s the case.

The car is stock.

At the next service I’ll get Dave to replace the thermostat in case that’s not opening fully and is causing insufficient water flow and bleeding problems (the car warms up fine on cold mornings, so the thermostat must be closing properly).

While I don’t have a proper temp sensor installed, I do have a laser temp gun which I was using to check the temp of the top radiator hose and the fins/top tank of the radiator. I assume these parts would fairly closely follow the temp of the water inside (heat transfer to radiator fins should be near perfect). The hottest temperature I recorded on the track day yesterday was 72 degrees. Perhaps this method isn’t perfectly accurate, but it wouldn’t be off by more than a few degrees.

Given that water boils at 100, and radiator caps are designed to keep the system pressurised up to over 100 (IIRC 130 degrees for an MX5) – it doesn’t seem to me that 72 is overly hot. Perhaps my gauge is sensitive?

Money is no object to the proper servicing of my MX5 - and I believe in preventative maintenance. I just need to get a steer towards what the issues might be so I don’t go throwing money at the wrong things. I think if the radiator change hasn’t fixed the problem, and the thermostat change doesn’t fix the problem, the only thing left which I can think of is a dodgy water pump.

TL;DR?

- What water temp is too hot and risks damage to a head gasket etc?
- If a system is hard to bleed, but the heater still blows hot, what might the cause be?
- If a stock MX5 is overheating under load despite a new radiator, what would you look at next?

Thanks
Greg

[Steampunk]

- What water temp is too hot and risks damage to a head gasket etc?
- If a system is hard to bleed, but the heater still blows hot, what might the cause be?
- If a stock MX5 is overheating under load despite a new radiator, what would you look at next?

Greg

- A little unsure of the "dead spot" range of the standard temp gauge, but if I am not mistaken, anything above 120deg C, it starts to move. Once it moves, it is of moderate concern. If it continues to move toward the 2 O'clock position it is of greater concern and it's legitimately overheating. I have no idea what the temp is when it's past this position though.
- Heater will always blow hot unless there is considerable blockage in the first heater pipe, thus BEFORE the core.
- close off all possible air escape routes; ie. look for gaps before and around the radiator. If the replacement rad is stock measurement specs, then simply buy and place some thick sponge (with an adhesive on one side) from Bunnings, Clark Rubber etc. to close the gaps.
I'm assuming you did not remove the metal air-redirection plate above the AC condenser?
Do you have the undertray installed?
What do you reckon is the condition of your thermostat?
Do you have an aftermarket grille?
Might sound silly, but it's easy to overlook, was your AC on when racing?

PS: Don't look too much into the size of the rad. Bigger is not always better. You have to keep in mind that a RADIATOR works by RADIATING heat away by the means of air-flow. If the rad is too big then air does not flow through it as well and thus it becomes more of a heat-sink rather than a radiator.

[OurCognitiveSurplus]

Thanks for the reply 1red5.

A little unsure of the "dead spot" range of the standard temp gauge, but if I am not mistaken, anything above 120deg C, it starts to move. Once it moves, it is of moderate concern. If it continues to move toward the 2 O'clock position it is of greater concern and it's legitimately overheating. I have no idea what the temp is when it's past this position though.

This is interesting. If the water temp is around 70 degrees, and the engine is actually over 120, that would suggest flow issues. So the thermostat or the water pump.

Heater will always blow hot unless there is considerable blockage in the first heater pipe, thus BEFORE the core.

Very helpful tip. Will it be relatively easy to get the mechanic to pressure-flush the heater core? Or is that a hard job?

Close off all possible air escape routes; ie. look for gaps before and around the radiator. If the replacement rad is stock measurement specs, then simply buy and place some thick sponge (with an adhesive on one side) from Bunnings, Clark Rubber etc. to close the gaps.
I'm assuming you did not remove the metal air-redirection plate above the AC condenser?
Do you have the undertray installed?
Do you have an aftermarket grille?
Might sound silly, but it's easy to overlook, was your AC on when racing?

The mechanic seems to have done a good job of re-sealing all the air-flow around the radiator. The undertray is installed and there is no aftermarket grille. The AC was off. I'm not sure what a metal air redirection plate above the AC condenser is but I assume Carco wouldn't take it out for no reason...

What do you reckon is the condition of your thermostat?

Potentially poor, I'll get it changed this friday.

[sailaholic]

I noticed a difference in my na Gauge when water temp was high 90.s according to the ecu.

Thermostat would be my first guess.

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[Lokiel]

What brand is the radiator?
Some China radiators and intercoolers don't flow particularly well.
Being smaller and thicker I'm betting that there's less air flow through it.

[NitroDann]

My NA6 shows dead centre at around 90c.
3mm warm at 100c, and starts moving at 115 odd.

The method with the infra gun doesnt work.

Dann

[Steampunk]

you can't compare the temp of the top radiator hose with true water temp as the sensor is located at the back of the engine. If anything, aim your gun at the heater inlet hose, it still won't match, but it will be a closer reading.

[Steampunk]

I noticed a difference in my na Gauge when water temp was high 90.s according to the ecu.

I wouldn't trust the Adaptronic's temp gauge reading, as I'm sure you are well aware of Lokiel's problem.

The highest temp ever was 95deg C, this was after 5 laps at QR, 26deg ambient air temp, and I had a grille installed. Without the grille it reached 93deg C.

The needle (in a normally functioning engine) should not go past 11:30-11:45 O'clock, this is the "normal" position. If it starts moving higher during normal driving, especially in your case, I would be quite concerned.

[project.r.racing]

partly blocked somewhere. whether it be the thermostat not openning fully or rust/crap in your heater core.

very surprised the thermostat wasn't replaced when the radiator was done. now you have to drop most of the coolent and refill the system again.

that's like replacing a timing belt kit, and not doing the $65 water pump underneth it.

as above, the needle should be 1-2mm under horizontal, this is around 90C.

[Lokiel]

I noticed a difference in my na Gauge when water temp was high 90.s according to the ecu.

I wouldn't trust the Adaptronic's temp gauge reading, as I'm sure you are well aware of Lokiel's problem.
:

I trust mine fully now that I've finally calibrated it myself

sailaholic, if you haven't done your temperature calibrations yet I would recommend that you do so, just to be safe - give me a yell if you want to use my "high-tech" equipment since I've only done it recently and have a spare ECT sensor you can use so you don't have to flush your coolant and remove the sensor from the rear of the engine.

[sailaholic]

Sounds good lok.

I should point out it wasn't normal driving. It was sitting at middling revs with the AC on full while I was doing some light throttle tuning. Noticed the temps were up went for a quick drive with no AC and normal revs and it dropped quickly.

Gauge was still just below half for memory. Still doesn't hurt to check.

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[OurCognitiveSurplus]

partly blocked somewhere. whether it be the thermostat not openning fully or rust/crap in your heater core.

Is flushing out the heater core going to cost an arm and a leg, or no big deal?

[NitroDann]

Stationary revving is the fasteat way to heat up an engine, take it from a guy whos tried a few radiators and tested a heap ducting.

Dann

[Okibi]

My NB8A the thermostat opens at 180"F and the fans kick in at 210"F, temp never really goes beyond there as the fans start to cool things down.

[Steampunk]

Is flushing out the heater core going to cost an arm and a leg, or no big deal?

If you really think there is a blockage in the core, you can simply bypass it for the time being and drive around without it.
Bring it back to your mechanic at your convenience and he can test the flow through the core.
Saves the hassle of taking apart the dash just to find out there might be no blockage.