can a bad heater core cause a car to overheat? even in the spring?

The heater is just a small radiator, in parallel with the main radiator. You have a problem elsewhere, and the heater core when working removed enough heat to keep things under control. Once the heater core no longer removed heat, THEN the system was overloaded. It means you have a problem with other parts of the main cooling system which you did not see while the heat removal by the heater masked the problem, and then when it no longer removed heat, resulted the overheating of the engine. A heater by itself has NO effect on the operating of the engine other than to take some heat and shunt it into the passenger compartment for your comfort.

Your heater core will still leak even though the blower motor or temperature selector switch is not on. When ever your engine is on, the water-pump circulates the coolant through the engine AND heater core before the thermostat cracks open regardless where the dash switches are adjusted. There is an easy trick you can do in the mean-time. Any NAPA or Carquest has a heater-core jumper adapter fitting for under five bucks. Its a 5/8 -3/4 plastic adapter that joins the two heater core hoses at the firewall. All you need are two worm-drive stainless steel hose clamps and your done! The only problem is you will have no heat inside the car. Goofy cooling system sealers do not fix leaky heater cores.

Think of the heater core as a small radiator because that's what it is. The hot coolant flows through the inside of it while the fan blows air through it and into the passenger compartment. The only way it could contribute to overheating would be if it was leaking causing a loss of coolant. This should be easy to spot since there's going to be coolant either under the car or there will be coolant on the floorboards when the engine is running and the heater is turned on. The usual tip-off to a bad heater core is the smell, the slightly sweet odor of hot coolant and sometimes the lack of heat when its turned on.

Your problem could be a bad water pump, or a clog somewhere in the coolant system. Have you tried flushing the system with a cleaner?

could be radiator, water pump, and the heater core, is there any leaking? or do you replace the antifreeze often? could also be the head gasket. Its best to have someone look at this, I have a 99 that does the same thing and found out its over 1k to fix in a shop. Mine is the head gasket. If yo do not have the $ to fix like me, I put in a leak stop it was like 30$ and i really haven't had too many problems for now ( knock on wood!!) But I know it isn't a permanent fix only temporary.

I don't know about the heater core, but when my old car was doing that I had to replace the thermostat about 5 times before I got a good one.

Good luck!!! I remember too well what that one feels like.

Causes Of Vehicle Overheating

not likely not but a dirty radiator would a loose belt a bad hose a bad fan heater core is not a have to have thing car can and wil run without heat remove radfiator take to a radiator shop have it dip or boiled out wil be like new again

car overheats, first by lack of water, if radiator is full all the time, you're answer is no, though heater core leaking, you can maybe by-pass it so it doesn't leak

Not likely...bad heater cores usually either leak, or are plugged delivering no heat.

just got 1990 ford 150 lariat overheats, previous owner just changed thermostat, fan, no water in oil, idles very high in park, water looks to be circulating when looking in overflow? what could the problem be?