Nissan Patrol 4x4 (Africa)

I had a similar experience on mine too many years ago. On the expansion cap bottle, there is a rubber pipe that has a bend in it that sits in the water, so it can be sucked back into the radiator as the radiator water cools, but where the pipe attaches to the underside of the cap, it was not sealing properly so as the radiator cooled, air was being sucked back into the radiator. The amount of air being sucked back into the radiator was not enough to show any overheating at all. The reason for this is that when the water is building up its heat it just pushes the air back out to the expansion bottles, and sucks it back in when cold.

On mine I put a proper clamp and tightened it up properly..... since then no problem, all fixed. My Money is on the sealing of the pipe under the expansion bottle cap !!

Cedric, I was also having my doubts about that cap on the expansion bottel. I replaced the bottel when I installed the new radiator and have never really investigated if it seals properly or not. Will look at all the pipes and see if it leaks somewhere.

Michael, on mine it looked like the pipe was fitting ok, but it could swivel around the yellow plastic peilitjie thingy on the underside of the cap that the pipe fits over. If it doesn't seal perfectly it can only suck air as the suction from the radiator is so slow that it gives plenty time for the air to seep through any crack or gap on the pipe.

Well, I'm really going to start investigating a couple of things and doing some tests.

The pipe between the "overflow bottle" (actually found out this morning it's not an expansion tank, but an overflow bottle.....didn't know there was a difference) is still 100% and has brand new wide spring clamps so it seals without a doubt.
Radiator cap seems to seal as well. Pressurized the cooling system from the overflow pipe side and sprayed soapy water on the cap.....no leaks there.

I will check the rubber pipe under the overflow bottle cap and make sure it seals there. Will maybe give it a clamp as well.
I will also have the radiator pressure tested to make sure it is still ok.
Will look for bubbles in the radiator when idling this afternoon as a head gasket is still not off the table.

This morning when I stopped at work I slowly opened the radiator cap to check if there was any air in the system, but the radiator was filled to the brim with water so seems ok.

Although a head gasket failure cannot be ruled out, I think you would be seeing other issues like higher water temps, especially seeing as you can monitor the precise water temps via the Madman. I would think that as the water gets hotter the crack is the weak point and will start to get bigger(longer) after each hot cold cycle showing up and even higher water temp. The vicious circle would have shown up much harsher symptoms by now in my opinion. One other thing is to check the bolts holding the water pump onto the head/block. Our diesels, shake rattle and hum quite a bit and a tiny leak could be causing the air ingress when the head/block cools, but seal when hot ( expanded ).... however I think this is unlikely.

I still have my money on the pipe on the underside of the, now known as "overflow bottle"..

I just had a brain fart.... you mentioned earlier that the block was a bit dirty and now with the new coolant/antifreeze, the block is looking better, or cleaner. Maybe with the "cleaning effect" of the coolant, the dirt that was assisting with the sealing has now been cleaned out exposing a potential "weak point" in the sealing of some component ( possibly the waterpump for example ). A small leak will be very difficult to see if under the water pump as its difficult to even get to the bolts to tighten it up, nevermind trying to see a watermark.....

ricster wrote: ...
Our diesels, shake rattle and hum quite a bit and a tiny leak could be causing the air ingress when the head/block cools, but seal when hot ( expanded )
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What Cedric says here is paramount. If you speak with virtually any coolant / radiator guy, they will tell you that you generally have to diagnose leaks on cold vehicles.

Case in point - following the fitment of my new radiator - there would be absolutely no fluid at all in the morning. Then I'd start the vehicle and drive the kids to school. And then all of a sudden I'd be dripping coolant for a very short while, after which the leak would go away again.

It turned out to be the pipe connecting the expansion tank (the tank at the top of the radiator) with the thermostat on the radiator side. When cold, the thermostat would be closed. As soon as the thermostat opened up, and the pipe was under pressure, it would leak for a short while. As soon as the expansion tank had warmed up, the "expansion" of the metal resulted in a good seal between the pipe and the tank, and the leak would go away.

I replaced the clamp, and the issue has been sorted for ages now.

Sorry - I am quoting myself here, but I made a typo here which may be misunderstood. Clarifying it.

iandvl wrote: ...
Case in point - following the fitment of my new radiator - there would be absolutely no fluid LEAKS at all in the morning. Then I'd start the vehicle and drive the kids to school. And then all of a sudden I'd be dripping coolant for a very short while, after which the leak would go away again.
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