1995 MX3. After replacing two thermostats, the symptoms remain the same:

1. If leave the car at idle for 15 minutes or so, the water temp goes up into the normal range nicely, and the air blowing inside is toasty warm.

2. As soon as I start moving and air starts flowing around the engine, the water temp drops way below normal and the air temp blowing inside drops to a lukewarm state--sometimes more cold than warm the outside temp.

Suggestions?

 

I've been trying to figure this one out. The cooling system is quite simple. I think the water pump may be bad or you have a major air bubble in the system.

 

Water pump was replaced 6 months ago. The problem was still there at that time.

How to get rid of air bubbles if they exist? Do I run the car for a while without the radiator cap on?

Some suggest a plugged heater core. However, I don't think i would be getting any heat at idle if that was the case.

 

First: When the radiator and heat don't work but the engine is running seemingly normal than you have a overheating problem: the block and head are getting hot nad the heat is not transported away. That's why i thought the pump may be failing. That's not likey now that yo tell me it has benn replaced recently.

It is more likey there is an airlock. Watch this video:

If your heater core is clogged try this method:
Wow!!
I thought I would pass this along, seeing as how there are quite a few people that are having problems with their heaters in there Metros.
Last week I had finally come to the conclusion that I needed to pull my heater core and replace it, but low and behold, I mentioned it at the junkyard that I frequent quite often, and I was given a suggestion before spending the 4-8 hour job of pulling the dash to do it.
I still can't believe it was this easy:
Clamp the 2 hoses that come from the heater core on the firewall,(so you don't lose coolant) and then pull the hoses off.
Here's the Easy part, use a pressure sprayer to unclog!!! Yup, it's that easy!! Spray full pressure into the right side tube first,(as your facing the firewall from the front of the car) as that is the outlet of the heater core. You may be suprised how much crap comes out of the core, I ended up sparaying in both tubes to get it cleaned out completely.
I am absolutely astounded that it was that easy, and it took a grand total of 5 minutes to do it.
I went from absolutely NO heat, to the hottest heat I have ever had in a Metro!!
Now I realize not everyone has a pressure sprayer at home, so my suggestion is, try your local car wash.

Hope this helps someone, as it saved me the depressing job of pulling the dash, and purchasing a new heater core.

Enjoy, and let there be heat!!!!!

 

I did what you suggested and bleed the system. I removed the radiator cap and added coolant until it was full; not much more than a half quart. I turned on the car and let it idle for a while. (There were a few tiny bubbles that surfaced but as the engine warmed up, they went away. As noted before, the temperature gauge went up to the normal range. Air in the cabin was nice and warm--though not blazing hot like my other cars when the engine is at operating temperature.

I moved the temperature selection dial in the car from cold to hot and the cabin air temperature fluctuated accordingly from very cold (it's 7 degrees F outside) to quite warm--again not hot. I felt the radiator hoses, and they were hot to the touch. I felt the heater core hoses at the firewall and they felt quite warm.

So I took the car for a test drive. Within approximately 1/4 of a mile, the coolant temperature indicator went from normal operating temperature to bottoming out in the cold range; cabin air blowing was cool as well with the temperature selector all the way to hot. When I got home--with the car running--I opened the hood and felt the radiator and heater core hoses and all were lukewarm to the touch.

Is it possible that the second new thermostat is bad? I'm really stumped on this one.

 

Half a qt in the radiator is displacing a significant amount of air. See if there is again an air space in the radiator. If so look for a leak.

 

How about a new rad cap to bring the pressure back up for better heating.
You were down a bit in coolant?
It's a cheap try and may be the solution.

 

I do think he has a leak. After refilling the radiator things seemed to work. After running a while the old symptoms returned.
My wife's Contour had a coolant leak somewhere. I was to lazy to find out where exactly and I used
and it is working fine for now. When the leak returns I will take it to the shop. Coolant mix is so *^%$*^%$ expensive! And we recently had the system serviced.