So I rebuilt the upper part of the my wife's 95 Blazer. In hopes to save some money in this economy, I used some marine heads that had only a few hours on them. I had them magna-fluxed prior to replacement, and also asked for several opinions about using the marine heads as to if there would be any complications in using them (besides not having a temperature sensor hookup). So here I am with the truck at the shop. The truck has no heat whatsoever. Well, I guess it does if you really spike the rpms. Prior to taking the truck to the shop, I did replace the heater core and thermostat. My friend at the shop also replaced the water pump. Anyone out there have any suggestions or theories at to what may be wrong? Thanks....any help is greatly appreciated.