250 six overheating

[Big D]

It must be the time of the year for overheating!

This weekend I replaced the intake/exhaust gasket on my 70 GMC w/250 six. everything went fine and the exhaust leak was fixed, but now the truck pukes out antifreeze after being driven for around 7 miles. I drove it 15 miles on friday and it used about a half gallon of water, but no temp light and the motor didnt seem that hot.

Today I replaced the thermostat and radiator cap, NO change. The radiator hoses arent colapsing, I pulled a couple a plugs and they dont look green.
about the only thing I have not tried yet is a water pump.

Could it be running too lean? I drove it all last week in warm weather without a problem, and drove it home 140 miles and it it loosing any antifreeze.


Any ideas?

['68OrangeSunshine]

How high is your water/anti-freeze ratio? I had to thin my mixture out and lowered my temps in a 292 that way. It was fresh built and just off break-in. I was running about 70% prestone and was indicating 220-230*. Above boiling but not in a closed system. I changed my t-stat out to 195*. Now it's running about 180* -195*. I have 3-tier V8 style radiator. V8 radiators are about 6" wider and have more cooling area than the original radiators GM put in L6 equipped trucks.
You might try the prestone engine flush kit if you haven't done this in a while.