could i still have a blown head gasket

[00RangerShavedAndDropped]

hey everyone i recently bought a 00 ranger with a 3.0 in it guy said had blown head gasket so i drove it home it didnt overheat or anything today i got the block test thats supposed to test for blown head gaskets and i took the cap off made sure the coolant was low and after it started to warm up coolant started to overflow top of radiator waited untill thermo opened and sat there with the leak test on it for at least 5 mins and the blue liquid never turned color. on the tester it says shouldnt take longer than 2 mins. anything else i could do to check for blown head gasket idk why the guy sold it to me saying it had one when i dont see a problem yet. last thing is there is a knock sound every so often i get in it either when turning on or off didnt know if that would have anything to do with it thanks again guys

 

[adsm08]

You did pump the bulb to pull the gas up in, and didn't just let it sit there, right?

 

[RonD]

So overflowing coolant before t-stat opens could be a sign of a blocked by-pass.

Not sure on the 2000 3.0l
But all cooling systems work pretty much the same way.
The water pump pulls water out of the lower rad hose pushes it thru the engine and then out the upper rad hose via the t-stat.
When t-stat is closed the water pump can't pull coolant out of the lower hose because there is to much suction pressure because upper hose is blocked by t-stat.

So there is a by-pass system, usually part of the heater system, water pump pushes the water thru the engine, when it hits the upper rad hose(closed t-stat) there will be a smaller "heater" hose that the coolant can flow thru instead, this allows coolant to flow though the by-pass and then to another smaller hose that goes to the water pump, so circulation is complete.

Some vehicles use the heater core as the by-pass, my '94 4.0l is this way.
Some have a by-pass valve, 4 smaller hoses attached, usually near the firewall heater connection.

If by-pass is blocked or partially blocked then that could cause coolant to be push backwards into the lower rad hose causing the rad to overflow if cap was off.

 

[00RangerShavedAndDropped]

yeah i pumped the bulb had to wait till truck heated up fully cause before that i kept sucking in coolant as the coolant level kept rising and overflowing out the radiator. after it ioened and the coolant level dropped i kept pumping the bulb watching the blue liquid bubble with every pump and after 5 mins or more it still didnt turn colors. could the fact that the radiator kept oerflowing be a problem?

 

[00RangerShavedAndDropped]

and i get a weird almost knock sound when i start the truck sometimes.it only knocks about once or twice then nothing sounds fine lasts for less than a second. and sometimes when turnong off it will do the same thing for a second or two as the motor turns off