we did two on mine one at the house then took it to garage just to make sure we had not made a misstake both came back the same could not make it out we all thougt the same as you are saying it would show up
have just had similar problem with overheating & rock hard hoses.have had to replace h/gaskets ,both heads pressure tested and skimmed,new stat fitted,now seems to be all ok. 4.0 engine with 124000 on clock