Help needed coolant issues

I have a 98 mitsubishi triton v6 3.0, I am confused with a issue I am having, coolant heats up and is put into reservoir tank as normal, but when car cools down it does not suck it back into radiator.

Have purchased new radiator cap
No coolant on ground or any visible leaks in engine bay
No white smoke when starting car or running
No sweet smell of coolant burning
No leaks inside heater
Car doesn’t overheat, just pushes more coolant Into reservoir
Slight bubbles in radiator when cap off
No coolant in oil
Put a bottle of radiator leak fix in, no improvement,

Only thing is that 3 out of 5 intake manifold bolts are stuffed, they grab but don’t get a tight fit, could this cause the vacuum leak not returning the coolant to radiator

After you let it fully cool off, and take the cap off (presuming you can see into the radiator), is the fluid level in there down? If it is, but hasn’t sucked fluid back in, maybe there is some kind of a leak in the overflow hose…???

Possible head gasket failure comes to mind.

Just been to the radiator specialist, he’s booked me in for more test as the block test didn’t show a head gasket issue

So the hot coolant moves into the bottle when the engine is hot, but it doesn’t go back into the radiator as the engine cools? Just curious, what caused you to discover this?

Bubbles in the coolant when looking into the radiator could mean a head gasket leak. That would be exhaust gas getting into the coolant causing the bubbles. Since exhaust gas is mostly CO2, and CO2 combines with water to form carbonic acid, it seems like if you measured the pH of the coolant, that might provide a clue. If it is abnormally acidic, that could mean exhaust gas in the coolant as a result of a head gasket leak.

But it’s possible for small bubbles to appear in the coolant in the radiator if there’s some trapped or dissolved air in the coolant, like if you’ve recently added new coolant say. But then the coolant wouldn’t measure acidic either. So that’s the way to tell.

@melott 's comment about a possible leak in the overflow hose is a good one. Have you ever tried to suck a soda straw for a sip of coca-cola only to discover you can’t b/c there’s leak in the straw? Same thing, that hose has to be leak free for it to be able to draw the coolant back into the radiator. So ask your shop to test for that, and that all the connections are open. They could use one of those hand-held vacuum pump to easily test for that. On some cooling systems the overflow tank is designed to be pressurized too. So if your overflow tank developed an air leak, maybe this could result, not sure, but worth asking your shop about. Best of luck.

“as the block test didn’t show a head gasket issue”

You naysayers need to step off now

There have been some among you that said the term block test was not correct

OP just used the term, and he doesn’t even know me

That should prove it’s common and correct usage