Hello guys.
I have swapped a D16Z6 engine in my EK Civic 2 months ago. There are two very strange issues after the swap. Coolant level drops ~50ml every ~500km and the temperature gauge's level stays just a little bit above 1/3 when idling. When I accelerate and keep the rpms above or on 3k rpm, the gauge moves upwards very little, maybe about 1-2 millimeters. The gauge is always in 1/3 and 1/2 area. When I let off the gas pedal, it drops to the idling level - a little bit above the 1/3.
There are not visible coolant leaks or white smoke. The radiator pipes are soft and the radiator fan works. I have also changed the water pump.
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There could be the smallest of leaks at a hose connection, or in a hose, which leaks when operating temperature / pressure is reached. How old are the hoses / clamps?

 

They are the original ones. We've checked with my friend when idling but as you said maybe it leaks only on pressure.
What about the temperature? Is it normal to go up and down just a little bit? With the D14 engine, it was steady I think.
Could it be a bad head gasket? And if it's a bad head gasket, won't there be more symptoms?

 

The gauge reading could vary from sensor to sensor. My original D15B7 would go up to the bottom of the little thermometer symbol, while driving. When I had a rebuilt engine installed, the dial would read slightly above that, but never had moved more than a needle width up. For a head gasket leak, first sign would be a little white exhaust, which would take a while to stop, as things heat up and seal the head. I had that just before my original engine turned into an honorary steam engine. Does your overflow coolant tank leak at the cap? does the hose from the radiator to the cap leak? I put clamps on both ends of mine. How new is the radiator cap? How old is the radiator? Is the floor damp on the passenger's side from a possible leaky heater core.

 

It doesn't. It is the original one.
It doesn't when idling. I also have clamps on both ends.
We've checked the heater core and there aren't visible leaks. We are gonna check again just to be sure.
The radiator is original. But as I said the coolant wasn't disappearing with D14 engine with the same radiator. I've noticed that the small hose from radiator to the overflow tank is hot. I think it doesn't have to be like that, right? I am gonna change the radiator cap Tuesday and I will update the thread. If it continues to disappear after the change, I am gonna do the compression test and test the radiator for exhaust gasses.

 

The overflow feed line could be normally hot, excess coolant at operating temperature goes there. In my experience, Honda radiators only seem to last 5-8 years, before developing a leak, where the plastic tops are connected to the main body.

 

I totally forgot about this thread. I've changed the radiator cap with a 0.9 bar one but it is still losing coolant. Me and my friend did a combustion leak test from the radiator and everything was fine. Is it possible that something is wrong with the engine after this test? And what else is left to look at? Could it be a bad thermostat? I am assuming it is not a radiator issue because everything was fine just before the engine swap.

 

A bad thermostat is possible to cause your problem I would think. If it's sticking your not getting that steady flow of heated coolant to circulate causing the temp sensor to give you slight fluctuations on the needle reading. Also make sure the system has been fully bled of air.
For what it costs and how easy it is to change the thermostat, I would do it to eliminate its possibility of your problem and just for reassurance. Make sure it's installed with the jiggle pin at the 12 o'clock position!
One last thing I think that might help for longevity is to add a rad flush treatment to your coolant, let it circulate a couple heat cycles to help de-scale the system and then do a full flush with new coolant before changing the thermostat.

 

We finally solved the problem. It was a small radiator leak. We've changed the thermostat and the radiator a month ago and coolant loss stopped. Thanks @95coupeRI and @Maxcapacity.