I'm stumped and hoping for the best. Suburban is over heating immediately upon warming up. Replaced the thermostat. Thermostat works, I've tested it. No oil in the water and no water in the oil. Still overheats with new thermostat. To be sure I've tested with 3 brand new thermostats. Still over heats. Here's the kicker, when I bleed the water thru the system I get massive bubbling in the reservoir causing the water to spill out. The thermostat seems to be opening although there also seems to be low flow thru the lower hose. My mechanic friend hasn't seen this before and is looking into it. He did suggest that there could be a leak in the head compressing air and exhaust thru the line hence the massive bubbling. I know there is a test that can determine if the coolant has exhaust mixed in it but mechanic is not quite ready to run that test. We know it could be the water pump &/or the radiator, but we jusy dont see that very often with this engine. Has anyone across this before?

 

Bubbling in coolant is exhaust getting into the coolant. Probably needs head gaskets.