Search the Community

Every day driver is a 1941 GMC truck. We call it a little mule and it has been turning over and running as good as it probably did the day it was pulled off the lot 80 years ago. Only swap out was the flathead 6 for a 71 350SB (done by prior owner), and 6 to 12 volt system. The issue: Overheats (never use to) Details: Warms up nicely to 180° within 16 to 20 minutes - with the electric fan running during idle. At roughly 25 minutes weather pressing the excelerator to force some more coolant into the block or hitting the road and driving, it creeps up to 190 (we know 190 to 200 is often said as a healthier engine temperature BUT it is always ran at 180 and never went beyond that until recently) my 25-30min. Prior to 35 minutes when driving or tapping the accelerator when at idle the temperature gauge will drop down to 180, then back up to 190, then back to 185 when tapping excelerator, then up to 195, and then after 35 to 40 minutes tops it slowly continues to rise to 200, 220 and keeps going up beyond 230 (we have course back off and work on calling it down once it starts creeping beyond 200). What we have done during diagnosis: Replaced hoses, replaced thermostat, drilled two small holes inside of thermostat before installing to allow ongoing circulation (the old thermostat had this done as well by prior owner), replaced radiator cap, replaced water pump, flushed radiator and block with radiator flush (twice), burped of course after replenishing coolant, and remedied any minor leaks. When running top coolant hose is hot as it should be. Bottom coolant hose is very warm but not hot (as I believe it should be given it personally called fluid into the water pump and block). As we see the temperature gauge start to climb above 190° we realize that the bottom coolant hose seems to be almost as hot (based on from touch) as the top coolant hose. ***Is this a possible sign that the radiator is not cooling the fluid well enough once reaching higher temperatures? Signs: No white smoke is coming out of tail pipes, motor turns over nicely at cold start, no traditional signs of a blown head gasket (although we’re not narrowing out anything at this point), not seeing any leaks in the colon system. The 350 just doesn’t want to stay below 190 as mentioned above. Where the trouble started: Was on the road on a hot day and overheated. We’ve been in these conditions before and it stayed strong at 185. When we pulled over and checked under the hood the lower coolant line popped off the bottom of the radiator. Not sure if that was just due to a loose bracket, for too much pressure built up somewhere in the corn system due to blockage and forced it off. At this point we allowed at the cool, reconnected the lower coolant hose, checked all other hoses and fittings, refilled with coolant, burped, and everything was running nicely for about two weeks; thereafter the overheating begin we begin our attempted diagnosis in changing out basic parts to eliminate possibilities. Many thanks, Jason and the AOA team