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Is the high flow version thermostat better?

Finally, it is an internal leak. No.1 spark plug is little bit rusty, and after head was removed, I saw No.1 cylinder chamber is cleaner. It should be the head gasket, but the mechanic said head still needs to be tested to see if it is cracked or warped. Also I ask mechanic to change timing chain and water pump by the way, almost no additional labour cost. Thank all of you again for suggestions and support, very kind of you.

I bought a pressure tester. When it is sitting and cold, it can hold 18 psi for couple hours. It seems no leak while sitting and cold. I connect pressure tester to recovery tank and start engine with no pressure. Like other two cars I checked, engine takes 20 minutes to warm up and thermostat opens. But after thermostat just opens, the pressure read on my car is lower than the other two, and it needs maybe 20 more minutes idling to reach the pressure read on other two cars.

Thanks for reply. There are some more important clue, it can pass pressure test no matter engine is cold or hot and running or not, in the first half hours after start engine the coolant system pressure is much lower and building up more slowly than other cars. It seems coolant is leaking only after engine start and when engine is running at low rpm. Where the crack could be? Exhaust valve?

My car doesn't have a rad cap, only can measure from a recovery tank.

I always took a photo of coolant level before I started the cold engine, then started engine and did the test, then took another photo of level after engine is completely stone cold, so it doesn't concern the coolant expanding. If always city drive every day, coolant recovery tank will be empty after 1 week. If it is a long trip on freeway, it seems losing much less.

Yesterday I started engine and left immediately for a 10 miles nonstop driving, after engine cold I saw the coolant level didn't drop any! Last week I started engine and just left it idling for half hours, then after engine cold I saw coolant level dropped 0.5cm. So it seems that the car is losing coolant only when engine is idling or running at very low rpm, especially in the first half hours after engine starting. What could it be?

Thank you. I will visit his channel to have a look.

Yes, or just put a bottle of bars leak to see what will happen. Have got tired with this tricky problem.

Today there is a bad news, I checked another same model car of my friend, when his engine was warming up, the water drips at the tailpipe is also slightly fluorescent when under uv light, wtf, I got crazy. His car is not losing coolant at all. Lost an important evidence. But I found there are many things different between his car and my car. Firstly, his car has much less and thinner damp smoke than mine after cold start, and there is completely no condensation on the underside of oil cap. Moreover, I felt his engine is running more smoothly when I put my hand on the car. Besides, a much higher pressure can be built up in his coolant system, the pressure meter on his car showed 8.5 psi after engine was completely hot (thermostat opened). For my car, the pressure just stayed at 4.5 psi after engine was hot and thermostat opened, and then it needed to take 30-40 more minutes to slowly get to 8 psi with engine idling. So I think the low pressure may be caused by a coolant leak during engine warming up. Now these are all the symptoms which seem still not obvious enough for a blown head gasket or engine crack. So confused, maybe have to remove the head?

That is true. I just noticed the damp smoke (more like steam) from my car is obviously more than many other cars.

The dealer said they checked the spark plugs, but I maybe they didn't checked all of them. Or the coolant is not leaking into combustion chambers but straight going to the exhaust passage.

So if the coolant is not present in the cylinder chambers, is the head probably cracked that causes the coolant straight going to exhaust passage and being burned?

I put the uv dye in and still couldn't find any leak by using uv torch. But during the engine was warming up, I found the water drips at the tailpipe are with slight UV sensitive. The water looks like just normal clear water without any color, but looks slightly fluorescent under the uv light, but it is not as green and bright as the fresh coolant under uv light. And this disappeared after smoke and drips stoped coming out and tailpipe became dry. If there should be nothing else with UV sensitive in the car, I think it must be coolant which was burned to vapor and lost its color first but still coming out with a very slight UV sensitive. What do you think?

Today I noticed that no matter cold start or hot start, no matter start in the early morning or sunny afternoon, much damp white smoke and few drips always appear at the tailpipe after engine start and till engine completely warms up. The debate about stop leak products is always there, I would try if my car is very old, but it has done just 100,000km.