The motor is a D16Z6. I noticed that increasing amounts of coolant are being overflowed in to the reservoir to the point that the reservoir is overflowing after about an hour on the highway.

Compression check showed 175psi in all four cylinders. Cooling system pressure check held 16psi and the cap was replaced with a new OEM part since the old one tested bad. The problem still persists.

Is it likely that the head gasket is still leaking between the cylinder and cooling system even with good compression numbers? No leak down was done.

 

I have about the same problem. I did have a leak down test and it check out ok. The shop is still trying to figure out the problem.
https://honda-tech.com/zerothread?id=1859381

 

Intake manifold gasket maybe? There's a coolant passage that runs through the head by the #4 cylinder and into the IM.

 

easy way to check, is if you have a coolant pressure tester, put it on before you crank the car, and when you crank it watch the gauge, if it goes up at all when the car is car, then you know its the headgasket.

 

FYI, yeah it was the head gasket. All fixed and healthy now.