Had a minor scare over the weekend. The good old cooling system again I hadn't put any water in for a couple of months but had noticed that my overflow bottle to the expansion bottle had an inch or so in it. So that meant the pressure was exceeding 20psi and pushing the water out. And I had noticed the fan was coming on a bit more than normal, but I had put that down to the hot weather. That's normally indicative of an air lock (trapped wind :-)) So I broke the golden rule of 'if it ain't broke don't fix it', I decided to try running the engine up to temperature with the cap off the expansion bottle so it would circulate better and not be under pressure. To stop it pouring out I wrapped a cloth around a funnel and stuck that in the top of the expansion bottle. Started the engine, and as expected the water level rose and then sat about 1/3rd of the way up the funnel. Once she was up to temperature I squeezed all the hoses, and some air was expelled.
But then a most odd thing happened. The level in the funnel suddenly rose all the way to top. I rushed around the car to stop the engine, but then it suddenly dropped all the way down to inside the bottle. But then a few seconds later if rushed up again, so quickly that it actually spilt a little bit before I shut the engine down. At this point I just added a little bit to bring the level in the expansion bottle up to it's normal level (about 3/4" below the top), put the cap back on and put her to bed. Then my daughter's partner suggested we take her for a quick run to test if my lifting the back end had cured the exhaust mounting grounding. So we drove to the Colsterworth roundabout (3.5 miles, 5 mins) and I flew her around that, forcing the passenger side down and success, no grounding. But as we drove back I noticed the temperature gauge was on the 'A' of NORMAL and climbing, and the fan was on. Now. normally she takes 20 mins to get to temperature so this was obviously worrying. In fact the temperature carried on climbing and was just entering the red as we got back home. What was a bit confusing was that when the fan comes on I normally feel a steady stream of warm air coming up through the footwell. This time I could only just feel a bit of warmth.
We took the bonnet off and I expected the engine to be red hot, but oddly, I could rest my hand on it and it was not that hot. And the top metal pipe was also not very hot. But what had happened was that the overflow bottle was now full of boiling water. (A an aside there was also water dripping down at the front of the car which was very scary until I realised it was actually coming off the overflow bottle and running down between the chassis tube and the body before finding a hole.) So on the face of it it was simple problem in that the expansion bottle cap was not properly seated, so the water boiled over and was dumped into the overflow bottle, lowering the level and thereby heating the engine. So I decanted the water back into the expansion bottle, made sure the cap was properly seated and repeated the run. This time the temperature was under control although the fan was on permanently so it obviously wasn't perfect. Thought about it overnight and hypothesised that the odd behaviour of the water when I was running with the cap on could have been caused by a long-standing air lock suddenly being released from deep within the engine and moving to the top of the engine. And because the cap was not seated properly the level dropped so much that the temperature sensor was now in free air so went up into the red even though the engine itself was actually not that hot. So I removed the top pipe from the radiator and added some water. She took TWO LITRES !! So put everything back together. Drove her 12 miles (20 mins) this morning and she appeared to be perfect. But obviously I need a much longer journey to give her a proper test.