I have a 2005 Excursion with a 6.8L approximately 192K on it. I took it on a 2k mile trip and it ran fine the entire time. A few days after I got back, I had an issue when driving the truck there was luke warm heat and then heat was lost all together. The coolant reservoir was empty and it looked like it had push coolant out of the cap. I filled the reservoir and heat came back and no other issues. A few days after that after taking someplace with out any issues, I went to leave and after driving for a few minutes the heat gauge climbed quickly to overheating. I pulled over and shut it down and it was pushing coolant out of the cap. I restarted it and the temp gauge went to normal temp and it ran fine for the rest of my way home. I then changed the thermostat out and the issues persisted. I brought it to my friends garage and he sniffed it and found no combustion gases. I put a new water pump in it and let it come up to temp and drove it around with out any issue. The next time driving it after a few minutes the temp gauge climbed, I lost heat and it began pushing coolant out of the cap. I stopped shut it down I felt the thermostat open and coolant move through the upper radiator hose, so I start the truck and it was fine.

After getting home I let it sit overnight, topped off the tank, and let it idle in my drive way. It had heat, and as the temp rose the coolant level rose to the cap, I lost heat and it pushed a little out of the until the thermostat opened and the level dropped and heat came back.

I let it sit over night again, did not top off the tank and let it idle in the driveway again, same thing happen but it didnt push coolant out because the level was low to start with.

I let it sit and get cold again, topped it off, then started it and drove it after driving for a few minutes the temp began to climb above operating temp I lost heat, stopped, and it was pushing coolant out. I shut it down and felt thermostat open, started it and temp was normal with out any other issues for the rest of the drive.

If I let it idle up to temp before driving with the coolant level low it never pushes coolant out but I always loose heat just before the thermostat opens.

If I jump in it and drive it with the coolant topped off or low it will spike temp, I'll loose heat and push coolant out until the thermostat opens then it will be fine.

I want to know how the coolant system circulates coolant prior to the thermostat opening? In a normally functioning system does it circulate coolant through the heater core constantly preventing the pressure from building in the system until the thermostat opens? If so could the heater core be plugged up or restricted preventing the coolant from flowing enough to build pressure in the system prior to the thermostat opening and that's why I'm seeing pressure spikes prior to the thermostat opening?

Thanks in advance for any guidance.

 

A clogged heater core isn't going to do that. Since it was ran low on coolant, try purging all the air out of the system. Let it run with the cap off until it gets up to temp. Also try replacing the cap. If it continues to happen you likely have a pressurized leak, in that combustion gasses are making it into the coolant passages. I would not rely on the sniff test, there are chemical test kits for this.

 

Here’s the procedure to burp the cooling system
https://www.ford-trucks.com/forums/1...l#post14031875
hope this helps

 

Thanks. I'll do it again but what I did burb the system after the water pump was installed. I also installed a new reservoir cap when I put a thermostat.

I should also say, I never lose coolant. It always stays at whatever level it was at, unless it pushes it out the cap. Also my oil shows no signs of contamination from coolant and I dont see any white smoke from the exhaust or smell coolant from the exhaust.

 

...you can have what I had...exhaust gasses making it into the coolant system. Not all head gasket failures result in contaminated oil or vapors in the exhaust. The chemical block test is the only way to know for sure if you have exhaust gasses leaking into the coolant system.

 

Chemical block test was negative for exhaust gas. I let it run up to temp with cap off. Same thing happened, I lost heat just before the thermostat opened. It did not push coolant out, but it never does if it let it just idle up to temp.

yesterday I started it and just started driving stopping when the temp gauge started to rise and you could tell it was building pressure. I drove a little further and I lost heat so I stopped and it was just starting to push coolant out and I lost heat again, the thermostat hadn't opened yet. I shut it off and let it sit a minute then started it again, still no heat and the system was under pressure. I kept it running and held it at 1500 for a minute, it started to push a little coolant, still no heat. I shut it down again, gave it a few seconds then started it again. The thermostat opened i got heat back, and everything was fine after that. I parked it on a steep incline while running and took the cap off and let it run for a few minutes. Then parked it again.

To me it seems like a flow issue prior to the thermostat opening if driven before it opens.