We just put a water pump and thermostat in a 2008 bmw 328 i,.....took for a drive yesterday about 10miles ...all good.,,,let set overnight and no leaks...put the shield back on underneath and took up on interstate and it got hot coming back...light came on luckily not far from home....light went off as pulled under car port....radiator was hissing....we left it set a while and my husband loosed the cap and bit and of course anti freeze spewed all over.,...tightened it back up a bit....letting it cool now ....what could cause the overheating? someone said something about bleeding the system?