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Originally Posted by Shreyans_Jain It was standard practice to stop to let the engine cool down after a few hours on the road, top up the water and then drive on. Fortunately, modern cars have become a lot more reliable and tolerant to heat.
... Observations over the past week- The engine overheat warning lamp has never lit up.
- The AC has been functioning normally. Cooling is proper, even when idling for 5-10 minutes while waiting. It has never tripped.
- Car has shown no sign or indication of overheating whatsoever. It has felt, smelt and driven just like at any other time.
Again, it has been a case of the car running out of coolant, overheating and showing no indication of any kind. |
In the days of the Ambassador and Fiat, the coolant system was usually not pressurized (the rubber gasket of the radiator cap was invariably removed), did not use coolant (just tap water sufficed, though some of us used distilled water), and did not use a thermostat valve to control the circulation (there used to be a valve, which was removed at the first opportunity by most mechanics). This meant that the circulating coolant fluid (plain water) would run in an open system, and would boil if part of the water leaked out or the engine was overheating for some reason, and the conversion from liquid to steam would carry some of the heat (latent heat) away - until all the water boiled off, and the temperature gauge would rise to the red zone. Before that could happen, it was standard practice to stop the car and add some more water to top up the radiator.
The temperature gauge was (and is) placed into the bottom tank of the radiator - the coolest part of the system. When the water boiled, there was a slight rise of the temperature gauge needle, until all the water evaporated, when the needle would suddenly shoot all the way to the 'H' end of the gauge.
Then came pressurized cooling systems, with the coolant not being allowed to boil & evaporate. At roughly 15 psi (which is the pressure that the pressure cap of the radiator holds), the boiling point of the coolant rises to ~125 degrees C. Any little evaporation that happens, is allowed to be redirected to the expansion tank, where it can condense back to coolant. In any case, coolant at atmospheric pressure boils at over 106 degrees C.
So, if this closed loop cooling system is intact (no loss of coolant or pressure), the temperature can well rise far enough to be detectable on the temperature gauge, such as in case of a failed cooling fan (the commonest occurrence) or a broken V-belt (water pump not running = no circulation of coolant = no heat exchange) or a thermostat valve that is jammed close. One may notice that the coolant level in the expansion tank has risen more than the 'High' mark, or even overflowed a bit.
However, if the coolant is leaking out (e.g. broken hose, punctured radiator), the coolant pump will continue to circulate the remaining coolant, until there is too little of it to circulate (level below the radiator bottom hose, so the coolant pump cannot suck it up). There is no more coolant to remove the heat from the engine block, while the bottom tank has residual coolant that cools down due to the airflow below the car - ergo, the temperature gauge remains in the normal zone, or even shows a drop in temperature. However, the engine continues to overheat until it blows a gasket (or worse).
As the engine overheats due to loss of coolant, the electronics kick in. In old cars, the usual giveaway of an overheating engine was the knocking (pre-ignition) sound on accelerating - modern fuel injected ECM-equipped engines have knock sensors that dynamically alter the ignition timing and / or the fuel injection timing and quantity, effectively countering the pre-ignition, so the driver fails to realize there is something wrong. This is until the overheating protection system turns the AC compressor off (intermittently at first, then completely), and then tries to reduce power to the engine, which is when the driver senses something is wrong.
Therefore, the best indicator to immediately check for coolant loss due to leakage is when the AC is suddenly not cooling optimally, and the car seems to have lost some power / acceleration. Opening the bonnet might or might not show where the leak is, but the smell of coolant vapour is often present (either releasing as steam from the leak point, or evaporating after coming in contact with the hot engine block). The temperature gauge is not going to tell you anything until it is too late.
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We were parked in Devprayag having lunch when I detected a little whiff of the smell of coolant from the car. I checked but nothing was apparantly amiss.
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This can be explained by the phenomenon of heat soak. Once the hot engine is stopped (hotter than normal because of low coolant), it tends to boil the coolant present in the cooling gallery around the block (remember, the system is not fully pressurized any more because of the leak, so coolant will now boil at a lower temperature, and come out as steam through the leak). The coolant vapour has a typical odour, which is what you must have smelt - but saw no trace of coolant on the ground.