Hi I have a 91 240dl that is overheating and I think losing Coolant somewhere. I read some where that you can tell if you need to change the thermostat by the top radiator hose being cold and the bottom being hot. Is this correct? I ran the car yesterday for about 7 min and the bottom hose was very hot and the top was still cold. Also, I had no coolant in the Reservor tank and no time. Do I need to add coolant also. I just need to know if I need to change Thermnostat? Thanks

With no coolant in the tank, how do you know how much coolant is in the engine? It might just be low on coolant. Start with that. If the car is using coolant, there has to be a leak somewhere or evidence that the coolant is getting into the oil or burning out. Check the oil. If it's foamy or whitish in color, you might have a head gasket problem. If the car is blowing white "smoke" out the tailpipe, again, it could be a head gasket. If the thermostat is stuck closed, then yes, the top hose will be cool and the bottom will be hot.

First thing is to add coolant.

Next pull the dip stick and make sure the oil is not milky or foamy. That is as already posted a sign of a bad head gasket. Do not drive the car is that is what you find.

A stuck thermostat will cause an engine to overheat in a hurry. But so will an engine that is low on coolant. After you top off the tank you can pretty quickly sort that out.

Start the car and get a flashlight and check for water leaks as the engine warms up. A bad hose, cracked or leaky radiator, busted heater hose, water pump gasket, leaky heater core, or leaky head gasket would be the things to look for. Check and see if the needle has moved and as it does feel the top hose, if this is still cold and the needle has started to move that would point to a bad thermostat. If the temp starts to soar cut the engine off. That is an Al Head and excess heat will get you started on that head gasket job sooner that you should have to.

You want to order a gasket or make sure there is one in the box before you start replacing a thermostat. Replace both hoses and do a complete new fluid refill if you do have to go into the system. I replace hoses and belts every two years and do a complete drain and refill also. They are cheap and what happens if those fail is most of the time a bit more on the pricy side and tend to fail at the worst possible time.

If you have a leak from the heater core inside the car a sweet smell when you kick the heater on is one of the signs. Wet carpet up front is another one.

Leaks on the backside of the engine would point to the heater hoses that run from the back of the water pump to the passengers side inlets, and back to the engine block.

Leaks in the front of the engine would be water pump gasket, water hoses, thermostat gasket, or the thermostat housing is eaten up.

All else fails take the car down and let a pro pressure test the system. They have a system when air is pump air is used to raise the internal pressure. The leak will often show right up. For the tricky ones they add some UV and break out the UV googles and light.

Good Luck,

Paul

Thanks Jim and Paul. I did notice the smell of Coolant when I turned on the heater yesterday. Do you think I could replace the hoses to the heater if this is the case?

You need to sort out if this is the connectors or the heater core itself before you know what to replace. Time for a flashlight and pulling back some carpet.

Art has a good write up on cleanflametrap. I think this makes the Blower Motor look like walk in the park just looking at it. But that is a whole lot cheaper than what pro is going to charge.

Good Luck