Engine Overheated

[scott.mohekey]

So I was on my way home after a weekend away.. about 300kms into the journey, when the engine suddenly overheated.

I pulled over and popped the bonnet to find the radiator overflow FULL of intensely boiling water. I left it to cool for some 20 minutes, and while I was waiting, found the vacuum hose on the top of the throttle body had popped off. There was also some steam coming up from around the water pump and an clearly audible boiling noise coming from the block.

After leaving it to cool for a further 30 minutes or so, I limped to the next town (about 2 or 3 minutes drive) and topped up the water. After a few more minutes letting it cool down again, I drove off and got about 300 meters down the road before it was overheated again.

By now I'm spitting tacks. It's a good hours drive home yet, and the car seems in no state to drive, so I call road side rescue and arrange a tow truck. They'll only take me as far as the next nearest town with a garage (which isn't super helpful seeing as I need to get home, but that's another rant).

Once there, I check the oil level (as I'd done a complete change with amsoil just a month and a half earlier, I didn't think that that could be an issue), only to find it was a good 5mm below the bottom notch. So I topped up with oil, and seeing as the engine was now well cold (after the tow), I waved good bye to the towy and set off once again...

... only to get 10kms further.

This time I'm seeing something is seriously wrong, so I arranged for another two at my expense for the rest of the journey. We had to wait a good 3 hours for this one to arrive, so by the time we got home it was 11pm or so, and I went straight to bed without even looking at the car.

What the hell could be wrong with it? Why was the oil so low, when there is no sign of any leaking, and no sign of it burning oil either (no smoke etc).

[Kenneth]

I don't think you should be worrying about the oil just yet, if it was overheating then it would have been stressing the oil and quite possibly causing a good amount of it to turn to vapour.


If you start your car and leave it running, after a while (and you can tell by holding the bottom radiator hose) the engine will get to temp and start flowing water past the thermostat.

Make sure you sit in the car until it gets to temp, and make sure it stabilises before getting out to check under the bonnet. If the heat goes directly up to 3/4 turn it off and go check the bottom radiator hose. If the thermostat is stuck, then there won't be coolant flow and the bottom hose should be fairly cool.

If the temp does stabilise then go and check the radiator. The bottom hose should slowly warm up and then eventually the fans should start.
Keep going and checking the engine temp (this will take a while) and see which comes first, over-heating or fans start up. If it overheats before the fans start, then that could be a problem. (this isn't likely as fans aren't usually necessary at highway speed)
If the the fans start, then wait around and see if it overheats again. If so, you could have a partially blocked radiator or blown head gasket.

If everything works as usual and the car doesn't overheat, take it down the road and see if you can get it to overheat with some abuse (don't go far from home) keep an eye on the temp though. If it doesn't overheat idling at home, but does driving then you could be looking at a head gasket.

[scott.mohekey]

I wondered about the thermostat, but wouldn't that restrict the boiling water from getting to the over flow tank? Or can it get back through the top pipe?

[Kenneth]

Or can it get back through the top pipe?

Correct.

[scott.mohekey]

Ok, I'll try this when I get home tonight.

In either case (head gasket or blocked coolant flow), is there likely to be any damage caused from it overheating? Or will I get away with just locating and fixing the problem?

[Kenneth]

Ok, I'll try this when I get home tonight.

In either case (head gasket or blocked coolant flow), is there likely to be any damage caused from it overheating? Or will I get away with just locating and fixing the problem?

Depends on how bad it ended up overheating. If it is head gasket, then the bores etc will be checked when the heads are off. If thermostat, then just drive it and see.

You do at least have the security of knowing you had decent oil in there, if not and it had burnt up etc then you would definitely need a new engine!

[Nick Mann]

Overheating can cause damage. If it does overheat, the best advice is to pull over, let it cool down for a good amount of time, then add water and drive it home. Unless you are home! Either way, adding a lot of cold water to a very hot engine is also a risky thing to do. A small amount into a radiator 3/4 full with hot water is less risky. Zero risk is adding cold water to a cold system.

[scott.mohekey]

I only put about half a liter in to top it up (the overflow tank was showing low).

Well, heres hoping it wasn't too bad. Otherwise the vr4 engine will have to be swapped in sooner than expected.

[Kenneth]

The other option (if you are at home) is to boil the jug, and then add that to the radiator. (if your engine is hot)

Overheating can cause damage. If it does overheat, the best advice is to pull over, let it cool down for a good amount of time, then add water and drive it home. Unless you are home! Either way, adding a lot of cold water to a very hot engine is also a risky thing to do. A small amount into a radiator 3/4 full with hot water is less risky. Zero risk is adding cold water to a cold system.

[Nevman]

Some time ago someone's told me that a bit of overheat may be good for engine as it may help to remove deposit.
I think it's boll***s but maybe there is something in it? Has anyone heard anything about it?

[OnBoost!]

Nah i have never heard anything like that before...

[scott.mohekey]

Ok, I've narrowed it down to the thermostat. i have a spare one in the vr4, and have taken the casing off but am not sure how to remove the thermostat itself. Anyone know how?

[bradc]

If you can see the thermostat, just pull on it or tap it from side to side with a screwdriver

[scott.mohekey]

Ok, I've replaced the thermostat, and I have the same problem. Interestingly, the thermostat I pulled out was largely dry around the seals. What could this mean?

[bradc]

With the top hose disconnected from the radiator does water come out of it once the car starts to get warm, ie does the pump come on?

[scott.mohekey]

I'll try that tomorrow night. Thanks/

[scott.mohekey]

Which way does the water flow through the thermostat? I'm pretty sure the bottom radiator hose was dry yesterday when I swapped the thermostat over.

[bradc]

the water should flow out of the engine into the top of the radiator, then out of the bottom of the radiator back into the engine.

[martin_y]

My moneys on a blocked radiator.

[shaun1978]

My moneys on a blocked radiator.

If so try taking the radiator out and try flushing it out both ways