Learned Something Today


Okay, not a car fire, but in the worst extreme it could have been.

Important lesson 1:  If you top up the coolant reservoir in an overheating car, the car (at least my Miata) has to cool down before it will draw the coolant from reservoir into radiator.

When I was getting the Miata back on the road after the Explorer was killed by a truck (nobody was hurt; let’s get that clear from the start, but the insurance, given the Explorer’s age, totaled it out), I had a couple of more serious problems to deal with (brakes!). As a result checking coolant level fell through the cracks. While out doing some shopping today, I noticed engine temp going up past the top end of the scale. Eep. (Important lesson 2:  Pay attention to your gauges).

So I stopped and bought some coolant (gas stations and parts stores are all over the place to it was easy to find). Reservoir was essentially dry so I filled and started out again.

Engine temp continued to rise. Eep2.

At my next stop, I popped the hood and checked the coolant reservoir again. Still full. Level not a millimetre lower than it was when I’d filled it.

“Oh,” thinks I, “this is not good. Not good at all.”

I inspected the hose between reservoir and radiator thinking maybe there’s damage or a leak causing the coolant to not be picked up. Nope. Nothing wrong I could see there. I popped open the radiator cap and…no coolant visible. So, I poured more coolant into the radiator. Only, it was still hot so I got some bubbling and mini-geysering. It was a bit of a challenge to get the cap back on without getting burned/scalded but I managed. Started up the engine again and, temperature promptly fell back into normal range.

When I got home, I parked and let the engine cool for a bit. Then checked the coolant level in the reservoir. About half full, so coolant was drawn from reservoir back into the engine. I topped it up again with the bottle of pre-diluted coolant I had in the garage.

And that, boys and girls, is why you always, always check your fluid levels. Having those checks “fall through the cracks” can lead to bad things indeed.

5 thoughts on “Learned Something Today”

  1. That sounds like a bad thermostat.
    The thermostat (at least in cars I’ve owned) is a valve that opens once the engine is up to temperature to allow cool water to circulate from the radiator into the engine.

    Engines like to run at a temperature above a certain level, so the thermostat stays closed while the engine itself warms up, then opens slowly to allow water in from the radiator without causing a precipitous drop in the temperature in the engine. Result: the engine comes up to its preferred temperature sooner than if there were not thermostat valve in place at all.
    But if the thermostat is broken, it may take its own sweet time opening, or decline to open at all.


    1. No, thermostat is fine. My experience with them is that they usually fail “open” leading to a car that’s very slow to warm up. The problem was simply coolant level. The car had been sitting garaged for years. I noticed that the cap was dislodged from the reservoir as well. That provides plenty of opportunity for the water in the coolant to evaporate. Once I refilled the cooling system everything was fine. And neither “milkshake” in the oil nor clouds of white smoke from the exhaust so head gasket is still sealing.

      So I got lucky.


      1. Most thermostats “fail closed,” which is what made the Stant “Fail Safe” thermostat so different – it “failed open.” Overheating tends to fail a thermostat, so I advise replacement.

        As far as adding coolant directly to a hot engine – when pouring it into the radiator, either trickle it in while the engine is running (allowing the cold coolant to mix with the hot coolant, and not “shock” any metal castings, which can crack them) or allow the engine to cool for a while (2-4 hours) before adding coolant. As you noted, adding coolant via the overflow won’t let any into the engine until the system cools down and the air contracts, pulling a partial vacuum in the radiator and drawing coolant in from the bottom of the reservoir (for this reason, the reservoir should be removed and cleaned out from time to time, or you end up sucking that crud into your radiator. I usually do it when changing coolant, so about every five years. Same interval for changing brake fluid.)

        And, be sure to mix the coolant with water, if it’s not premixed – oddly, mixing with water increases the boiling point and depresses the freezing point of the coolant mix, over coolant alone. DO NOT USE TAP WATER. (God alone knows how much time I’ve spent cleaning the crud from tap water out of people’s radiators…)

        Also bear in mind that overheating can screw up the springs in your radiator cap that govern system pressure – and replacing them is a cheap and easy fix. Better safe than sorry.

        (Dr. Phredd has been swinging wrenches for 40 years. I won’t say I’ve seen it all – someone always manages to bring me something new – but I’ve seen most failures by now…)


      2. OK, my experience was that one failed “closed”.
        Just to add injury to insult, it opened while I had the radiator cap off and my arm in the path of the steam.
        FWIW, second-degree burns hurt.


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