89 formula. Car was originally TBI but was converted to carb. Having a problem with the engine temp regulating. When drivin around the city and at lower speeds engine gets up to temp and the heat works. As soon as I get on the highway the gauge drops way down and it blows cold air!! Have tried everything. Car runs great. Just replaced the stat and problem still exists. Is it possible the coolant temp sensor is bad? I thought from reading all the posts that the ecm is disabled when changing to carb. Do I need to unplug it under the dash? Like I said car runs mint and everything works as it should. Confused!!

 

Your temp gauge has nothing to do with your heating problem. The temp gauge can be disconnected and you will have heat.

I would start by checking for air bubbles in your cooling system. Doesn't sound like you have that problem since you get heat when not on the highway. But it is something to check.

Another thing to check is your vacuum. The control valve that routes coolant through the heater core as well as the flaps the divert air through the heater core are vacuum powered. If you are having vacuum issues that may cause issues.

 

I guess the other possibility is that your new thermostat is defective and is stuck open. This has happened to me before with Autozone thermostats. When you first start the car check to see if you are circulating coolant through the top radiator hose that goes into the thermostat housing. This should not happen until the thermostat opens at the proper temperature.

 

Mine didnt heat well for a while, got tired of the cold air after replacing everything with the cooling system, ended up being a crappy old heater core, new one allows the fan to blow some nice heat

 

How do I check vacuum? Just took the car for an hour test ride. Sitting at an idle heat blows super warm. As soon as I hit the highway it slowly starts to get colder and colder until its plain cold!! Then as I come off the highway it takes about 10 minutes of drivin to warm up again! When I change the heat controls from vent to heat to defrost and so on it does as it should. Really getting agrivated with this issue. Even when the heat is blowin cold all the heater hoses are pipin hot! Getting ready to burn the car to the ground, someone please help!! Have the hvac vacuum line goin to the manifold, should it be goin to the carb?

 

You can get a vacuum gauge from any auto parts store. Just pop a vacuum line off the manifold and hook it up. Start the engine and read the gauge.

It really sounds like your thermostat is stuck open.

 

If the stat was stuck open, would take a while to warm up right? The heat will work fine for about 20 minutes of city drivin then starts to get cold. Hit the highway and its a sure bet I only have about 2 minutes of heat left. Took off the heater core box cover and all the doors are moving as they should. Maybe I am losing vacuum after a while and the doors are starting to move but even so I still should have heat coming out somewhere because the heater core is hot and so is every hose!!! Never been this stumped on any vehicle I have ever worked on. If the heater core was clogged I cant see how I would have heat for a while, then none at all. Car has been changed from tbi to carb. ECM still connected. Possibly that the case? Someone please save all my neighbors from FORMULA 350 body parts all over!!!!!!!!!!!

 

Ok fixed it!!!!!!!!!! Anyone having this problem,make sure the heater hoses are in the right spot. When the previous owner changed to carb from tbi the hoses were not installed right. Here is how it needs to be... The heater hose needs to come from the intake and not the water pump. Mine was coming from the pump so as soon as I started drivin the water was being cooled even as it went through the core. Plugged the line at the top of the pump and ran it to the intake,now the car gets to 210 and stays there for the most part. Heat is hotter than hell and stays hot!!