90 Toyota P/U RE...here is the issue...engine over heats when driving, then cools down, then thermostat cycles hot to cool to hot etc.. Puddle of antifreeze under truck when stopped. New radiator, new pump, no antifreeeze moisture in the exhaust (head gasket is fairly recent) head was surfaced when new gasket was installed, new thermostat, no water in oil. Question, can this head crack between exhaust port and water jacket? It appears air is getting into coolant system (hot gases) from exhaust. Takes about 3 weeks and coolant is low,low,low. Thoughts?

 

Which thermostat are you using? Are you sure you are getting all of the air out of the system when refilling? How did you determine exhaust getting into coolant,i.e.bubbles? What brand of head gasket was used?

I guess head could crack and allow it, but seems that if your head was resurfaced at a machine shop it would have been checked for cracks. Could just as easily be a head gasket issue.

 

where's the leak coming from? maybe new radiator cap? is your thermostat installed backwards?

 

Ive found that non dealer t-stats never work well. I just went through the ringer with mine and it was over heating the truck new out of the box and it built so much pressure it was pushing antifreeze past a lot of connections. bought one new from the dealer for $19 including a new gasket and that solved my problems

 

Whenever there is cooling issues, always your first step is to pressure test the system. It will tell you:

A. If the system maintains pressure
B. Will show you where the leak is.

Go from there...