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I noticed coolant was leaking and drived home slowly. By the time I got home I noticed there was no more leaking so could be all coolant gone. It seems to be from the radiator however I could be wrong. I drove for the following reasons:

I assumed that since the in car heater was working during my return home, this meant there is still sufficient coolant in the system. As far as I'm aware if coolant becomes low the in car heater would have given cold air.

I also drove slowly(under 3000 rpm). I assumed even if coolant is not there, so long as you drive 2000-3000 the engine won't overheat, its only if you rev higher.

I kept my eye on the temperature gauge and assumed that so long as it doesn't show overheating into the red area, which it didnt, i should be safe.

Were my assumptions correct, if not please explain why. Also could head gasket now be gone or would that only happen if your temperature gauge goes to the red region?

Thanks

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I assumed that since the in car heater was working during my return home, this meant there is still sufficient coolant in the system. As far as I'm aware if coolant becomes low the in car heater would have given cold air.

Depends on the exact car but generally yes this is correct - at the very least it indicates that there is enough coolant in the system to have some passing through the heater matrix. It doesn't always mean there is sufficient to provide adequate cooling for the engine though.

I also drove slowly(under 3000 rpm). I assumed even if coolant is not there, so long as you drive 2000-3000 the engine won't overheat, its only if you rev higher.

Not quite - what generates heat in the engine is when it is under load (which is not always the same thing as high revs), and driving slowly can actually be counter productive as this will significantly reduce airflow to the radiator and engine bay which is badly needed to cool the engine when you are marginal on cooling. This can be seen when cars that have cooling system issues overheat in slow traffic or stationary. Ideally you want to travel as fast as you can with as little application of throttle as possible - lift and coast as much as you can!

I kept my eye on the temperature gauge and assumed that so long as it doesn't show overheating into the red area, which it didnt, i should be safe.

Not always - the "temperature gauge" in most cars is actually a measureing the temperature of the coolant at a certain point in the cooling system. If there's no coolant, or if there is an airlock you could see an artifically low reading that doesn't reflect the reality of what the engine is experiencing. If there's still coolant in the system and you're seeing a "normal" reading that's encouraging but not cast iron.

Were my assumptions correct, if not please explain why. Also could head gasket now be gone or would that only happen if your temperature gauge goes to the red region?

It's possible that the head gasket could have gone but in my experience this would be unlikely from one short period of low coolant - although it very much depends on the engine in question as some are less robust than others. My old car (S6 with a V8 engine) had a thermostat failure and I limped some 20+ miles (with multiple stops to cool down and top the boiled off coolant) and the headgasket was absolutely fine - try that in something with a K-series engine and the results would likely have been a headgasket failure.

Besdt course of action now is to avoid driving the car (if possible) and get the leak tracked down and fixed. Once that's done get the system emptied, filled with fresh coolant, bled and watch it like a hawk for a week or two. Keep an eye out for any of the classic signs of headgasket failure:

  • erratic temperature readings
  • steam from the exhaust
  • oil in the coolant (looks like a brownish floating scum in the coolant expansion tank)
  • coolant in the oil (looks like dirty mayonnaise on the underside of the oil cap)

And if you see any of these get it checked out as soon as possible.

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Overheating my not occur immediately depending on where in the system/radiator the leak is and how big it is. If the leak is only a small one, higher up in the rad and further away from the top return hose you may be able to drive for quite a while without any problems, although the temps must be monitored at ALL times. If the leak however is lower down in the radiator you'll have less time to carry on driving because as well as loosing coolant via innitial pressure expulsion, you'll also have the basic draining issue. Monitoring the heater temperature is a good observation, so you did well in that regard.

If the heater radiator is still producing heat then there is likely coolant remaining in the system, but it could still be critically low and the movement of the vehicle along with whatever is moved by the pump may just be enough to swill a little coolant through the heater rad every so often, giving the impression that the rad still has a good amount of coolant. Normally though you would notice the heater rad blowing hot then cool alternately, before it finally blows totally cool as the system empties.

Driving slowly and at lower RPMs may make a difference if there is still coolant present, as less heat is being produced by the engine under less load. Be aware though that if you are leaking coolant even driving at low RPM will not prevent overheating! Consider that even a vehicle full with coolant can still overheat, say it is stationary & the cooling fan isn't working etc.

Cooling only works when there is coolant and airflow present through the radiator, if either is reduced cooling is negatively affected.

Assuming the car has not actually overheated then the head gasket 'should' be ok. However after having the leak is repaired a coolant pressure test can be carried out to detect any further possible issues before test driving etc.

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Your engine dissipates heat by transferring it via coolant to the radiator. Even the smallest of leaks will #1 Change the boiling point of the fluid (it increases with atmospheric pressure) as most cars run a 15 lbs. pressurized system. #2 can cause the remaining coolant that has not leaked to not circulate. #3 Depending on the location of the coolant temp sensor, give a false reading of actual engine temp as it needs to be submerged to work properly, if there is an air pocket it will read cooler temps then actual. #4 The temp sensor also controls the electric cooling fans on most vehicles, causing your situation to be further compounded by not only coolant loss but possible radiator fan shut off as well. #5 It does not take long once a car starts overheating to burn up head gaskets which is a very expensive fix. #5 Triple A is very cheap to get a vehicle towed instead of driving a car losing coolant. Should not drive a car losing coolant as there are other considerations as well as polluting the ground water, possible poising of animals, they tend to drink anti-freeze, it’s a lubricant on roads causing other vehicles to loose traction and it can cause a unexpected visibility hazard as your putting pressure thru a damaged area and if more gives suddenly and then steam is emitted while driving in your visibility area... then what? NO do not drive cars leaking coolant.

  • I have driven cars that have been loosing coolant - some drastically, and got to home or the garage ok. You need to be aware of the issues. Did it on a v8 (also several 4cyl engines) and the head gaskets were fine... So head gasket damage is not guaranteed... – Solar Mike Oct 1 '18 at 6:15

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