My Neon runs cold longer then usual and throws a P0128 code. As the car warms up, it starts to overheat. It has a new water pump, thermostat, temp sensor, radiator cap, head gasket etc (All changed within the last year). Problem seemed to start after water pump change (changed couple weeks ago). Started out having no heat. After water pump change and flushing out, heater works fine now. Also I noticed when it overheats that coolant is spilling over from the coolant overflow tank.

 

Welcome to the 2nd Gen Neon forum.

I see you never received any replies, its a bit late & you've probably figured it out by now but never the less I'll see if I can get things started. It kinda sounds like you might not have gotten a good cooling system air burp.

Try parking with the front end up hill, the steeper the better, such that the radiator cap is the high point in the cooling system. or other wise raise the front end as much as you can.
Top up & mark the coolant overflow tank coolant level. Turn the cabin heater temp control to Max heat. Start & idle the engine for about 5-10 minutes until the thermostat opens, then run it at about 2000 rpm for 4-5 minutes, so to chase any entrapped air to the radiator. Leave the front end elevated & shut the engine down with the hood up, so it'll cool quicker.

After it completely cools, check the overflow tank coolant level to see if it dropped, if it did, the system burped some air & has topped itself up, so fill the overflow tank up to the mark again & repeat until the cooling system stops burping & the cooling system should be full & purged of any air blockage & ready to do its cooling thing. Some beginning thoughts for consideration, let us know how it goes.

 

I forgot to ask if you used a thermostat with the built in air by-pass poppet valve design, as that can help the system burp entrapped air. If you didn't use that design, it'll be very important to make sure you have the front end elevated & run the engine long enough for the thermostat to open, so any entrapped air can pass & get back to the radiator & pressure cap so it can be burped out when the system cools.

 

Thank you so much for the information! I replaced the thermostat again and cleared the P0128 code and it seem to be running okay. It warms up quickly now and has not overheated. I have heard that Neon's are difficult to burp out all of the air and just a little air in the cooling system can cause problems. What exactly is a thermostat with a built in air by-pass poppet valve. I am not familiar with that type of thermostat. I thought they all were basically the same. Do you have a specific part number for that type of thermostat? Your assistance with the issues with my Neon is sincerely appreciated...

 

Good feedback & to hear you seem to have your anomaly put right & things are back to normal.

If you installed the specified Mopar thermostat it'll have the air by-pass design valve. It's just a simple popett/jiggle design nothing fancy, but it is effective in helping us get the air out after a coolant drain & refill. One type looks like this Stant made Carquest thermostat, that's easy to see what it looks like, where its located & how it works. It's the brass colored jiggle thing centered in the hat brim flange of the thermostat assembly. https://shop.advanceautoparts.com/p/...4209/5653525-p