Sorry about the length of this post.

I've been having overheating problems with my TP Sedan (Auto, carby) for six months or more. Prior to that, the temp. gauge never went over half-way in any conditions at any speed. Now there is an overheating problem at any speed over 100kph. It is worse when the ambient temperature is high. I've managed to get through the winter ok but the coming summer will be difficult.

Around town, even on the hottest day and in the heaviest traffic, it will run all day without a problem. On the open road, it is fine until I exceed 100 kph for more than a couple of kms. It will go up long, continuous steep hills at 95 without a problem but if I go down the other side at 110 it will start to overheat. Once the temp. gauge goes over half way, the only way to get it back down is to slow to 80 kph or less. Otherwise it will continue rising until it eventually gets into the red.

I've replaced the thermostat twice and tried removing it altogether temporarily. I've replaced all the radiator and heater hoses. Replaced the radiator cap twice. Installed a brand new water pump (there was nothing wrong with the old one). Swapped the radiator from my wagon (made no difference). De-scaled the radiator with citric acid. The fans are definitely working properly. Also replaced the air and fuel filters.

Initially I suspected a blown head gasket or cracked head but have eliminated that. It runs smoothly, starts easily and has plenty of power. If anything, it is running rich rather than lean. Fuel consumption is not good - 11.5 litres per 100km country driving, often worse. It does not use coolant unless it overheats significantly, in which case the coolant gets blown out of both the reservoir and radiator. If it only overheats slightly, the radiator replenishes itself from the reservoir, so that system is working properly. It does not use any oil and there is no contamination of the oil or coolant. I haven't touched the ignition timing and it runs too well for the timing to be significantly out.

I'm stumped. What else could it be?

I'm considering a brand new aftermarket radiator. They sell on eBay for between $250 and $400. However I am reluctant to keep throwing money at the car since it is barely worth that much. Also the current radiator was swapped from my wagon and the wagon had no problems with it. Some of the aftermarket radiators are copper/brass and some are aluminium. The aluminium ones are supposed to have greater cooling capacity. Which do you think is better?

Is the radiator overflow actually pumping out steam when you stop and the needle is high?
If not, suspect the temperature sender on the engine, or the temp gauge itself. If your fuel gauge goes up at the same time as the temp needle it will be the voltage regulator on the back of the instrument cluster.

Is the radiator overflow actually pumping out steam when you stop and the needle is high?
Yes it does. On Thursday, I was being monstered by a truck on my way home from Tamworth, so had to keep the speed up. It didn't actually boil until I got home and stopped but when I did, most of the radiator contents blew out. It also popped the top off the reservoir. If this keeps happening, I will end up cooking the head gasket.


If not, suspect the temperature sender on the engine, or the temp gauge itself. If your fuel gauge goes up at the same time as the temp needle it will be the voltage regulator on the back of the instrument cluster. Originally I did suspect the gauge or sender but t isn't that. It really does overheat.

Thanks for the reply.

pop into a radiator shop

some can test if there is carbon dioxide in the coolant

did you replace the radiator cap

Yes, I replaced the cap a couple of times just in case I got a bad one. There are no radiator specialists in the vicinity but might be one in Tamworth. I will look it up. Thanks.

The part I don't understand is why it only happens at high road speeds.

I had this on my old TN wagon, for the winter it was normal, then on a trip to Melbourne it just gradually creeped up and up. I could let it idle in my driveway all day and it cooled fine.

Turned out it was my radiator, was blocked enough that a long trip it got hotter and hotter, but on a short trip around town it let enough water through to keep it cool.

Thanks, everyone. I will roll the dice and try a new radiator. Only question is whether to pay extra for aluminium. If there is a fault in a copper/brass one, I can re-solder it myself whereas I can't do that with aluminium. Does anyone have any experience or opinions on which is better?

Before you replace the radiator, take it out and flush it - upside down, garden hose, full force - see what comes out. Also look at the water pump - might have corroded vanes on the impeller - and tension/condition of the belt.

Before you replace the radiator, take it out and flush it - upside down, garden hose, full force - see what comes out. Also look at the water pump - might have corroded vanes on the impeller - and tension/condition of the belt.

Done all that. New water pump. Belt is ok. Flushed the radiator when I swapped it from the wagon. Removed and flushed it (and the block) again after de-scaling with citric acid. The de-scaling seemed to make a slight difference. Really, I don't think the radiator is the problem but if I replace it with a new one I will know for sure. Just not sure about these aftermarket radiators. Anybody had one?

Check the bottom radiator hose. If its soft and squishy it may be collapsing when the water pump is sucking hard.

You could be onto something there. I replaced both hoses but the bottom one is very much softer than the top. The old one I took off was similar and so is the one in my wagon. Last time I checked in Supercheap at Tamworth, I found one in stock but didn't buy it because it was just as soft. They are made by Gates Rubber, a good brand, so I just accepted they must be ok. Maybe I could put a spring inside it to prevent it collapsing.

This is just to let everyone know that the problem is fixed and MadMax was right. It was the soft lower radiator hose. I had considered that earlier because it was the only fault I could think of that matched up with the symptoms. i.e. overheating only above a certain road speed or rpm rather than amount of power applied. However I rejected it as a possible cause because I knew the hose was new.

Thinking back on it, my first overheating episode happened when the upper radiator hose got a hole in it through rubbing on the hose clamp underneath it. When I replaced the top hose, I also replaced the bottom one as a precaution, but kept the old one aside. Ever since then, I've had this overheating problem at speeds over 100 kph. (see my first post above for all the things I've replaced or swapped trying to fix the problem).

After MadMax's post, I checked again and confirmed that the new lower hose is very soft, easily compressed by squeezing between finger and thumb. The old one I had taken off is much better, as is the one on my wagon, though neither is as good as the top hose. Anyway, on Tuesday I put the old hose back on again. Before doing so, I wrapped it with tape to help reinforce it. Yesterday I went on a 400+ km trip to Coolah Tops National Park and back with lots of hills and lots of open, flat road. No problems now at any speed, sustained as long as necessary. Then today I went to Tamworth and back (150km). It has been a hot day and, despite the big hills, the temp gauge never went over half way. It now sits just under half way on the gauge as it always used to do.

So the moral of this story is: Be careful when buying radiator hoses. Check before you buy and don't accept them if they are easily compressed by squeezing them. I think I know where I bought mine and if I can find the receipt, I will go back and give them a roasting and tell them to pass it on to their supplier. Thanks for the help everybody, especially MadMax.