What is wrong with my car. A cooling system and heater question

Okay, I have a '91 Mazda 323 that has been acting very funny lately. One day I heard a hiss from under the hood after parking and took a look. The radiator has a plastic screw in plug at the top that has a crack in it. It has a pin hole leak that is spitting steam, so I took some pliers and tightened it up a little and the leak, while still there, is much smaller than it was before. The liquid level in the radiator is fine, and I have been checking it often.

The car has also been running hot lately. When sitting at a light, the temp dial will raise up to the 80% mark. When moving, it cools down, but it seems to be pretty much random while moving. The other day it seemed to run hot going down hill, then cool off going up hill, but I did not have enough time to experiment with it.

Also, the heater only works when the car is moving. To me, that makes no sense, but it is the truth. I can crank the heater to full blast and it only blows air that is slightly warmer than outside. Then once I start moving, it starts to roast me toes.

So, do any of you know what is up with the car, or at least have an idea? Could a pinhole leak in the radiator plug cause what I am seeing?

Thanks

Unfortunately, I’ve had more experience with cooling system problems than I care to.

If your heater is working on and off and your temperature gauge is moving up and down, I suspect you have air in your cooling system. A big enough air bubble down deep in the system somewhere, and your radiator coolant level could continue to look full.

Now, as to WHY you’d have an air bubble in what’s normally a closed system, I’ve had it turn out to be from several different causes, ranging from annoying but easily fixable all the way through “HOW much did you say it would cost???”

if you have a “small” leak in your radiator, you may have a bigger problem than you may think. your cooling system is pressurized, meaning its a closed system that relies on a certiain pressure to attain a certian temperature. if you let pressure out of that system, the system cannot operate properly, sort if like letting the steam out of a pressure cooker.
There are several components to your cooling system that can break down and cause overheating problems. You need to have a qualified mechanic check your cooling system possisbly in this order (from cheapest to most expensive): thermostat, heater hoses, freeze plugs, water pump, heater core, head gasket. Your problem should be very simple to diagnose if you can find a good mechanic.

The crack mght have formed from too much pressure in the system as well. So there.

I’m thinking circulation problem which would most likely be caused by either a thermostat or a water pump. I’m suspecting the latter (because it’s more expensive and it’s not my car) because if the pump is failing, it could be requiring a higher RPM to make it move juice. This would explain your heater problem (no fresh hot water circulating in when at idle) as well as the counterintuitive cooling while under load (uphill is usually going to be in a lower gear, and thus at a higher RPM)

Usually running hot while sitting still is due to the radiator fan not operating.

When you say you only get heat while moving, if that actually means when the engine speed is higher than idle, then the most likely cause is an air pocket, as kunilou suggested. It’s conceivable that the air pocket is causing the rest of the symptoms you describe (other than the leak) as well. The leak you saw on top of the radiator might be the root cause (air could have been sucked in through it), although alternatively it might be a result of overheating. Cooling systems can be tricky to diagnose because many symptoms are like that, in that they could either be the root cause or a result of the problems effected by the root cause.

It’s unusual, but not impossible, to see coolant up to the top of the radiator when there’s a significant air pocket in the system. It’s important to check the coolant level IN THE RADIATOR (cold engine only!), NOT IN THE OVERFLOW JAR. Many leaks will suck air into the system instead of sucking liquid out of the overflow jar, so you can have an engine and radiator dangerously low while the jar is full.

As you may know, if the engine severely overheats, it’s likely to cause expensive damage. In cases where there’s a big air pocket in the cooling system, the temperature gauge may not always give a reliable reading. Towing it to a repair shop is cheaper than replacing a head gasket damaged by driving it and letting the engine overheat.