You can have a sneaky leak around a clamp, like the bottom of the coolant tank, and it be hard to see, amy only leak while driving. Pressure test coolant system? The cold bleed practice of unbolting the tank, and raising it while bleeding helps.

 

Buzz check out my full method. All I'm doing is opening the t valve and filling the overflow tank till it comes out the t valve. Then starting the truck and get it up to operating temps and bleeding the t valve. Can I start the truck with the overflow lid off and check for air bubbles getting into the tank? I think I need to start it with the cap off and filling it after the t stat opens to a normal level. Can I do it this way or will it pull air into the system will the cap removed? Thanks for any info.

 

It won't pull air with cap removed. The air is not coming from the quantity in the tank, it just never has been allowed to escape and/or more is sneaking in around a clamp or HG is allowing exhaust gas into system.

You can raise the level of the tank, which pushes more air out of the system.

 

Try a brand new cap. I know from recent experiece that weird problem can happen with a old degraded cap.

The other way that air can get into the system is a leak where your coolant level gradually lowers.

Recommend purging out air, pressure testing system to look for leaks, and getting a new cap.

 

Hay that seems like what happens. It will be fine for months then I hear a little gurgle from the heater core and its low. If I add water it's fine. When it gets so low it will overheat. If it has a hg is wouldn't I see some air bubbles coming into the tank? I will swap the cap. Thanks for the info guys. Thank god I got a warranty from ford ESP on the lr3. Already paid for itself after the last visit. Lol

 

ok had a chance to try to work on it. Got the front end put back together and verified the electric fan is working. I filled the overflow until water came out the t valve with the screw. I let it idle for about 15min without overheating. I made sure the system was topped off and took it for a drive. I drove it about 30 miles at speeds around 50-60mph and it didnt overheat at all. Had hot heat the entire time without any gurgling under the dash. Do you guys think its fixed or just havent driven it enough. I would think that that long of a drive at that speed it should be good. I think im just going to sell it for 3200 or so and let the next buyer know that it could still have issues. I figure since im selling it about 2k below blue book would give the next buyer some extra money in case it needs headgaskets. thanks for any info Kevin

 

It does not sound like a very big leak and a cheap pressure test would tell you where its leaking from. If its a easy hose clamp then great its fixed if not then you know what the problem is.

 

If you still had air in the system that managed to make its way to the expansion tank, then as the vehicle cooled it would have drawn the coolant in to the system leaving the tank very low. Regarding the flow of coolant into the tank when the cap is opened, is this when the coolant is hot? If so, this normal. The system is under pressure when hot. If you let the pressure out by opening the cap, the coolant will flow in.