Hi guys, I really have read a lot of posts before writing this topic, read a lot of information and tried a lot of stuff said here, but the problem seems to come back in a few.

I own a '99 Civic Sedan, D16Y7 engine, all stock. Last November I unfortunately had a broken timing belt, sent the car to a known mechanic, the mechanic sent the whole car to the machining shop, where they disassembled the head, changed all wrecked valves and stuff, machined the head, changed the head gasket, checked the block surface for warping (they said it was good), assembled and tested. So I paid for the service and got my car back again.

All seems fine for at least 6 months, idling OK, no overheating, no coolant leaks, heater working fine, rad fan kicking in at right temperature (measured with infrared thermometer at thermostat housing when hot), when I noticed the level of expansion reservoir dropping a little bit (1 inch in 2 weeks) and noted a little spill of coolant in the engine bay. I found out I will need to change 2 hoses (heater out and bypass, still OEM, very worn bc of the age of the car). Ordered OEM hoses by internet and topped the reservoir when needed until the hoses arrive.

By the beginning of June (when temperatures get much colder where I live) I noticed that, when at the highway at 60-70 mph, the temp gauge descends and stays there for all the ride, so when I step off the highway and stop at a traffic jam/signal, the temp gauge goes to the middle and stays there. I thought "there is no thermostat in this engine". So I ordered an OEM new thermostat. On a weekend I did all the job: drained coolant, changed hoses, installed thermostat as per service manual, refilled coolant, did the burping with a spill-free funnel until there were no bubbles coming, topped rad and reservoir with coolant, installed rad cap and went to the road.

Temp seems fine at all, but some days later when I was checking the level of the fluids (since I overheated another engine I am paranoid with fluids) I noticed that the level on the expansion reservoir - with cold engine - have RISEN instead of dropping. Opened the rad cap and there were no coolant up to the top. Fill the rad up to the neck and did the whole burping procedure again. Worked fine but 5 days later, same issue. No sign of smell of combustion gases in the reservoir or in the spill-free funnel when burping the system.

I have read some users commenting that this is normal behavior of the engine (draws some coolant to the reservoir and sucks back later), and other users commenting that this is NOT normal behavior (rad MUST be filled to the top IN ALL CONDITIONS). Maybe the machining shop did not do its work well, leaving some "minor leakage" in the head gasket that builds air bubbles noticeable only within a week? Is it really normal behavior?

If some could give me technical advice on this it will be greatly appreciated, this question is really freaking me out.

Thanks in advance.

 

if it were me, I would install a new radiator cap and coolant temp sender (signal to the gauge). you can pressure test the cooling system also. testing kits aren't too expensive if you want to DIY, or have a shop do it for you.

 

If replacing the radiator cap provides no joy, then next rent the AutoZone block tester kit to check for exhaust gases in the cooling system.

 

When you replaced the hoses did you bleed the system?