head gasket concern

in first glance it looks fine, ive cut the tip in an angle so it wont suck against the walls.
how do i realy check the integrity of the connections?
ive put some zipties here and there, dont know if its a fair seal

This sounds like my experience of head gasket failure. When you put the replacement engine in did you change it's head gasket or just assume it was OK?

well i didnt, but i worked her couple of trips and was fine. started acting up mid june whenwe got some heat waves.
the most annoying part is that i just cant get the co2 test to change color. the previous engine was 100% positive for the test. current one gives me hard time. feels like a new engine is in order. got pretty hard to come by a good m13a, would be nice to put in a m15a but for insurence reasons might be not recommended where iam from.

Or replace the head gasket it might not be as cheap but less work.

If the system isn't staying pressurised and you can't get a result for combustion gases then it might not be the head gasket.

I'd take another look at the radiator cap. Some aftermarket caps aren't correct.

There are 3 seals in the radiator cap and two one way valves.

1) the top seal. This seals against the very outside flange/rim of the radiator, if this doesn't seal then coolant can weep out under the cap, and worse air gets drawn back into the radiator in preference to coolant from the expansion tank.

2) the pressure valve. This seals against the lowest flange/rim at the bottom of the radiator neck.  When the coolant warms up it expands increasing the pressure until this seal lifts up against the spring and allows coolant out of the radiator and into the pipe to the expansion tank. As the pressure falls it snaps shut again, so the coolant stays pressurised, which raises the boiling point. If it leaks, too much coolant goes into the tank and it will boil at a lower temperature, most likely the temperature gauge will read slightly low.

3) vacuum relief/non return valve. As the engine cools, the coolant contracts causing a negative pressure, which will flatten the hoses. External air pressure on the coolant in the expansion tank forces it back into the radiator through this valve. AFAIK the plastic part floats on the coolant and seals until the coolant starts to pressurise and keeps the valve closed, or there may be a light spring keeping it up against the seal, mine just dangled loose and it sealed and worked just fine.

If the head gasket weeps, gases from the cylinders get pumped into the coolant under pressure, so when the engine is cold there is a pocket of pressurised gas in the radiator that keeps valve 3 closed, coolant stays in the expansion tank, and you hear a Pssssht! when you undo the radiator cap as the gas escapes.
If there is a big enough leak at the gasket, or a cracked head this gas forces water back inside the cylinders and you don't get any residual pressure. If things are that bad it will blow large bubbles in the coolant, use loads of water (probably excessive white "smoke" from the exhaust) and the compression will be down on the affected cylinder(s)